How sharp is the sharp Archean Moho? Example from eastern ...geology.rutgers.edu/.../Levin_et_al-2016-Geophysical_Research_Lette… · disparate Archean terranes suggests that it

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How sharp is the sharp Archean Moho? Example from eastern Superior Province.

Vadim Levin, Jill A. VanTongeren, Andrea Servali

Department of Earth and Planetary Sciences, Rutgers University


Abstract:

The Superior Province of North America has not experienced major internal

deformation for nearly 2.8 Gyr, preserving the Archean crust in its likely original state. We

present seismological evidence for a sharp (less than 1 km) crust-mantle boundary beneath

three distinct Archean terranes, and for a more vertically extensive boundary at sites likely

affected by the 1.2 – 0.9 Ga Grenville orogeny. At all sites crustal thickness is smaller than

expected for the primary crust produced by melting under higher mantle potential

temperature conditions of Archean time. Reduced thickness and an abrupt contrast in seismic

properties at the base of the undisturbed Archean crust are consistent with density sorting and

loss of the residues through gravitational instability facilitated by higher temperatures in the

upper mantle at the time of formation. Similar sharpness of crust-mantle boundary in

disparate Archean terranes suggests that it is a universal feature of the Archean crustal

evolution.

Key Points:

1. Teleseismic receiver functions used to measure Moho thickness in eastern Superio

Province.

2. Sharp (~1 km) Moho likely is an original feature of Archean continental crust formation

3. Density sorting and delamination due to high mantle potential temperature explain sharp

Archean Moho


Introduction:

The transition from the seismically slower material of the crust to the faster material

of the mantle, rarely accessible for direct sampling, bears the (somewhat shortened) name of

its discoverer Andrija Mohorovičić, and has been a subject of seismological studies for over a

century (Prodhel et al., 2013). A degree of ambiguity remains in associating the change in

seismic speed at the Moho with a change in lithology at the crust-mantle boundary (O’Reilly

et al., 2013). Nevertheless, Earth’s crust is most commonly delineated by seismological

means (e.g., Cook et al., 2010).

In global surveys (e.g. Christensen and Mooney, 1995) and regional studies (e.g. Yuan, 2015)

the depth to the Moho within stable continents is ~40 km, with oldest areas (cratons) having

systematically smaller values. A recent survey of diverse seismological definitions of the

Moho (Abbott et al., 2013) argues for systematic differences in its character as a function of

the continental consolidation age, with the oldest Archean-age cratonic crust having a larger

fraction of “sharp” Moho observations. Criteria used for the “sharp” designation depend on

the methods in specific studies, and include confident detection of PmP reflected waves in

controlled seismic source profiles, as well as shapes of P-to-S converted phases from distant

earthquakes.

Here we use the teleseismic receiver function technique to probe Moho structure at

six sites within the eastern Superior Province of North America. We present evidence for the

Moho being sharp (~1km) within all undisturbed Archean terranes, even those with different

formation ages and compositions, and notably more diffuse (> 4 km) near the ~1.0 Ga

Grenville Front. We also confirm that the crust is thinner than the global average. In both its

thickness, and the vertical extent (sharpness) of the Moho, the Superior Province crust differs

from the typical continental crust generated in the Phanerozoic.


Regional Setting

The Superior Province is the largest of the Archean cratons (Card, 1990), made up of

terranes with origin dates as early as 3.3 Ga, assembled mainly during 2.72 Ga and 2.68 Ga

collisional events (Percival et al., 2007), and further modified as late as 2.51 Ga (Moser et al.,

2008). Seismic studies of the Superior province (e.g., Calvert et al., 1995; Calvert and

Ludden, 1999; Mereu, 2000; Musacchio et al., 2004; Darbyshire et al., 2007) showed its crust

to be 40 km or less, with velocities typical of felsic lithology, and with many instances of

“sharp” Moho.

