Azores Triple Junction + Hotspot

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Dynamics of Mantle Flow Around the AzoresTriple Junction: Constraints from Bathymetry and

Gravity DataRavi Darwin SankarFlorida State University

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Recommended CitationSankar, Ravi Darwin, "Dynamics of Mantle Flow Around the Azores Triple Junction: Constraints from Bathymetry and Gravity Data"(2009).Electronic eses, Treatises and Dissertations. Paper 2086.hp://diginole.lib.fsu.edu/etd/2086
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FLORIDA STATE UNIVERSITY

COLLEGE OF ARTS AND SCIENCES

DYNAMICS OF MANTLE FLOW AROUND

THE AZORES TRIPLE JUNCTION:

CONSTRAINTS FROM BATHYMETRY AND GRAVITY DATA

by

RAVI DARWIN SANKAR

A Thesis submitted to the

Department of Geological Sciencesin partial fulfillment of the

requirements for the degree of

Master of Science

Degree Awarded:Spring Semester, 2009

Copyright 2009

Ravi Darwin SankarAll Rights Reserved


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The members of the Committee approve the Thesis of Ravi Darwin Sankar defended on

March 06th

, 2009.

Jennifer Georgen

Professor Co-Directing Thesis

Jim TullProfessor Co-Directing Thesis

Vincent Salters

Committee member

William ParkerCommittee member

Approved:

__________________________________________________

Leroy Odom, Chair, Department of Geological Sciences

The Graduate School has verified and approved the above named committee members.


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To the four pillars of my life: God, my wife and my parents. I am not skilled to

understand what God has willed or planned but walking with Him and you through this

journey has given me strength. Danika, we were joined in marriage during the course of

this study. I love you. You mean everything to me, without your love, encouragement and

understanding I would not be able to make it. My parents, you have given me so much.

This is your degree. Mom, thanks for continuous faith in me, and for teaching me that I

should never relent. Dad, you always told me to never be satisfied with anything

ordinary. Thanks for inspiring my love for earth sciences.

We made it.

For I know the plans I have for you, declares the LORD, plans to prosper you and not to

harm you, plans to give you hope and a futureJeremiah 29:11-13


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ACKNOWLEDGMENTS

I would like to express my sincere appreciation to my advisor, Dr. Jennifer Georgen forher patience, understanding, professional and personal support throughout the duration of

this project, as well as funding for the latter part of the research. Thanks for giving me the

opportunity to be part of this work. It has been a tremendous honor working with you.You made excellence a habit. Thank you!

My advisory committee: Dr. Jim Tull, Dr. Vincent Salters and Dr. William Parker. I

admire your intelligence and leadership. I am grateful for your comments andsuggestions. Dr Stephen Kish, thanks for your presence in the lab and your

encouragement.

I am eternally grateful to the United States Department of State for the award of the

FULBRIGHT Faculty Development Scholarship. You allowed me to integrate with a

nation, university, people and culture that I have come to love and appreciate. JacquiGregoire, Amy Whitish, Renee-Hahn Burke, thanks for the second chance. There are not

enough words to describe your excellence. Pastor Matt and Laura Cates, thanks for your

friendship and support. It was a tremendous pleasure serving in ministry with you.

Great is your faithfulness oh God! Your grace is enough for me. Thanks for neverforsaking me. Thank you Father, Son and Holy Spirit. I owe you everything!


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TABLE OF CONTENTS

LIST OF FIGURES.....vi

ABSTRACT.vii

1. INTRODUCTION and MOTIVATION1

2. GEOLOGICAL SETTING OF STUDY AREAMid-Atlantic Ridge.3

The Azores Triple Junction.4

The Terceira Rift.6Mantle Plumes and the Azores Hotspot..8

Alternatives to the Thermal Plume Hypothesis.11

3. METHODOLOGYBathymetry and Free-Air Gravity Data.....13

Mantle Bouguer Anomaly.14

4. RESULTS

MBA Patterns along the Mid-Atlantic Ridge16

MBA Patterns for the Azores Archipelago and along the TR...18

5. DISCUSSION

Azores/Terceira Rift Waist Width.....20Influence of Triple Junction Geometry. 22

6. CONCLUSIONS..24

7. APPENDIX A...25

8. REFERENCES.37

9. BIOGRAPHICAL SKETCH..42


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LIST OF FIGURES

Figure 1: Location of primary study area in reference to the Mid-Atlantic Ocean. Base-

map produced from data obtained by Smith and Sandwell (1997)....25

Figure 2: Map of the ridge-ridge-ridge (RRR) Azores Triple Junction indicating thebranch locations of the Mid-Atlantic Ridge and the Terceira Rift, in reference to the

Azores hotspot...26

Figure 3: Simplified Azores Triple Junction geometry and ridge configuration, indicating

variations in ridge spreading rate and plate motion with respect to the triple junction.....27

Figure 4: Regional location map of the Azores plateau indicating the existing position of

the triple junction, the islands comprising the Azores volcanic archipelago as well as the

major fracture zones in the region.............................................................28

Figure 5: Isotope systematics of Terceira Rift lavas.29

Figure 6: Averages of crustal thickness and Na8 at various mid-ocean ridges, as well asfor the Azores region confirming the negative correlation of Na8 with

Fe8..............................................................................................................30

Figure 7: Regional scale bathymetric map for the Azores Triple Junction, indicating

shiptrack coverage, V-shaped bathymetric features as well as the location of the Azores

hotspot, in reference to the triple junction.....................31

Figure 8: Free-air anomaly (FAA) map for the Azores region, reflecting the density

contrast of the seafloor in the area.32

Figure 9: Map of Mantle Bouguer anomaly correction, generated from bathymetry

data.33

Figure 10: Mantle Bouguer anomaly (MBA) map, calculated by subtracting the mantle

Bouguer correction from FAA, while assuming a constant 5 km thick reference

crust34

Figure 11: Axial bathymetry profile (11a) and filtered MBA profile (11b) along theTerceira Rift, with increasing distance from the Azores Triple

Junction..........................................................................................................35

Figure 12: Along-isochron widths of residual bathymetric anomalies W, versus half-

spreading rates U (12a). Figure 12b is a plot of along-isochron amplitudes of MBAplotted against ridge-hotspot distances..36


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ABSTRACT

Mid-ocean ridge interactions with hotspots strongly affect mantle flow processes. This

study analyzes the geophysical anomalies produced as a result of the interaction between

a hotspot and an oceanic ridge-ridge-ridge triple junction, in close proximity to one

another. The complex three dimensional (3D) nature of the Azores Triple Junction

(ATJ), in which two near-collinear faster-spreading ridges are joined orthogonally with a

slower-spreading ridge, provides an excellent opportunity to quantify the effect of triple

junction geometry on along-axis magmatic accretion and mantle dynamic processes as a

result of the interaction with a hotspot. For the ATJ, the faster-spreading ridges are twobranches of the Mid-Atlantic Ridge (MAR), and the slower-spreading ridge is the

Terceira Rift (TR).

Using shipboard bathymetry and satellite free-air gravity data, we obtain mantle Bouguer

anomaly (MBA) by eliminating from free-air gravity the attractions of seafloor

topography and a reference crust. Along the TR, the Azores hotspot has a maximum

MBA axial gravity low of 100 mGal, suggesting localized crustal thickening, elevated

mantle temperatures and/or low density mantle. The entire Azores plateau along the TR is

associated with a large (~ -80 mGal) broad MBA low. Dispersion of plume material

along the TR, a distance in the range of 550 km, is likely minimized by the rift systems

obliqueness, immature nature and ultra-slow spreading rate, as well as the presence of the

Gloria FZ. Further, along-axis profiles along the TR suggest that MBA shows a strong

dependence on the tectonic segmentation of the ridge axis.


