www.elsevier.com/locate/epsl

Earth and Planetary Science Le

Two-phase opening of Andaman Sea:

a new seismotectonic insight

P.K. Khana,*, Partha Pratim Chakrabortyb

aDepartment of Applied Geophysics, Indian School of Mines, Dhanbad, Jharkhand, IndiabDepartment of Applied Geology, Indian School of Mines, Dhanbad, Jharkhand, India

Received 17 May 2004; accepted 1 November 2004

Editor: E. Boyle

Available online 15 December 2004

Abstract

High-resolution reconstruction of Benioff zone depth–dip angle trajectory for Burma–Java subduction margin

between 28 and 178N Lat. reveals two major episodes of plate geometry change expressed as abrupt deviation in

subduction angle. Estimation of effective rate of subduction in different time slices (and then length of subducted slab)

allowed drawing of isochrones in Ma interval through these trajectories for the time period 5–12 Ma. With these

isochrones, the deformation events on the subducting Indian plate are constrained in time as of 4–5 and 11 Ma old.

This well-constrained time connotation offered scope for the correlation of slab deformation events with the well-

established two-phase opening history of the Andaman Sea. While the 11 Ma event recorded from southern part of the

study area is correlated with early stretching and rifting phase, the 4–5 Ma event is interpreted as major forcing

behind the spreading phase of the Andaman Sea. Systematic spatio-temporal evaluation of Indian plate obliquity on the

Andaman Sea evolution shows its definite control on the early rifting phase, initiated towards south near northwest

Sumatra. The much young spreading phase recorded towards north of 78 Lat. is possibly the result of late Miocene–

Pliocene trench retreat and follow-up transcurrent movement (along Sagaing and Sumatran fault system) with NW–SE

pull-apart extension.

Nonconformity between plate shape and subduction margin geometry is interpreted as the causative force behind Mid-

Miocene intraplate extension and tearing. Enhanced stretching in the overriding plate consequently caused active forearc

subsidence, recorded all along this plate margin. Initial phase of the Andaman Sea opening presumably remains concealed

in this early–middle Miocene forearc subsidence history. The late Miocene–Pliocene pull-apart opening and spreading was

possibly initiated near the western part of the Mergui–Sumatra region and propagated northward in subsequent period. A

temporary halt in rifting at this pull-apart stage and northeastward veering of the Andaman Sea Ridge (ASR) are related

0012-821X/$ - s

doi:10.1016/j.ep

* Correspon

E-mail addr

tters 229 (2005) 259–271

ee front matter D 2004 Elsevier B.V. All rights reserved.

sl.2004.11.010

ding author.

esses: pkkhan@indiatimes.com (P.K. Khan)8 partha_geology@yahoo.co.in (P.P. Chakraborty).

P.K. Khan, P.P. Chakraborty / Earth and Planetary Science Letters 229 (2005) 259–271260

with uplifting of oceanic crust in post-middle Miocene time in form of Alcock and Sewell seamounts, lying symmetrically

north and south of this spreading ridge.

D 2004 Elsevier B.V. All rights reserved.

Keywords: depth–dip angle; benioff zone; isochrone; obliquity; rifting; pull-apart

1. Introduction

Collision between the Indian and Eurasian plates

[1–5], coupling and decoupling of different platelets

[6], crustal movement along strike-slip faults [7–15],

rotation of continental blocks [16–19] and opening of

Fig. 1. Map on the left shows the regional tectonic framework of the weste

rotation of south Sumatra is from Ninkovich [16], counter-clockwise rota

north Malaysia are from Hall [19]. Left bottom open arrow indicates Indi

motion with respect to Siberia since the Miocene. The tectonic setup of

subducting Indian lithosphere showing on the right after Curray et al. [7

polygon demarcates closely spaced structure contours.