We investigate the vertical extent of the crust-mantle transition in three different

terranes within the eastern part of the Superior Province (Fig. 1). Two stations (WEMQ and

NMSQ) are located within the La Grande terrane, which contains the oldest rocks of the

eastern Superior Province, with 3.3 Ga to 2.9 Ga continental basement, and juvenile

magmatic rocks emplaced between 2.75 and 2.70 Ga. One station (YOSQ) is located within

the Opinaca subprovince in late Archean paragneiss (Reed et al., 2005). Further to the south,

three stations (MATQ, LSQQ, CHGQ) are within the Abitibi granite-greenstone belt, known

for its abundance of komatiitic lavas considered by some to be indicative of plume-sourced

oceanic plateaus in the Archean (Fan and Kerrich, 1997). Station MATQ is located on a

sequence of late Archean volcanic rocks, whereas stations LSQQ and CHGQ have

experienced post-Archean events. LSQQ is located directly on top of a dike from the late

Proterozoic Abitibi dike swarm (1141±1 Ma), and CHGQ is only 30 km west of the 1250-980

Ma Grenville front (Reed et al., 2005).

Measuring Moho sharpness

We investigate the vertical extent, or sharpness, of the Moho transition using


observations of shear (S) waves present in the coda of first-arriving compressional (P) waves

from distant earthquakes. The receiver function (RF) analysis technique (Ammon, 1991)

assumes that such S waves originate by the process of mode conversion from P waves at a

contrast in seismic properties. The constraint on the vertical extent of the boundary comes

from the consideration of the pulse shape of a P-to-S converted wave. Figures 2a&b illustrate

expected effects of the vertical profile of seismic velocities across the crust-mantle transition.

Three cases are considered: an instantaneous increase, a 4 km wide gradual increase, and a 5

km wide sequence of layers with alternating properties. At frequency 0.25 Hz (corresponding

to S wavelengths ~15 km, Figure 2b) resulting time series are virtually identical. Differences

emerge as higher frequencies (shorter wavelengths) are introduced. A vertically

instantaneous change yields a pulse with progressively diminishing width. A smooth vertical

gradient results in a pulse with a characteristic width that does not change after a certain limit

is reached. Finally, a complex crust-mantle transition zone results in a set of distinct pulses

arising from individual contrasts within it.

We adopt the wavelength of the P-to-S converted wave that departs from the simple

pulse shape as a measure of the boundary width. This approach is similar to the consideration

of pulse shapes of waves reflected from closely spaced boundaries (Widess, 1973), which

informs a widely accepted ¼ wavelength rule for the vertical resolution of seismic reflection

data. The time separation between two vertically propagating P-to-S converted waves from

horizontal boundaries separated by h (Figure 2c) will be

, where Vp,Vs are constant velocities of P and S waves between the boundaries, and

. If we set

, where is the wavelength of the P-to-S converted wave,

then

for =1.75.

Simulated P-to-S converted waves in Figure 2a show that a departure from the simple pulse


shape becomes obvious at the f =1 Hz corresponding to km, a wavelength

commensurate with the width of the velocity transition. This observation is more in line with

the h~P/2 estimate (P – P wavelength) proposed by Bostock (1999) and based on

amplitudes of mode-converted waves in zones of generalized velocity heterogeneity. In our

context this will yield h~ measure of the converting boundary width.

Data and Analysis

Continuously operating seismic observatories in the Superior Province (Figure 1)

provide data for the study. We select groups of nearby earthquake sources (see Supplement

Figure 1) from datasets containing between 90 and 140 records per site. All records chosen

for one site in the frequency domain are stacked to form a single RF time series. All sources

used to form this RF have similar ray parameters and backazimuths, eliminating possible

distortions of the converted wave pulse shape due to lateral changes in Moho properties and

variations in the incidence angle. We use a multitaper spectral correlation variant of the RF

technique that affords an exceptional resolution of higher frequency RF components (Park

and Levin, 2000). For each site, the RF time series with different frequency content (Figure

3) are constructed in order to examine the resulting pulse shapes of the P-to-S converted

waves from the Moho (designated Pms). We identify the highest frequencies at which these

pulses appear “pure” (i.e. as predicted by an abrupt change in properties, Figure 2a), evaluate

their wavelengths, and derive two estimates of the vertical distance over which seismic

properties change, an optimistic one assuming a constant velocity layer (i.e., ), and

a conservative one assuming a complex vertical transition (i.e. ).