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CHAPTER 1

INTRODUCTION AND MOTIVATION

This investigation focuses on the dynamics and generalized characteristics of mantle flowprocesses in the vicinity of the Azores Triple Junction (ATJ). The ATJ is an oceanic

ridge-ridge-ridge (RRR) triple junction formed at the intersection of the North American

(NA), African (AF) and Eurasian (EA) plates. The ATJ is made up of northern and

southern branches of the Mid-Atlantic Ridge (MAR) and a third branch to the east of the

MAR referred to as the Terceira Rift (TR). The TR represents a unique oceanic rift

system situated within thickened, relatively old oceanic lithosphere and it exhibits both

oceanic and continental features (Beier et al., 2008). The Azores archipelago is the

surface manifestation of a hotspot that has come into contact with a mid-ocean ridge.

Studies have shown that where a ridge and a plume coincide, a large quantity of melt

production in the mantle, amplified crustal thickening, and unusually robust seafloor

volcanism take place. The stable configuration of the ATJ offers an exceptional

opportunity to study the interaction of three spreading centers with varying divergence

rates in close proximity to a hotspot.

The focused pattern of mantle upwelling characteristic of mantle plumes has a direct

effect on the density structure of the upper mantle. It has been hypothesized that regions

of ascending plume flow are related to negative density anomalies that are a consequence

of high mantle temperatures and thickened crust. These variations are reflected in mantle

Bouguer anomaly (MBA) maps (Kuo and Forsyth, 1988; Lin et al., 1990). MBA maps

yield important information regarding mantle flow dynamics and can also help to

characterize the interaction between the Azores hot spot and the ATJ.

This research combines an analysis of shiptrack bathymetry and satellite altimeter gravity

data in order to determine MBA for an area in the central Atlantic Ocean. The region of

study for this investigation is the zone bordered by 350

N 450N, 25

0W 35

0W

(Appendix A, Figure 1) with significant emphasis placed on the Azores hot spot complex,

located between 340

and 450

N along the MAR, as well as on the Terceira Rift (Appendix


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A, Figure 2). The unique setting of the Terceira Rift in the Azores Plateau, with an

ultraslow spreading axis above a melting anomaly, also allows this research to address

the fundamental question of how melting processes along extremely slowly-diverging

ridges are influenced by mantle dynamics.

This analysis reveals that the Azores plateau is characterized by a large regional MBA

low, implying substantial crustal thickening and/or anomalously low mantle densities

(e.g., due to an uncharacteristically warm mantle). An examination of the MBA

signatures allowed us to delineate a 550 km length of the TR affected by the Azores

hotspot. The amplitude of the anomaly along the TR is ~ -100 mGal. The presence of

long-wavelength bathymetric and gravity anomalies, extending several hundred

kilometers away from the Azores hotspot along the SMAR (South Mid-Atlantic Ridge)

and NMAR (North Mid-Atlantic Ridge) axes, suggests that ridge-hotspot interaction near

the triple junction occurs over a broad, three-dimensional region.


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CHAPTER 2

GEOLOGICAL SETTING OF STUDY AREA

Mid Atlantic Ridge

The Atlantic Ocean is the worlds second largest and youngest ocean, succeeding the

Pacific Ocean, and is surrounded by the European, African, North and South American

continents, as well as Antarctica. The Atlantic Ocean is characterized by an Sshaped

elongated basin that extends from the Arctic Ocean in the north, southward into the

Indian Ocean. Centrally located between the continental shores of the Atlantic is an

underwater mountain range, known as the MAR. The MAR is a divergent boundary that

separates the North American plate from the Eurasian plate in the North Atlantic, and the

South American plate from the African plate in the South Atlantic. The ridge sits on top

of a progressive bulge, referred to as the Mid-Atlantic rise, which proceeds along the

entire length of the Atlantic Ocean. A relatively small number of volcanic islands located

in close proximity to the axis of the MAR comprise the Azores archipelago.

The origin and subsequent development of the Atlantic Ocean commenced with the

break-up of Pangaea during the Triassic-Jurassic period about 230 million years ago.

Around 200 Ma magmatic injection in the form of giant dike swarms began almost

synchronously (Salters et al., 2003), providing a legacy of basaltic dikes and lavas over a

vast area along the margins of the North and South American continents as well as on the

Iberian peninsula of the European continent, referred to as the Central Atlantic magmatic

province (CAMP) (Olsen, 1997).Rifting of the southern portion of the Atlantic, resulting

in the separation of Africa and South America, initiated 135 Ma (Fowler, 2005). Northern

Atlantic rifting continued through the Tertiary, leading to the separation of Greenlandfrom North America and Eurasia (Fowler, 2005). Separation of the Atlantic continues

today at the rate of several centimeters a year along the MAR.

Seafloor magnetic data have been used extensively to aid in the determination of past and

present spreading rates along the MAR. Asymmetric spreading has resulted in a west-


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southwest migration of the MAR since 36 Ma (Searle, 1980). The current spreading rate

of the MAR is on the order of 1 to 4 cm/yr half-rate, which categorizes it as a slow

spreading ridge. The branch of the MAR immediately to the north of the ATJ is

spreading at a half rate of 1.2 cm/yr, while the southern branch is diverging a little

slower, at a half rate of 1.1 cm/yr (Georgen and Lin, 2002) (Appendix A, Figure 3). The

spreading kinematics of the MAR in the vicinity of the ATJ have been well constrained

for the past 10 Ma (Luis et al., 1994). There has been little along-axis variation in

spreading rate during the last 7 Ma (Yang et al., 2006).

As mentioned above, the ATJ is the site where the African, North American and Eurasian

lithospheric plates meet. Along the MAR, the Pico (PFZ) and East Azores (EAFZ)

fracture zones are located to the south of the triple junction, whereas the North Azores

fracture zone (NAFZ) lies to the north of the triple junction (Appendix A, Figure 4).

Between the NAFZ and the PFZ lie the Faial fracture zone, the Acor fracture zone and

the Princess Alice fracture zone.

The Azores Triple Junction

A distinguishing feature of the global mid-ocean ridge system is the location where three

plate boundaries come together at a single point. Such a geological setting is referred to

as a triple junction. Triple junctions allow us a unique opportunity to study the

geodynamics of the mantle and lithosphere (Georgen and Lin, 2002). Triple junctions are

made up of three branches, each of which may be a transform fault (F), trench (T), or a

ridge (R). There are many different triple junction configurations (McKenzie and

Morgan, 1969), and these are thought to induce variations in along-axis flow and thermal

patterns (Georgen and Lin, 2002).

The two fastest spreading ridges of the ATJ have nearly equal spreading rates and are

roughly collinear, forming a trend which the slowest spreading ridge, the Terceira Rift,

intersects quasi-orthogonally (Appendix A, Figure 3). High resolution studies of RRR

triple junctions have shown that, rather than meeting at a strict geometrical point, the


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three spreading branches often fail to connect (Georgen and Lin, 2002). The ATJ is an

example of a region of such non-connectivity. The slowest spreading branch of the ATJ

is detached from the triple junction by a zone of diffuse deformation, leading to the

suggestion that spreading takes place across an expansive area, rather than linearly along

three well defined ridges (Searle, 1980). The inability of the slowest spreading ridge to be

attached to the faster ridges is expected to minimize the overall upwelling beneath the

triple junction, and thus counteract some degree of the enhanced upwelling caused by

triple junction geometry (Georgen and Lin, 2002).