marginal basins on various scale [7,19,20] framed the

Tertiary tectonics of southeast (SE) Asia. Models

favoring both decoupling with back arc stretching

(Mariana Trough; West Philippine Sea) and transform

with pull-apart extension (Japan Sea) are sighted as

operative plate kinematics to explain the opening of

rn part of southeast Asia after Tapponnier et al. [22]. Clockwise 208tion of south Malaysia and north Sumatra and clockwise rotation of

an plate motion vector and right top open arrow is for major block

Andaman Sea with structure contour map for the top part of the

], Curray and Munasinghe [65] and Dasgupta et al. [63]. Note the

P.K. Khan, P.P. Chakraborty / Earth and Planetary Science Letters 229 (2005) 259–271 261

marginal basins through this highly dynamic geo-

tectonic province. Palaeomagnetic clue, bathymetry

data [21] and theoretical reasoning [7,18,20,22,23] on

origin of such basins, however, visualizes their

episodic evolution [24–26]; episodes may vary both

in operative tectonics and basin opening rate.

The Andaman Sea, concern of the present study,

occupy an unique position as a marginal basin in SE

Asia geography and offers scope for understanding

both intra- and inter-plate kinematic controls along

Burma–Java subduction margin in the backdrop of

other coeval tectonic structures in the region viz.

subduction of the Indian plate under the Eurasia, roll

back of the Java trench, large-scale dextral motion

along Sagaing fault, extrusion activity in SE Asia

and rotation of continental blocks (Malay peninsula

and Sumatra) [10,27,28]. A two-phase evolution for

the Andaman Sea is established in literature with an

early (Oligocene–early Miocene) rifting and subse-

quent (post-middle Miocene) active spreading [7]. It

is proposed that in post-middle Miocene Andaman

forearc witnessed three major tectonic divisions in

north–south transect; the central basin of the Anda-

man Sea with short spreading rifts and transforms

(with NW–SE spreading at the rate of 1.6–3.72 cm/

year; [21]) and right lateral shear movement in its

north (along Sagaing fault) and south (along

Sumatran fault system), respectively (Fig. 1) [7,21,

29,30]. Recent works [21,31] though provided

significant insight on spreading history (V4–5 Ma,

i.e., late Miocene–early Pliocene onward) of the

Andaman Sea, offer only little account for its early

rifting phase associated with subsidence in the entire

forearc of this plate margin including the Andaman

Sea and Burma central basin [32]. The present study

aims towards understanding this poorly known early

Andaman Sea history with focus on an area between

Lat. 28 and 178 and Long. 928 and 998. Objectivesof this study are to address some of the long-

unresolved queries on this plate margin viz.: (i) If

subduction (of the Indian plate) process was oper-

ative at this plate margin since early Cretaceous, can

we constrain how and when rifting was initiated in

the Andaman forearc region? (ii) Does deformation

in the subducting Indian slab comply with the

subduction kinematics? (iii) is change in subducting

plate shape is reflected in changes of subduction

history or radius of arc? Answers related to these

questions must be sought in reference to regional

tectonic grains, Benioff zone geometry, Tertiary

volcanic trends and rotational history of adjoining

crustal blocks. Within the constraint of available

plate kinematic database reconstruction of Benioff

zone configuration through a systematic high-reso-

lution earthquake data analysis under the present

study allowed sequential modeling of the Andaman

Sea, in general, and its early rifting phase, in

particular.

2. Tectonics and geology

Opened on the northward-moving Indian plate, the

Andaman Sea, with its subduction boundaries towards

north, west and south, is one of the challenging and

poorly understood marginal basins with strange

tectonic setting. Continuation of Oligocene continen-

tal arc volcanism between Sumatra and Burma bears

evidence in favor of Oligocene–early Miocene trench

being very close to the pre-existing continental margin

on its east [8]. Active extension was presumed on this

continental margin from late Oligocene time in form

of opening of the Mergui–Sumatra basin (the dprotoAndaman basinT? of Curray et al. [7]). History of this

extension, however, did not last long as the basin

subsequently got aborted.

The late Oligocene–early Miocene limestone–shale

(with interstratified pyroclastics) dsequencesT off

Sumatra, interpreted as product of transgressive–

regressive cycles, document signatures of active

interplay between subsidence and eustacy in the

forearc area and considered to be valid all through

the southeast Asia during this time period [15,33,34].