Results.

With the exception of the site CHGQ discussed in more detail below, all sites within


the Superior Province show a simple Pms phase, with delay times in the 4.3 – 4.5 s range

(Figure 3). At three locations (WEMQ, YOSQ, MATQ) Pms phases display a progressive

decrease in width with increasing f. The highest usable frequency in our dataset is f~3 Hz,

and the corresponding ~1.2 km. At site NMSQ the Pms pulse does not decrease in width for

f>1.75 Hz, and at site LSQQ the highest frequency where the Pms pulse retains its pure shape

is f=1 Hz. Consequently, ~2.1 km for NMSQ, and ~3.7 km for LSQQ. Our estimates for

the maximum vertical extent over which seismic properties change at the Moho are ~0.7 km

for sites WEMQ, YOSQ and MATQ; ~1.2 km for site NMSQ; and ~2.2 km for site LSQQ.

Using a more conservative approach ( these estimates are 1.2 km, 2.1 km and 3.7 km,

respectively.

At site CHGQ there appears to be two positive pulses with properties similar to the

Pms phase at lower frequencies (e.g., 1 Hz), one at ~3.7 s and another at ~6.2 s. At higher

frequencies additional phases appear between them. Examination of the full dataset

(Supplementary Figure 1) shows that the pulse at 3.7 s is seen from most directions, and thus

is more likely to represent the P-to-S conversion at the crust-mantle boundary. This pulse

loses its pure shape for f>0.5Hz, or ~7.4 km, and thus our estimates of Moho width here

are 4.3 - 7.4 km. Presence of additional converted phases in 4 – 6 s time window points to

considerable complexity of the uppermost mantle down to depths of at least 50 km.

The PmS delay is a measure of the crustal thickness, the calculation of which is based

on an assumption of Vp =6.5 km/s (e.g. stacking velocity of Calvert and Ludden, (1999) for

the Superior Province), and Vp/Vs = 1.75 (see Supplementary Material for discussion). Using

these values we obtain crustal thickness values for the eastern Superior Province of 34.9 -

36.5km (Supplementary Table 1). Choosing global-average values of Vp=6.454 km/s and

Vp/Vs=1.768 (Christensen and Mooney, 1995) reduces these estimates by ~1 km, while

adopting values from controlled-source studies of the Abitibi province (Vp~6.6 km/s and


Vp/Vs=1.72; Mereu, 2000) increases them by 1.6-1.9 km (Supplementary Table 1).

Stacking of direct and multiply scattered Ps phases in RF timeseries provides a way to

estimate Vp/Vs and crustal thickness (Zhu and Kanamori, 2000), with an assumption of

either Vp or Vs value. Applying this technique (often called H-k stacking) to our data we

obtain tight constraints on the Moho depth, while Vp/Vs ratio is variable (Supplementary

Figure 2). While a choice of Vp value strongly influences the estimate of the crustal

thickness, for Vp<6.8 km/s our data favor crustal thickness values under 40 km at all sites

(Supplementary Table 2).

Discussion

Estimates of Moho sharpness rely on the ability to resolve RFs at high frequencies

(Park and Levin, 2000). In their review of controlled source reflection studies in Canada,

Cook et al. (2010) defined the Moho as “sharp” or “diffuse” using patterns of near-vertically

reflected waves. In cases of “sharp” Moho (e.g., at the northern end of Abitibi-Opatica line

near our site NMSQ, Figure 13 in Cook et al., (2010), originally from Calvert and Ludden,

(1999)) the change from strong reflectivity (in the crust) to no reflectivity (supposedly in the

mantle) takes place over ~0.2 s of two-way time. For a range of compressional wave speeds

of 6.7 – 7.5 km/s at the bottom of the crust (Musacchio et al., 2004), this translates into ~0.7

km of vertical distance, close to our optimistic estimate for site NMSQ,

Clarity and prominence of the PmP Moho reflection is another commonly used

indicator of a "sharp" Moho in controlled seismic source studies (cf. Abbott et al., 2013). In

western Superior Province Musacchio et al. (2004) report a PmP phase being best observed in

the 2-6 Hz pass-band, with a corresponding shortest wavelength in the lower crust of ~1.1 -

1.2 km, calculated as

, where Vp= 6.7 - 7.5 km/s. According to the 1/4 wavelength

measure of Widess (1973), this implies a velocity transition over ~0.3 km or less. Although


the frequency range of our current RF data is not sufficient to detect boundaries separated by

300 m, the behavior of the PmS pulse at site WEMQ (Figure 3) is suggestive – it becomes

progressively narrower with each increase of frequency. Given enough bandwidth in the

earthquake sources, RF resolution may be brought closer to that of controlled source studies.