There is evidence that the ATJ has existed since approximately 45 Ma (Krause and

Watkins, 1970; Searle, 1980). About 36 Myr ago, the ATJ migrated northward from the

meeting point of the EAFZ and the MAR to its current position at 38.550N, 30.00W

(Appendix A, Figure 4) (Yang et al., 2006). With this migration, the triple junction also

experienced a change in its configuration from ridge-fault-fault (RFF) to ridge-ridge-

ridge (RRR). Various authors have suggested mechanisms explaining the evolution of

the triple junction but aspects remain unclear. For example, Searle (1980) expanded on

two earlier theories put forward by Krause and Watkins (1970) and McKenzie and

Morgan (1969), saying that a reversal in the relative motion between the Eurasian and

African plates produced an element of extension across the EAFZ, which led to the origin

of the Terceira Rift.

The Azores volcanic plateau is characterized by the presence of topographical roughs that

protrude from the sea surface, leading to the formation of the islands of the Azores

archipelago (Appendix A, Figure 4). The islands are arranged along a WNW ESE

direction and cross the MAR obliquely, while being limited by the EAFZ to the south.

On the eastern side of the MAR, a series of en echelon basins extending from the West

Graciosa Basin to the Formigas Trough, as well as the islands of Graciosa, Terceira,

S.Miguel, S.Maria, S.Jorge, Faial and Pico, all combine to form the linear trending

structure that is the Terceira Rift. On the western side of the MAR, on the American

plate, lie the islands of Corvo and Flores (Miranda et al., 1998).


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As discussed in more detail below, the Azores plateau probably formed by a melting

anomaly in the mantle either as a result of a small thermal plume (Luis et al. 1998,

Cannat et al. 1999, Vogt and Jung, 2004) or of anomalously volatile-enriched mantle

(e.g., Bonatti, 1990). The current position of the postulated Azores plume conduit is

likely ~100300 km east of the triple junction and to the south of the Terceira Rift, in the

locality of Faial Island (Schilling, 1991; Zhang and Tanimoto, 1992; Georgen and Lin,

2002; Yang et al. 2006). Recent research has invoked the possible presence of a mantle

plume beneath the area surrounding the triple junction as the driving force behind plate

boundary reorganization in the ATJ(Yang et al. 2006). According to Yang et al. (2006),

southwestward movement of the Eurasian and African plates in relation to the hotspot

reference frame, coupled with hot upwelling mantle east of the Acor and North Azores

fracture zones, served to weaken the overlying lithosphere between the EAFZ and the

present-day Terceira Rift. This led to the generation of a new rift adjacent to the plume

center and the corresponding northward relocation of the triple junction toward the center

of the plume.

Interaction between a mantle plume and a triple junction may result in the formation of a

large igneous province (LIP) (Sager et al., 1999). The genesis of a LIP or an oceanic

plateau begins as plate migration brings a triple junction into the vicinity of a mantle

plume. As the plume fuses with the triple junction, it delivers to it anomalously warm

asthenospheric mantle material, resulting in voluminous magmatic activity and

anomalous geophysical and geochemical signatures (Georgen, 2008). It is possible that

interaction between the Azores hotspot and ATJ enhanced volcanism in the Azores

Plateau.

The Terceira Rift

The Terceira Rift is an integral component of the Azores Plateau. Located to the east of

the MAR, in the vicinity of the northeastern edge of the Azores Plateau, this 550 km long

axis forms the third branch of the RRR triple junction involving the North American,

Eurasian and African plates, and has been proposed to be an ultra-slow diverging


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spreading ridge (Vogt and Jung, 2004). The predicted opening rates along the TR based

on plate motion closure about the triple junction decrease from 0.45 cm/yr in close

proximity to the triple junction, to 0.39 cm/yr at the eastern end of the Azores Plateau

(Vogt and Jung, 2004). The Terceira Rift is a relatively recent feature; divergence likely

began only in the last 5 Myr (Luis et al., 1998; Vogt and Jung, 2004; Georgen, 2008).

Thus, the Terceira Rift is a unique example of oceanic rifting that exhibits both

continental (e.g. lithospheric thickness) and oceanic (melt availability, segmentation

pattern) features (Beier et al., 2008).

The Terceira Rift is characterized by unusual morphological characteristics (Vogt and

Jung, 2004). An earlier study (Searle, 1980) found that the Terceira Rift is made up of a

number of echelon basins, with volcanic massifs and ridges situated on both sides. Ultra-

slow spreading ridges often consist of connected magmatic and amagmatic accretionary

ridge segments that exist together over millions of years (Dick et al., 2003). Magmatic

segments of oceanic ridges may in some cases construct large volcanic structures, leading

to the evolution of volcanic islands. Three of the nine volcanic islands (Sao Miguel,

Terceira, Graciosa) of the Azores archipelago are situated directly along the axis and are

separated by deep avolcanic basins (Beier et al., 2008). Additionally, along ultra-slow

ridges, segments can transition between varying angular orientations in order to form

stable plate boundaries (Dick et al., 2003). The Terceira Rift consists of a continuous line

of ridge segments that vary in their obliqueness to the direction of relative plate motion,

ranging from 400

to 650

(Vogt and Jung, 2004).

Although the opening rates of the two MAR branches exceed that of the Terceira Rift

(Appendix A, Figure 3), it has been noted that the amplitudes and wavelengths of

seafloor topographic variations along the two ridges are similar (Vogt and Jung, 2004).

Also, the widths of the volcanic islands (30-60 km) are analogous to the MAR axial

valley widths (20-40 km). The inter-island basin depths lie within the range of other

ultra-slow spreading ridges (Vogt and Jung, 2004). The Terceira Rift valley is

approximately 1500 m deeper than the rift valley associated with the MAR; this is


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thought to be the result of the very slow opening rate, and hence thicker axial lithosphere

(Vogt and Jung, 2004).

Mantle Plumes and the Azores Hot Spot

The plume hypothesis was initially proposed by Wilson (1963) and Morgan (1971) to

explainthe clear age-progressive chains of volcanic islands that extend across the ocean

basins. Wilson postulated that these volcanic chains were produced by plates moving

over stationary hotspots in the mantle. Morgan reasoned that if Wilsons theory was

accurate, hotspots comprising mantle plumes should originate from deep in the mantle.

The Azores hotspot is likely to considerably influence the geodynamics of the ATJ

because of its close proximity to the triple junction. One of the strongest lines of evidence

that suggests the Azores hotspot has a direct impact on the MAR is the presence of off-

axis V-shaped ridge bathymetric features (Escartin et al., 2001; Yang et al., 2006). V-

shaped ridges can originate from a pulsing, dehydrating and radially flowing mantle

plume (Ito, 2001). More specifically, Escartin et al. (2001) contend that the bathymetry,

tectonic structure and gravity data around the ATJ demonstrate that the V-shaped ridges

result from the emplacement of anomalously large volumes of magma at the ridge axis.

These time dependent V-shaped ridges exhibit an east-west asymmetry (Appendix A,

Figure 4) that may be the result of sub-lithospheric flow between the MAR and an off-

axis plume (Yang et al., 2006). Also, although V-shaped bathymetric highs are seen to

extend southwestward along the MAR, they are not prominent along the MAR to the

north of the triple junction, suggesting the flow of plume material dispersed preferentially

in the southerly direction (Cannat et al., 1999).