Analogues of these dsequencesT can also be observed

in the northern part of this geodynamic belt at the

middle and north Andaman Islands [35]. Evidences

favoring this forearc subsidence are also found in

seismic reflection transects across other parts along

this plate margin [7,36,37]. The recently described

andesitic to dacitic pyroclastic tuff units in early

Miocene archipelago sediments of the middle and

north Andaman [38] presumably represent coeval

subaerial (on the then existing continental margin)

calcalkaline volcanic trend that spanned from Burma

in the north to Sumatra–Sunda arcs of Indonesia in the

south [26,39–41]. Except for some accreted on-land

P.K. Khan, P.P. Chakraborty / Earth and Planetary Science Letters 229 (2005) 259–271262

pyroclastic debrite (described from Andaman islands

as of distal subaquous origin), the absence of any

other evidence for this volcanic trend between Burma

and northern Sumatra is probably the result of rapid

subsidence of the forearc and its adjacent continental

margin in early Miocene time that subsequently got

transected by the active spreading ridge of the

Andaman Sea.

Existing models on the Andaman Sea heavily

stress on its middle Miocene opening through pull-

apart extension in a rift system on the eastern margin

of the subducting Indian plate [9,11,22]. Subsequent

to slowing down or cessation of convergence

through early Miocene time [3,15,42,43], the Indian

plate, perhaps with partly coupled Burma plate,

resumed its northward journey and increased its

obliquity and convergence rate [12,19,44] in middle

Miocene time. The resultant northward drag of

hanging slab by India through the surrounding

asthenosphere and northward movement of the

Burma platelet [9] and counter-clockwise rotation

of north Sumatra and south Malaya [10,12] are

figured as causative forcings behind pull-apart open-

ing of the Andaman Sea that mostly got accom-

modated through right-lateral Sagaing fault on its

eastern margin [10]. The active spreading that

resulted 460 km opening of the Andaman Sea

coincides with next major tectono-volcanic episode

in this plate margin, which witnessed explosive

volcanism [45], counter-clockwise (?) rotation of

northern Sumatra and southern Malay and partition-

ing of oblique convergence of the Indian plate into

an orthogonal and a strike-parallel component

[19,30]. However, this single, steady state post-

Miocene pull-apart extension model [22] alone falls

short in explaining many of the observed structural,

bathymetric and timing complexities of the Mergui–

Sumatra–Andaman basin as a whole viz. the east–

west extensional thinning of the continental and

oceanic crust, rapid subsidence in Mergui–north

Sumatra region in late Oligocene and Miocene time

[7,8,15,33,45–47], thick Oligo-Miocene clastic sedi-

mentary package, occurrence of which can be traced

at Mergui, forearc of middle and north Andaman and

central lowland of Burma [7,8,15,29,36,38,45,48,49].

The model favoring middle Miocene (~11 Ma)