At three locations we estimate the transition between the cratonic crust of the eastern

Superior Province and its lithospheric mantle as being ~1 km thick. Our results are very

similar for three locations with different early histories. Common to the Abitibi terrane,

which is dominantly mafic volcanics, and the La Grande and Opinaca terranes, which are

dominated by granitoids (Figure 1, and Percival et al., 2007) is the lack of tectonic activity

after their incorporation into the Superior province. Sites with considerably more diffuse (4-7

km) crust-mantle transition are LSQQ and CHGQ. Both have experienced post-Archean

crustal reworking such as dike emplacement and extensive faulting related to Grenville-age

continent-continent collision. A plausible interpretation of our findings is that the Superior

Province Moho was initially sharp, and was subsequently disturbed by tectonic events in the

Proterozoic. This disturbance did not relax over the subsequent ~1 Gyr. Thus, a sharp Moho

is likely an original feature of craton formation and stabilization in the Archean.

Many researchers suggest that mantle potential temperature in the Archean was up to

250°C higher than today (Davies, 1992; 2009; Korenaga, 2008a,b; Herzberg et al,. 2010;

Brown, 2007). Under these conditions the mantle is expected to melt to a greater extent,

thereby producing primary picritic-basaltic crust up to 40 km in thickness (Herzberg et al.,

2010; Herzberg and Rudnick, 2012). One consequence of the uniformly thick crust may have

been the inhibition of subduction sensu stricto by extreme bending stresses (e.g., Korenaga,

2006; Davies, 2009). However, preserved crust of the cratons is significantly smaller than

the 41.1 km modern continental average of Christensen and Mooney (1995), and seismic

imaging yields convincing evidence of subduction episodes during the Archean (e.g. Calvert


et al., 1995; Chen et al., 2009). Furthermore, the predominant TTG composition of the

cratonic crust (Moyen and Martin, 2012) likely requires multiple episodes of re-melting of

the primary picritic-basaltic crust (Johnson et al., 2014).

In contrast to preserved Archean cratonic crust, juvenile continental crust developed

in modern island arcs has a very different seismic structure, with higher absolute values of

compressional velocity, and a gradual change to upper mantle values over a distance of

multiple km (e.g. Holbrook et al., 1999; Kodaira et al., 2010). The expected RF signature of a

sample profile through the juvenile arc crust is illustrated in Supplementary Figure 3. In order

to transition from juvenile island arc to modern continental crust, a significant portion of the

lower crust and the mantle lithosphere must be removed (e.g. Jagoutz et al., 2011; Jagoutz

and Schmidt, 2013).

Examples of lithospheric instability during the Phanerozoic are inferred from exposed

terranes of island arcs, such as the Kohistan and Talkeetna arc sections (Jagoutz and

Kelemen, 2015). In Talkeetna, a ~100-400 m thick section of dense garnet gabbro crust is

the only remaining density-unstable lithology present under a 40 km thick crustal section of

granites and gabbronorites (Greene et al., 2006). It is proposed that a further 10-12 km of the

original crust were delaminated from the Talkeetna arc section prior to obduction (Greene et

al., 2006; Jull and Kelemen, 2001; Kelemen et al., 2004; Behn and Kelemen, 2006). Jagoutz

and Behn (2014) show that density sorting, and removal of density-unstable material, occurs

in the modern day crust on 0.5-5 Myr time-scales, and that higher Moho temperatures (e.g.

~1000°C in Talkeetna relative to ~700-800°C for Kohistan) allow for a sharper Moho by this

density sorting process.