The presence of the Azores hotspot also imparts prominent bathymetry and gravity

anomalies on the MAR axis. The most pronounced effect of the Azores hot spot on the

MAR occurs along a relatively short section of the ridge between 380N and 40

0N (Detrick

et al., 1995). In this area, the ridge axis rapidly shoals by more than 1000 m, crustal

thickening takes place, and the deep axial rift valley that characterizes the MAR in much


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of the North Atlantic disappears (Detrick et al., 1995). The Azores hotspot influence on

the MAR is also reflected in both the free air anomaly (FAA) and MBA. Detrick et al.

(1995) found that FAA is more positive toward the Azores while the MBA becomes more

negative, with the entire Azores platform being associated with a large (-60 mGal

amplitude) negative MBA. Residual bathymetry and MBA anomalies are a maximum at

the Azores hotspot and decrease southward, until becoming insignificant at distances of

approximately 500-750 km (Ito and Lin, 1995). In contrast, Goslin et al. (1999) found

that negative values of MBA extend north to 43005N, a distance of only approximately

300 km along the ridge axis. In general, the absence of geophysical anomalies associated

with the presence of the hotspot northward of 430N appears to be indicative of a weak

mantle plume influence along the ridge north of the Azores (Goslin et al., 1999; Yang et

al., 2006).

There is also significant geochemical evidence pointing to the influence of the Azores

hotspot on nearby ridge magmatism. Rock samples acquired from the vicinity of the ATJ

display enriched mid-ocean ridge basalt (E-MORB) signatures with distinctive

characteristics in trace element and isotopic ratios such as La/Sm, 87Sr/86Sr, 143Nd/144Nd,

206Pb/204Pb, and 3He/4He. For example, White et al. (1976) found generally high 87Sr/86Sr

ratios along the SMAR. The elevated 87Sr/86Sr ratios south of the ATJ were interpreted as

being the result of mixing of depleted mantle and plume material, consistent with the

influence of the Azores hotspot, and correlate well with changes in mid-ocean ridge

depth, suggesting a close link between underlying mantle composition and physical ridge

characteristics. Axial 87Sr/86Sr ratios over the NMAR display long wavelength variations

(Goslin et al., 1998), reflecting a broad pollution of the asthenosphere by radiogenic Sr

mantle material, which has been partly depleted of incompatible elements and may be

related to the partial melting and dispersion of the Azores plume (Fontignie and Schilling,

1996).

Beier et al. (2008) found that lavas from the volcanic systems of the TR return generally

elevated Sr isotope ratios but highly variable and distinct143

Nd/144

Nd ratios. The lowest

143Nd/

144Nd occurs in rocks from Sao Miguel with slightly higher

143Nd/

144Nd ratios at


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Graciosa, and the most radiogenic143

Nd/144

Nd in lavas from Terceira. Appendix A,

Figure 5 shows that on a diagram of143

Nd/144

Nd vs.87

Sr/86

Sr, all lavas of the Terceira

Rift (except Sao Miguel) lie on a broad positive correlation, while Sao Miguel forms a

well correlated array, orthogonal to the other Terceira Rift lavas. In general, Beier et al.

(2008) showed that each volcanic section of the TR forms a unique trend in Sr-Nd-Pb

isotope space in which the systems of the TR as well as the MAR converge at a

composition with87

Sr/86

Sr 0.7035,143

Nd/144

Nd 0.5129, and206

Pb/204

Pb 19.5.

Helium isotopic data for basalts from the MAR and Azores archipelago show that the

Azores has 3He/4He ratios both higher and lower than nearby MORB values (Moreira et

al., 1999). In general, the wavelength of He isotope variation is sometimes less than that

of other tracers such as Sr and Pb isotopes (Schilling et al., 1998) where plumes affect

spreading ridges. This has been attributed to a relatively deep degassing process within

the rising mantle plume, causing a strong peak in the helium anomaly in the vicinity of

the plume center while relatively degassed plume material that is still effectively traced

by Sr, Nd or Pb isotopes becomes laterally dispersed at shallower depths (Schilling et al.,

1998). In the Azores,3He/

4He ranges between 3.5 RA and 15 RA (Kurz et al., 1990;

Moreira et al., 1999; Moreira and Allegre 2002), with the highest values concentrated

near the triple junction or in association with the volcanic islands. There is also a good

co-variation of3He/

4He with Pb isotopes at the scale of the archipelago, indicating

considerable He isotope heterogeneity within the mantle source region (Moreira et al.,

1999). The lowest3He/

4He is associated with elevated

207Pb/

204Pb in basalts from Sao

Miguel while the highest3He/

4He occurs in lavas from Terceira that has

207Pb/

204Pb that is

closer to typical MORB. The low3He/

4He signature at Sao Miguel is attributed to

shallow level mixing between the plume and portions of continental lithosphere that

occurred during rifting and opening of the North Atlantic.

Major element geochemical studies provide further evidence for excess mantle

temperatures related to enhanced melting at hot-spot influenced ridges (Ito et al., 2003).

According to Langmuir et al. (1992) extensive melting can be triggered by high mantle

temperatures and/or variations in mantle composition. Extensive mantle melting leads to


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thicker crust and a low concentration of the incompatible element Na. Other elements

such as Fe are sensitive to the depth at which melting originates. Shen and Forsyth (1995)

showed that anomalously hot mantle begins to melt deeper and has a relatively high Fe

content with a relatively low Na content, consistent with evidence of a negative

correlation between regional averages of Fe8 and Na8 (Klein and Langmuir, 1987). For

the Azores region, observed values are lower in Na8 and higher in Fe8 (Shen and Forsyth,

1995) (Appendix A, Figure 6).

Alternatives to the Thermal Plume Hypothesis

There has been ongoing debate as to whether or not the Azores hotspot is caused by the

influence of a mantle plume. Several lines of evidence point to the hotspot as a plume-

like structure. For example, the hotspot is associated with bathymetry and gravity

anomalies (Detrick et al., 1995; Ito and Lin, 1995; Cannat et al., 1999). Finite frequency

seismic tomography, employing the use of P-wave velocity, may indicate the existence of

a distinct, deep, thermal plume in the vicinity of the Azores, although resolution issues

may introduce ambiguity (Montelli et al. 2004). Excessive volcanism associated with the

formation of the Azores islands lends further support to the existence of the Azores

hotspot (Moreira and Allegre, 2002).

Although the excess volcanism and crustal accretionary processes in the Azores region

may result from the effects of anomalously high mantle temperatures related to a thermal

plume, there is evidence to suggest that Azores hotspot volcanism may instead be caused

by compositionally distinct mantle. Bonatti (1990) found that the presence of H2O and

CO2 enriched domains in the upper mantle around the Azores hotspot, inferred on the

basis of geo-thermometry of basalt and peridotite data, served to lower the solidus

melting temperature of the hotspot mantle by hundreds of degrees and enhance partial

melting. The elevated enrichments in light rare-earth elements (LREE) and the high

La/Sm ratio of Azores hotspot basalts coupled with the abundance of incompatible large-

ion lithophile elements (LILE), serve as a catalyst for enhanced melting (White et al.,

1976), evidence for compositional plume involvement as opposed to a thermal plume.