spreading and opening of the Andaman Sea [7] also

referred in Hall [50] is strongly contested in recent

times through GPS survey [31,32] and reinterpreta-

tion of magnetic anomalies recorded from the Anda-

man Sea [21]. Based on the recently acquired data, it

is concluded that the Andaman Sea witnessed oceanic

crust generation through active spreading only during

past 4–5 Ma, i.e., from late Miocene–early Pliocene

time. This recent understanding of late Miocene–early

Pliocene spreading must be accounted in reference to

well-documented 450 km post-early Miocene dextral

displacement recorded from offset of the Myanmar

arc system [27] and offsets of the Chindwin and upper

Irrawady river systems [10] if we consider local-

ization of deformations caused by spreading only

towards a single boundary. Assuming initiation of

Sagaing fault system at 5 Ma and taking 20 mm/year

as instantaneous fault motion [51], maximum 100 km

total displacement can be surmised on the fault

system [32]. The natural outcome of this reasoning

is that the Andaman Sea had a long pre-spreading

rifting history, which is poorly constrained at the

present state of knowledge. Two-phase/sequential

opening for the Andaman Sea, though suggested by

different workers [7,45,48], was backed only sketch-

ily by documentation of geologic events through

process–product relation, particularly for its early

rifting phase. The kinematic reconstruction for the

Indian plate relative to Eurasia [32] in the context of

major Miocene tectonic events brought definite

signals in favor of intraplate deformation during this

time period that also fits well with the present motion

predicted by NUVEL I-A model [52,53]. These

findings necessitated a comprehensive model illus-

trating temporal and spatial variation in tectonic

milieu through this belt to explain the opening of

the Mergui–Sumatra Basin and Andaman Sea in a

sequential manner. The present study therefore aims

towards unraveling the history accommodating all

available signatures in a two-phase model; an early

east–west extension in late Oligocene–middle Mio-

cene time and a second phase of transtensional

stretching through trench retreat and pull-apart open-

ing since late Miocene–early Pliocene time.

3. Data collection and methodology

Earthquake data (mbz4.0; maximum recorded

depth up to 260 km) lying within a lateral width of

P.K. Khan, P.P. Chakraborty / Earth and Planetary Science Letters 229 (2005) 259–271 263

~200 km on the downgoing Indian plate and recorded

at 15 or more stations between Lat. 28 and 178 are

considered here for reconstruction of Benioff zone

configuration. The dataset was taken from the Interna-

tional Seismological Centre (ISC) Bulletin covering

the period between 1964 and 1999. For the purpose of

present study (i.e., to visualize deformation on

downgoing slab and along-strike variation in Benioff

geometry), the area was divided into five sectors on

the basis of geotectonic trend, gravity anomaly pattern

[54] and seismic activity (I to V; Fig. 2). The Benioff

zone trajectories (over different depth ranges) were

drawn for all the five sectors. A literature review for

slab geometry reconstruction from hypocentral dis-

Fig. 2. Sector-wise distribution of earthquake epicentres in the studied area

hypocentre on vertical depth sections. Thin lines represent the Benioff z

different sectors.

tribution revealed that the procedures followed for

Benioff zone reconstruction have inherent inconsis-

tency. The actual trend of Benioff zone trajectory can

be estimated either assuming the best fit of the slab

upper surface [55–57] or as a best fit down through

the hypocentral distribution [58–60]. The diffuse

shallow level seismicity in sectors IV and V did not

allow us delineation of upper surface for the slab.

Arguably, smooth curves drawn as best fits through

the hypocenters could serve as reasonable proxies in

such cases and followed here to configure the

trajectories. The slab dip angles were measured on

these trajectories for different depth intervals, which

allowed comparative analysis on depth-related slab

. Diagrams on right show sector-specific distribution of earthquake’s

one trajectory. Note variation in Benioff zone trajectories between

P.K. Khan, P.P. Chakraborty / Earth and Planetary Science Letters 229 (2005) 259–271264

geometry within each sector and spatial variation in

slab geometry recorded through trajectories of differ-

ent sectors.

Estimation of subducted slab length in each

sector for different time slices were attempted

through appreciation of spatio-temporal variation in

plate obliquity (discussed later) and corresponding

Indian plate subduction rate. Considering uniform

tectonic framework and plate convergence rate the

subducted slab lengths were estimated and iso-

chrones (in Ma interval) were drawn on the

trajectories for the time period 5–12 Ma so as to

compare the subduction history of different sectors

in well-constrained time frame.

4. Obliquity, Benioff zone geometry and variability

Oblique convergence of the Indian plate with

major temporal variations in its speed and direction

was claimed as principal driving force behind

different morphotectonic features on this plate

margin viz. trench-parallel strike slip faulting,

formation of sliver plate, extension and basin

formation on overriding plate [6,7,15,30,61]. Jarrard

[62] suggested that the major forces controlling

forearc shear faulting at oblique subduction zones

are the convergence obliquity, the strength of the

overriding plate and the degree of interplate coupling.