Thus, delamination of density-unstable volumes of primary Archean crust is a

plausible scenario to yield thinner Archean crust with an abrupt lower boundary (e.g.

Hamilton, 2013). The transition from density-stable to density-unstable crustal lithology is


pressure and composition dependent, corresponding roughly to the appearance of garnet in

the stable phase assemblage. Johnson et al. (2014) modeled the stability of primary crust

using the appropriate compositions for high MgO picrites and basalts from the Archean, and

showed that the removal of dense mafic lower crust by Rayleigh-Taylor instability occurs

rapidly in undifferentiated crust at mantle potential temperatures greater than 1550°C. Lighter

crustal material left behind after an episode of gravitational removal of dense residues is

likely to form an abrupt contrast with the underlying mantle. This conclusion is supported by

the similarity of Moho sharpness and crustal thickness in all Archean terranes investigated

here, regardless of dominant lithology, and suggests that the process of granitization does not

disrupt or alter the Moho signature.

Conclusions

At a set of locations broadly distributed within the eastern Superior Province we use

high-quality records of P-to-S converted waves from distant earthquakes to quantify the

length-scale of the contrast in seismic properties delimiting the bottom of the crust. Our

estimates of the vertical extent of the Moho boundary range from less than 1 km in

undisturbed crust to over 7 km near major post-Archean tectonic boundaries. We find short

vertical transition (< 1km) from the crust to the mantle in all Archean terranes probed,

regardless of their formation age or crustal lithology (basaltic vs. granitic). This finding

suggests that the sharp Moho observed in Archean cratons is a first order feature, likely

related to higher mantle potential temperatures resulting in hot (1000°C) Moho temperatures

during their formation. Our data also show that regions affected by Grenville-age tectonism

have a more diffuse Moho, suggesting that disturbances to the originally sharp Moho are

unable to relax on the >1 Gyr time scale.

Our observations of sharp Moho and relatively thin crust in the Archean terranes of


the eastern Superior Province are consistent with density sorting of the crust under the

conditions of higher mantle potential temperatures in the Archean. The formation of a sharp

Moho transition must be a primary and ubiquitous feature of Archean craton formation and

stabilization.

Acknowledgements

This work was supported by the NSF Earthscope grant EAR-1147831 and the Aresty Center

for Undergraduate Research, and made possible by the open data policy of the Canadian

National Data Centre for Earthquake Seismology and Nuclear Explosion Monitoring.

Discussions with C. Herzberg, O. Jagoutz , W. Menke and M. Bostock, as well as reviews by

Andrew Hynes and anonymous, helped us sharpen the arguments. Figures were drawn using

GMT (Wessel and Smith, 1995).

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Figure 1. A map of sites studied (circles) with the outlines of tectonic boundaries (from

Clowes et al., 2010). The locator map (inset) shows the study area as a red box. Four-letter

codes designate seismic observatories. Numbers next to sites show ranges of Moho thickness,

in km.


Figure 2. (a) Synthetic RFs computed in 1D layered velocity structures using a reflectivity

algorithm of Levin and Park, (1997). Time series are shaded according to the model (instant

step – light grey, linear gradient – grey, complex structure – solid). The shortest wavelength

is computed as

, where f is highest frequency, and VS = 3.7 km/s. An inset in (b)

shows the P-to-S wave pulse for f=3 Hz. (b) Values of shear wave speed for three vertical

profiles at the crust-mantle boundary used to produce synthetic RFs. (c) Raypaths and

waveforms of two P-to-S converted waves from closely spaced boundaries.


Figure 3. Observed RF arranged by frequency content for 6 locations within the Superior

Province. Pms pulses with progressively diminishing width are found at sites WEMQ, MATQ

and YOSQ. Sites NMSQ and LSQQ show pulses that have a simple shape up to f=1.75 Hz

and f=1Hz, respectively (marked by stars). Site CHGQ displays frequency dependence of the

Pms pulse suggesting complexity in the crust-mantle transition zone.

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How sharp is the sharp Archean Moho? Example from eastern ...geology.rutgers.edu/.../Levin_et_al-2016-Geophysical_Research_Lette… · disparate Archean terranes suggests that it