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The presence of compositionally-distinct material beneath the whole Azores Platform and

west of the MAR may explain the young volcanism of the two islands west of the MAR

by passive melting of enriched material within the mantle anomaly (Beier et al., 2008).

The lack of consistent tomographic evidence for all plumes has led to the notion that

hotspots could instead be the manifestation of shallow, plate related stresses that would

fracture the lithosphere, causing volcanism to occur (Foulger and Natland, 2003). With

the presence of three plate boundaries and a diffuse zone of deformation around the triple

junction point, it is likely that a complex pattern of lithospheric stress exists in the Azores

region. Further, the non-existence of a clear age progression along the islands of the

Azores archipelago, unlike whats observed along the Hawaiian island chain, is one of

the most potent arguments against plume involvement. A goal of this investigation is to

better constrain the excess volcanism around the ATJ, and particularly along the Terceira

Rift, to investigate the relative importance of plume vs. plate boundary magmatism in the

region.


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CHAPTER 3

METHODOLOGY

The unique geometry of the Azores, coupled with the magnitude of the buoyant

topography, the long wavelength bathymetric and gravity gradients, and the geochemical

anomalies observed along the MAR and Terceira Rift, provide an opportunity to explore

hotspot-related mantle flow and thermal patterns around an oceanic RRR triple junction.

This study adds further constraint to ATJ-Azores hotspot interactions by calculating

MBA and analyzing gravity anomalies along the Terceira Rift.

Bathymetry and Free-Air Gravity Data

The bathymetry data for this study were obtained from Smith and Sandwell (1997), in

which a digital, bathymetric map of the oceans was derived by combining sparse

measurements of seafloor depth from shipboard soundings with dense high resolution

satellite marine gravity information obtained from the Geosat and ERS-1 spacecraft

(Smith and Sandwell, 1997). This combination of data yields a global uniform level of

resolution (1), ideal for displaying major tectonic features, and serves to increase our

understanding of the accretionary processes along nearly all mid-ocean spreading centers

(Smith and Sandwell, 1997).

Data quality and spatial density are the most important aspects of bathymetric prediction.

The onset of international programs on ridge research has increased detailed surveying of

the northern Atlantic, with swath bathymetry measurements over the MAR domain as

well as the Azores Plateau. The plot of these shiptracks (Appendix A, Figure 7) shows

the ridge axis has very dense coverage. Around the ATJ, shiptracks are rather

concentrated on some lanes but nevertheless have a good spatial coverage for sufficient

bathymetric analysis.


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Free-air gravity data were taken from the satellite altimetry-derived gravity grid of

Sandwell and Smith (1997). The resolution of the map is 2 minutes. Free-air anomaly is

dominated by the gravitational effect of the seafloor topography, but also contains signals

from deeper interfaces, such as the crust-mantle density contrast. Appendix A, Figure 8

shows an example of a free-air gravity map which contains signals from sediments,

seafloor topography, crustal and mantle density anomalies.

Mantle Bouguer Anomaly (MBA)

In order to isolate the effects of sub-seafloor density structure on gravity in the ATJ

region, and to reveal the more subtle crust and mantle anomalies, MBA (Appendix A,

Figure 10) is generated by subtracting from free-air gravity data, the MBA correction

(Appendix A, Figure 9) due to the computed attractions of the seafloor/water and the

crust/mantle interfaces assuming a constant density crust (Kuo and Forsyth, 1988). The

gravity effects of the water-crust and crust-mantle density interfaces were calculated

using Parkers (1973) upward continuation two-dimensional fast Fourier transforms

(FFT) approach. The modeled crust was assumed to be 5 km in thickness while the

densities for seawater, crust, and mantle were assumed to be 1030, 2800, and 3300 kg/m3

respectively. The gravity effects were subsequently removed from the FAA at each point

along the ship-tracks and gridded in order to obtain MBA.

MBA variations can arise from differences in crustal thickness, variations in crustal or

upper mantle density, or a combination of these effects (Lin et al, 1990; Detrick et al.,

1995). Some of the factors that lead to an MBA gravity low are considerable crustal

thickening, anomalously warm mantle, or low crustal or mantle density due to

compositional effects. Thus, plume-affected areas are expected to have relatively low

MBA. Many studies involving MBA calculations often contain a subsequent step in

which the predicted, gravitational effects of the cooling of the lithospheric mantle is

considered. The thermal correction associated with this effect of cooling with age is

removed from the MBA signal and the resulting anomaly is called the residual MBA

(RMBA). An uncertainty in lithospheric ages in the vicinity of the TR made the


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determination of the lithospheric cooling correction for this investigation impossible and

as a result, RMBA was not calculated.

The calculation of MBA requires independent bathymetry and gravity data sets.

However, since both of the globally-gridded bathymetry and gravity data sets used for

this study contain information from satellite-derived sea surface height, it is necessary to

remove this component from one of the data sets. Accordingly, seafloor depths are

extracted from the predicted topography database only along lines constrained by

shiptrack soundings, using data flags provided by Smith and Sandwell (1997). The track-

line bathymetry data were then projected onto a 2n by 2m grid, where n and m are

integers, because the use of Fourier transforms in the MBA program requires the number

of grid points in the east-west (x) and north-south directions (y) to be powers of 2. For

this calculation, the number of grid points in the x and y directions were 512 and 256,

respectively, resulting in grid spacings of dx = 4.7 km and dy = 7.0 km. The MBA grid

for this study was filtered using a 40-km low-pass filter, to eliminate wavelengths below

which shiptrack gravity does not correlate well with satellite-derived gravity.


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CHAPTER 4

RESULTS

The ATJ is characterized by a large regional MBA low, with distinct intermediate to long

wavelength trends along the TR and the MAR, suggesting that the Azores hotspot

influences the thermal structure of accretionary processes, at least to the extent of our

map limits (30- 460N). Although the MBA lows seem to preferentially extend along the

ridges, there is an area beneath the intersection of the TR and the SMAR between 3250-

3300W longitude and 36

0-40

0N latitude, in which the anomalies appear to be

characterized by a circular gravity low pattern. Detrick et al. (1995) defined the shape of

the MBA low around the triple junction and the SMAR as a bulls eye, in agreementwith our observations.

MBA Patterns Along the MAR

Along the MAR, bathymetry and MBA values to the north and the south of the triple

junction differ considerably. Along the SMAR between 350N and 400N, figures 5 and 8

reveal bathymetric and gravity anomalies that extend several hundred kilometers along-

axis from the Azores. We follow Cannat et al. (1999) in suggesting that the long-

wavelength bathymetry anomaly extending southwards along ~700 km of the MAR in an

oblique V-shaped pattern is indicative of southward buoyant mantle plume material flow

from the Azores hotspot, during a period of highly focused magmatism. Our MBA map

for the same region shows that the waist width (W, the length of ridge showing hotspot-

related anomalies) of the on-axis gravity lows correlates well with the shallow areas of

the V-shaped ridge. We define the plume waist width along the SMAR to extend

southwards from the triple junction to a latitude of 340N. This agrees well with the

length of the strontium (87Sr/86Sr) anomaly along the same branch of the MAR (Ito et al.,

2003). The highest-amplitude off-axis gravity low (-95 mGal) is associated with the

region to the west of Faial Island at ~3300

W, 380

N and extends continuously along the

southern branch of the ridge to ~3260

W, 360

N, coinciding with the southern termination


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of the V-shaped ridge where MBA is approximately -85 mGal. The axial MBA values

progressively increase southwards to -60 mGal at the southern extent of our map limits.

Discontinuities between segments along the SMAR (Detrick et al., 1995) lead to short-

wavelength but significant variations in the amplitude of the gravity signatures. For

example, between 3240

W, ~350

N and 3250

W, ~350

N, there is a significant offset in the

ridge system called the Oceanographer transform. South of the Oceanographer transform

there is a large (-80 mGal) circular gravity low centered over the segment midpoint at

324.50 W. In general, the low MBA values coupled with the bathymetric trends observed

along the slow-spreading southern MAR domain indicate a pattern of segment-scale melt

focusing processes (Kuo and Forsyth 1988; Lin et al., 1990) leading to locally increased

melt production, superimposed over longer-wavelength trends associated with the Azores

hotspot.

Interpretation of the MBA signature along the NMAR in Figure 8 points to the

comparatively restricted influence of the Azores hotspot on ridge accretionary processes

north of the triple junction. From the intersection of the ATJ northwards to ~430N, MBA

magnitudes increase slowly, from -60 to -40 mGal. We define waist width along the

NMAR to extend from the intersection of the triple junction at 39.80N to ~42.8

0N, a

distance of approximately 300 km. This geophysical value ofWfor the NMAR compares

well to the geochemical waist width observed by Goslin et al. (1999), in which basalts

with increasingly lower (Nb/Zr) ratios were recovered progressively northwards of the

Azores triple junction, with an abrupt change from enriched (Nb/Zr = 1.6) to depleted

(Nb/Zr = 0.7) basalts at the 420N discontinuity. Consistent with our observations, Goslin

et al. (1999) placed the northerly limit of the Azores hotspot influence on the MAR

between a transition zone of 430N and 44

0N. In direct contrast, our value ofWfor the

SMAR is ~700 km away from the hotspot.

At 350N, the existence of the Oceanographer transform fault may serve to offset the ridge

out of the primary area of plume influence which acts to progressively decrease the

plume waist width (Detrick et al., 1999). Unlike the SMAR, however, no major


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transform discontinuity offsets the axis of the NMAR. Conceptually, the absence of

transforms over the northern domain would encourage the along-axis flow of plume

material thereby promoting longer plume waist widths (Georgen et al., 2001). The MBA

signature though, does not support this observation. Instead, following Goslin et al.

(1999) the influence of the plume processes on the NMAR appears to be closely

constrained along the ridge axis with minimal off-axis plume dispersion, indicative of a

steeper horizontal temperature gradient in the upper mantle over the northern domain.

Results from the bathymetric and MBA analysis along the NMAR suggest a relatively

thinner crust emplaced by lower magmatic output underlying the northern domain,

compared to the SMAR. Another significant difference between the structures of the

section of the ridge north of the ATJ and that observed south is the absence of V-shaped

bathymetric features on the northern flanks of the ridge, implying different temporal and

spatial variation in melt supply in NMAR-hotspot interactions compared with SMAR-

hotspot interactions.

MBA Patterns for the Azores Archipelago and Along the TR

MBA values obtained for the entire Azores plateau are large and negative, allowing us to

characterize the overall spatial extent of the gravity anomaly over the region as

expansive. From the intersection of the triple junction trending eastward, the MBA

pattern forms a broad, circular low (-100 mGal to -95 mGal) in the vicinity of Faial

Island (38.70N, 3310W). The islands of Graciosa (-75 mGal) and Terceira (-100 mGal)

return prominent MBA lows similar to that of Faial. Appendix A, Figure 11b shows that

the lowest value of MBA along the axis of the TR occurs at Terceira Island. The more

gradual slopes on either side of the MBA low at Terceira Island may mirror small

variations in crustal thickness or upwelling rate, and/or along-axis mantle temperature

gradients. MBA lows are also found in the region of the remaining volcanic features

along the axis of the Terceira Rift including Castro Bank (-60 mGal), Sao.Miguel (-70

mGal) and S.Maria (-45 mGal) (Figure 12b). The MBA signature stops abruptly at the

Gloria Fracture Zone (GFZ) with a regional high of -40 mGal, approximately 550 km


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away from the triple junction. This places the easterly limit of the influence of the

Azores hotspot on the accretionary processes along the TR at 3360

W longitude.

The varying amplitudes of the axial depth anomalies along the TR reflect significant

along-ridge variations in the rift morphology. The along-axis depth profile (Appendix A,

Figure 11a) of the TR from the ATJ to the GFZ indicates an alternating pattern between

topographic highs and lows consistent with the avolcanicvolcanic pattern observed by

Beier et al. (2008). From Graciosa towards the triple junction, the TR loses its

distinctiveness as evident by a decline in valley depth and width, as well as along-strike

relief. To the east, large magmatic segments along the TR, corresponding to topographic

highs, are spaced quasi-regularly in intervals of about 100 km (Vogt and Jung, 2004).

These volcanic ridges are characterized by excess crust and/or lower mantle density as

verified by regional MBA lows. The topographic valleys that separate the volcanic

islands and seamounts are deep basins and may be attributed to unfilled segments left as a

result of volcanic growth around them (Saemundsson, 1986), or may alternatively be

interpreted as segment ends (Vogt and Jung, 2004). These basins show valley depths

from 1 to 2 km, while the average axial anomaly amplitude observed at each volcanic

feature is 2 to 3 km.


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CHAPTER 5

DISCUSSION

Ridge-plume interactions strongly modify melting of the mantle under the ridge and

typically result in shallower seafloor and thicker accretion of oceanic crust (Schilling et

al., 1983) while some hot spot-ridge interactions construct huge volcanic edifices in the

form of oceanic plateaus (Ito et al., 2003). To place the Azores/TR system in global

context, Appendix A, Figure 12b shows the relationship between the amplitude of the

mantle Bouguer anomalies and ridge-hotspot distance. Hotspots in close proximity to

ridges such as Iceland (D < 50 km) display the most negative MBA (MBA) (-250 to -

340 mGal). At ridge-hotspot distances of 5001500 km, the hotspot signatures weaken to

the extent that they become almost non-existent and are impossible to differentiate from

standard variations related to ridge segmentation (Ito and Lin, 1995). Overall, therefore,

this figure points to a decrease in MBA with increasing D. Our MBA for Azores

hotspot-TR interaction returns a value of approximately -105 mGal at a ridge-hotspot

distance of ~100 km, roughly comparable to the present-day influence of Galapagos and

Easter on the Galapagos Spreading Center and East Pacific Rise, respectively.

Azores/ TR Waist Width

Appendix A, Figure 12a is adapted from the work of Ito et al. (2003) and is distinguished

by a grey area bounded by two curves. The curves on Appendix A, Figure 12a represent a

range of plume volume fluxes, Q, and illustrate the predicted relationship between half-

spreading rate U, along-axis plume width, and plume flux based on a scaling law given

by W=(Q/U)1/2

. This equation serves to reveal a scale for the width of the plume material

near the ridge axis (Ito et al., 2003). Observations at several prominent hot spot-ridge

systems from Appendix A, Figure 12a demonstrate a tendency of the along-axis widths to

decrease at greater spreading rates. Based on the figure, the maximum values ofWare

found along the slow spreading MAR near Iceland whereas values of Wdecrease with

increasing spreading rateto a minimum along the fast-spreading East Pacific Rise/Easter


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system. Although the Wversus Urelationship for the Azores/MAR system falls within

the boundaries of the predicted curve, our observed value for the Azores/TR system

along-axis width returns a value of ~550 km, visibly out of the predicted relationship.

This discrepancy is not due to plume flux, since the Azores hotspot Q is the same for the

Azores/MAR and Azores/TR systems. The following paragraphs explore possible

mechanisms to explain the disagreement between observed and predicted waist width.

Within the general vicinity of the Wvalue for the Azores/TR (Appendix A, Figure 12a)

lie comparable numbers for two other hotspots, located in close proximity to a ridge of

similar spreading geometry to that of the TR. The Marion and Bouvet hotspots are

adjacent to the ultra-slow spreading Southwest Indian Ridge (SWIR). Georgen et al.

(2001) suggested that Marion and Bouvet waist widths are shorter than expected because

long transform offsets along the SWIR curtail and compartmentalize the axial dispersion

of plume material (Vogt and Jung, 2004). With the exception of an offset across Sao

Miguel (Vogt and Jung, 2004), no transforms appear to be present along the TR.

However, the GFZ forms a major boundary on the eastern side of the Azores archipelago,

at the end of the TR. It is possible that this boundary is sufficient to prevent eastward-

directed hotspot flow.

Vogt and Jung (2004) suggest that sub-axial mantle convective processes, as opposed to

the role of surface tectonics, account for the accretionary structure along mid-ocean

ridges where there is an absence of major transforms. Segmentation along the ultra slow-

spreading TR is characterized by wide discontinuity domains with very short accretionary

segments, suggesting focused and low-viscosity mantle upwelling, with a limited magma

supply (Vogt and Jung 2004). This is similar to the SWIR and Gakkel ridges (Dick et al.,

2003), where isolated high-relief volcanic edifices suggest focused melting processes.

The axial undulations in MBA along the TR could reflect alternation between regions to

which melt is directed, and regions from which melt is directed away. Additionally,the

TR axis is interpreted as a continuous segmented line, in which the segments are strongly

oblique to the direction of opening (400 -650) (Vogt and Jung, 2004). The obliquity of the

TR plays a significant role with regards to melting processes (Beier et al., 2008).


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According to Okino et al. (2002), increased obliquity leads to decreasing effective

spreading rates, lower upwelling velocities and smaller melt fractions. We propose that

the reduced melt fraction along the highly oblique TR is one of the factors that acts to

inhibit the along axis dispersion of plume material.

Divergence of old oceanic lithosphere led to the generation of the TR about 5 Myr ago

(Vogt and Jung, 2004; Georgen, 2008). The relative youth of the slow opening TR

allows us to characterize it as an undeveloped rift system, in which only minimal

extension has occurred, preserving a cold, thick axial lithospheric lid (Beier et al., 2008).

Owing to the slow spreading nature of the TR, steady state conditions in the crust and

mantle have likely not been reached (Vogt and Jung, 2004), possibly resulting in an

overall slower export of volcanics from the rift zone, acting to minimize the width of the

plume material along axis.

In summary, a highly oblique, immature ridge system with intricate segmentation

geometry, very slow spreading rates and a thick lithosphere along the axis is thought to

affect the generation and eruption of melt along the TR. These factors could result in the

observed difference in W along the ridge axes between the MAR and the TR. It is

important to note that MBA cannot be used to differentiate between thermal and

compositional hypotheses related to plume involvement.

Influence of Triple Junction Geometry

The Galapagos system is a classic example of an off-axis hot spot interacting with a

single spreading center (Ito et al., 1997). A gradual shallowing of the Galapagos ridge

axis coupled with a simultaneous decrease in MBA along the GSC suggests anomalously

thick crust and low density mantle, direct evidence for ridge-plume interactions (Ito et al.,

2003). Our results obtained for the Azores show similar geophysical characteristics, but

there is a fundamental geological difference between the two settings. Whereas the

Galapagos plume is an off-axis hotspot interacting with a single ridge, the Azores plume

is a hotspot interacting with a triple junction.


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According to Phipps Morgan and Forsyth (1988), the flow fields and thermal patterns

along simple ridges are primarily two dimensional in nature (i.e., corner flow). In such

scenarios, flow from a near-ridge plume can be directed along the base of the lithosphere

to the rifting axis, resulting in anomalous geophysical signatures and capacious magmatic

activity (Schilling, 1991), as is the case with the Galapagos ridge-plume interaction. In

the case of the Azores, however, the unique geometrical style of three oceanic ridges,

each with varying spreading rates and flow fields, is likely to lead to a complex 3D

component of mantle plume flow along the ridge axes, with the possibility of the genesis

of a spatially extensive oceanic plateau (Georgen, 2008). The presence of long-

wavelength bathymetric and gravity anomalies extending several hundred kilometers

away from the Azores hot spot along the MAR axis, as well as the strong component of

predicted along-axis flow directed away from the triple junction along the TR (Georgen,

2008), suggest the possibility that the 3D flow disperses the Azores hotspot material over

a larger upper mantle region than if the hotspot interacted with a single ridge. Therefore,

plume flux inferred by accounting for Azores Walong the NMAR, SMAR, and TR may

over-predict actual plume flux.

Additionally, Beier et al. (2008) found that degrees of melting increase further eastward

with distance away from the ATJ. Our results, though, point to an increase in the

amplitude of the MBA towards the triple junction. This suggests an apparent paradox as

increased degrees of melting are sometimes associated with large negative MBA

signatures. One possible explanation for this contrast is the presence of a compositional

plume anomaly coupled with plate boundary effects toward the triple junction.


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CHAPTER 6

CONCLUSIONS

Our synthesis of along-axis variations in topography and gravity anomalies reflects the

dynamics of mantle flow around the ATJ as well as the influence of the Azores hotspot

on the geodynamic processes of three nearby spreading centers, allowing us to draw the

following conclusions from this investigation:

1) The Azores hotspot, approximately 150 km southeast of the triple junction,

imparts a high-amplitude (~ -100 mGal) expansive mantle Bouguer gravity

anomaly low to the Terceira Rift, implying low density mantle and considerable

crustal thickening towards the triple junction. Low MBA values coupled with V-

shaped bathymetric features along the slow-spreading southern MAR correspond

to a southward propagation of a large magmatic anomaly originating at the plume

source, indicative of an increase in crustal accretion toward the Azores hotspot.

MBA analysis along the NMAR suggests a relatively thinner crust emplaced by

lower magmatic output.

2) The strong predicted along-axis flow directed away from the triple junction along

the TR, together with long-wavelength anomalies over several hundred kilometers

away from the Azores hot spot along the MAR and TR axes, suggests three

dimensional ridge interaction with a hot and/or compositionally distinct mantle

plume.

3) Along-axis dispersion of plume material along the TR (~550 km) is minimized by

the rift systems obliqueness, youthful nature and hyper-slow spreading rate.

Wide discontinuity domains, characteristic of ultra-slow spreading centers, act as

thermal and mechanical barriers to along-axis plume transport, thereby decreasingplume waist width along the TR. We postulate that the presence of the Gloria

Fracture zone also acts to inhibit the eastward flow of plume material along the

TR.


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APPENDIX A

FIGURES

Figure 1: Predicted bathymetry map of the Azores region, showing the general setting of

the study area. The Azores triple junction is located in the Central Mid-Atlantic, formed

at the intersection of the North-American, Eurasian and African plates. Red dot indicates

the postulated location of the Azores plume.


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Figure 2: Location map based on free-air gravity data for the Azores triple junction.The position of the hotspot in reference to the triple junction is indicated with a star.

Free-air gravity data were extracted from the global satellite altimetry database of Smith

and Sandwell (1997). Ridge abbreviations are: N.MAR = Northern branch of the Mid-

Atlantic with respect to the Azores Triple Junction, S.MAR = Southern branch of theMid-Atlantic Ridge with respect to the Azores Triple Junction, Ter.R = Terceira Rift

(from Georgen and Lin, 2002).


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Figure 3: Tectonic ridge geometry of the Azores triple junction region, reflecting thetrend of each of the spreading branches associated with the junction. U 1 represents the

half-spreading rate for the northern branch of the Mid-Atlantic Ridge, designated as R1

due to its fastest rate of divergence. U2 represents the half spreading rate for the SMAR

designated as R2. U3 represents the half spreading rate for the Terceira Rift, designated asR3. Ridge configuration for the Azores Triple Junction indicates the two fastest spreading

branches (R1 and R2) are virtually collinear. The slowest spreading branch, R3, intersects

the other two branches almost perpendicularly. Arrows indicate relative plate motionwith respect to the triple junction (from Georgen and Lin, 2002).


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Figure 4: Existing position of the Azores triple junction (solid black circle) arrow south

of the Terceira Rift indicates the plate motion direction of the African and Eurasian plates

with respect to the hotspot reference frame. The map also shows the position of all nineislands along the Azores archipelago, with Flores and Corvo to the west of the triple

junction, and Terceira, Graciosa, Sao Jorge (SJ), Pico, Faial, Sao Miguel, and Santa

Maria to the east of the triple junction. Dashed line indicates the uncertain location of theAfrican-Eurasian plate boundary near the MAR. The names of prominentfracture zones

are also labeled. Along the Northern Mid-Atlantic Ridge is the North Azores fracturezone. South of the triple junction are the Acor fracture zone, Princess Alice fracture zone

and Pico fracture zone. South of the Terceira Rift is the East Azores fracture zone.COV2, CDRO, PSJO, PSCM, CMLA and PSMA are broadband seismic stations on the

Azores islands used to record tele-seismic body waves for calculation of P-wave velocity.

Map modified from Yang et al. 2006.

V-shapedridge

V-shaped

ridge


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Figure 5: Isotope systematics of the Terceira Rift lavas. Lines show linear arrays of the

islands along the TR. Arrows indicate the trend toward Sao Miguel lavas (Beier et al.,

2007).White circles represent Pb spike analyses. Dashed circles indicate possible mantle

source end-members for each mixing array. Samples taken from the Mid-Atlantic Ridgeare indicated by MAR (Dosso et al.1999) (from Beier et al, 2008).


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Figure 6: (a) Averages of crustal thickness and Na8 at various mid-ocean ridges

(Langmuir et al., 1992). Dashed curve shows predictions of the melting model of Klein

and Langmuir (1987), and dotted curve is the prediction of McKenzie and Bickle (1988).

(b) Negative correlation between regional averages of Na8,0.1 and Fe8,0.1 (Shen and

Forsyth, 1995). To correct for source compositional effects, Shen and Forsyth (1995) use

correlations between Na8 and Fe8 and K2O/TiO2 in order to estimate the Na8 and Fe8content. These compositions are Na8,0.1 and Fe8,0.1. (Figure from Ito et al., 2003)


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Figure 7: Bathymetry map for the Azores Triple Junction, showing the southward

propagation of V-shaped ridges. Ridge abbreviations are as follows: N.MAR = Northern

branch of the Mid-Atlantic ridge, S.MAR =Southern branch of the Mid-Atlantic Ridge,

and TER.R. = Terceira Rift. Filled white star indicates the postulated location of theAzores hotspot. Thin lines indicate shiptrack coverage of the bathymetry data. Orange

lines on either side of the Mid-Atlantic Ridge represent isochrons during different stages

of evolutionary rifting. Grid spacing is 2and contour interval is 1000 m.


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Figure 8:Free-air anomaly map for the Azores region. The free-air gravity map contains

signals from seafloor topography, sediments, and crust and mantle density anomalies.

The free-air anomaly is dominated by short wavelength variations which reflect thedensity contrast at the seafloor. Gravity data are plotted at 2 spacing and the contour

interval is 200 mGal. The position of the Azores hotspot is marked by a solid white star.


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Figure 9: Map of Mantle Bouguer anomaly correction, generated from bathymetry data.

MBA correction was calculated using the method of Kuo and Forsyth (1988). The crust

was assumed to have constant thickness of 5 km that follows the seafloor relief. Theassumed density of the crustal layer is 2,800 kg/m, and that of the underlying mantle is

3,300 kg/m. Contour interval is 200 mGal. The position of the Azores hotspot is markedby a solid white star.


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Figure 10: Map of MBA calculated by subtracting the mantle Bouguer correction from

the FAA, assuming a constant thickness, 5 km reference crust. Grid spacing is 5 and

contour interval is 50 mGal. Postulated position of the Azores hotspot is marked by asolid white star. Red areas correspond to shallow regions with prominent negative

gravity lows and may result from the combined effects of thicker crust, lower densitymantle, and/or higher temperature mantle than the surrounding regions.


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Figure 11: (a) Top panel shows axial bathymetry profile along the Terceira Rift, with

increasing distance from the Azores Triple Junction. (b) Bottom panel shows filtered

MBA profile along the same region of the Terceira Rift axis, with increasing distance

from the Azores Triple Junction. Windicates the total along axis width of plume materialalong the Terceira Rift. The amplitude of mantle Bouguer anomaly, MBA, denotes its

total along-axis variation. Note that the majority of localized MBA lows correlate tobathymetric highs. The long-wavelength trend in the MBA with a minimum at ~250 km,reflects enhanced crustal thickness and/or lower mantle density, possibly caused by rapid

upwelling over the plume source (from Georgen and Sankar, 2008).


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Figure 12: (a) Top panel shows along-isochron widths (W) of residual bathymetricanomalies, versus half-spreading rates U. Two curves show predictions of scaling laws

for a range of plume volume fluxes, Q (from Ito and Lin, 1995). The observed value for

the along-axis width of the Azores/TR system, represented by a yellow star bordered byan orange outline, falls visibly out of the predicted relationship. (b) Bottom panel shows

along-isochron amplitudes ofMBA plotted against ridge-hotspot distances (from Ito

and Lin, 1995). MBA for the Azores hotspot-TR interaction, represented by a yellow

star bordered by an orange outline, returns a value of approximately -105 mGal at aridge-hotspot distance of ~100 km.


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BIOGRAPHICAL SKETCH

Ravi D. Sankar

Ravi Darwin Sankar was born in San Fernando, Trinidad on the 15th

of April, 1981. He

attended the San Fernando T.M.L primary school, San Fernando Government Secondary

and San Fernando Senior Secondary high schools and completed his Advanced-level

education in April of 2000 from the Marabella Senior Comprehensive School. He

received his Bachelor of Science degree in Physics and Environmental Physics (First

Class Honors) from the University of the West Indies, St. Augustine, Trinidad in the

spring of 2005. He went on to Master of Science studies in Petroleum Engineering at the

University of the West Indies, St. Augustine, for one year before being awarded an

international FULBRIGHT Faculty Development Scholarship (2006) from the U.S

Department of State to pursue graduate studies in the United States. He enrolled at

Florida State University in the spring of 2007 where he studied marine geology and

geophysics under the guidance of Dr. Jennifer Georgen. He completed his Master's

degree in the spring of 2009.

Documents

Azores Triple Junction + Hotspot