Conforming this view, Diament et al. [44] considered

an increase of obliquity from the southern tip of

Sumatra to the Andaman Sea as major control on

spatially variable displacement gradient along Suma-

tran fault zones and arc parallel extension along this

belt. To assess the role of this obliquity on rates of

Indian plate subduction at different parts on this

subduction margin (between 28 and 178 Lat.) and to

evaluate its control on temporal variability in depth–

dip angle trajectory, Benioff zones were drawn for

different studied sectors (Fig. 2). The depth ranges

over which Benioff zone trajectories were drawn in

five studied sectors span between 3 and 129 km (57

hypocentres; sector I), between 4 and 194 km (135

hypocentres; sector II), between 25 and 149 km (76

hypocentres; sector III), between 14 and 260 km (146

hypocentres; sector IV) and between 4 and 208 km

(377 hypocentres; sector V). Inhomogeneity in seis-

micity distribution is apparent amongst the sectors;

sector V records maximum concentration of seismic

activity that decreases phenomenally in sectors IV and

III. Towards north, earthquake concentration further

increased in sector II and drops abruptly to a

minimum in sector I. With variation in concentration,

the seismic activity continued all through the Benioff

zone for the entire depth range except in sector IV

where a near seismic gap is observed between 159 and

217 km depth (line D–D in Fig. 2).

Comparison between these trajectories reveals

important spatial and depth-wise variations, which

include the following: (i) From sector I to V, flexing

of plate occurred at successively lower depth (~35

km variation in depth can be noticed between sectors

I and V). Corroboration in favor of this observation

came from closely spaced structure contours for the

top surface of the seismically active subducting

lithosphere between 58 and 68 Lat. (Fig. 1) [63] thatcoincides with sectors IV and V of the present study

area. (ii) Dip angle saturation reached at much

shallower depth (~25 km) towards south (sector V).

(iii) Drastic separation of trajectories for sectors I, II

and III from those of sectors IV and V at a shallow

depth (around 30 km; marked by arrow A; Fig. 3)

with sharp increase (from ~288 to ~608) in slab dip

angle. Depths, at which these separations recorded,

however, vary; trajectory in sector I separate at much

shallow depth (~22 km) in comparison to trajectories

in sectors II and III (~34 km for sector II and ~44

km for sector III, respectively). (iv) Sectors IV and

V, though follow near coincident trajectories at

shallow depth, deviate smoothly in their dip angle

around 70 km depth that becomes an abrupt one

around 140 km depth. Interestingly, the seismic gap

in sector IV nearly coincides the depth at which the

dip–angle changes abruptly. (v) After reaching a

maximum value, decrease in plate dip angle is

apparent in sector IV at the highest depth range

(depth greater than about 225 km). The two most

significant variations in depth–dip angle trajectories

that emerged from the above discussion are (i) abrupt

increase in dip angle in sectors I (up to 428), II (upto 328) and III (up to 288) at a shallow depth (around

30 km) and (ii) smooth to abrupt deviation in dip

angle between sectors IV and V at a greater depth

(varying between 70 and 140 km).

Considering each Benioff zone trajectory as

cumulative record of Indian plate convergence for

Fig. 3. Downdip dip angle variation of the Benioff zone trajectory for different sectors with superimposed isochrones (in Ma scale) for the time

period 5–12 Ma. Subducted slab lengths for different time steps are recalculated after Replumaz and Tapponier [28]. Note sharp variation in

subduction angle (i) in sectors I, II and III approximately coinciding with 5 Ma time line (arrow A) and (ii) between sectors IV and V coinciding

with 11 Ma time line (arrow B).

P.K. Khan, P.P. Chakraborty / Earth and Planetary Science Letters 229 (2005) 259–271 265

more than 15 Ma, necessity was felt to isolate the

plate subduction history over different time slices

and also to assess its spatial variation between

different sectors studied under this work (Fig. 1).

The time steps in our reconstruction was constrained

through great strike-slip faults and opening of major

basins in southeast Asia. We consider 5 Ma as

preferable interval for time stepping (i.e., 15–10 Ma,

10–5 Ma and 5–0 (present) Ma) assuming near

similar subduction history (cf. Replumaz and

Tapponier [28]) in each of these steps. Relative

velocity and azimuth of the Indian plate motion

(against stable Eurasia), obliquity of the Indian plate

and its trench-normal subduction rates were derived

(Table 1) for each of these time steps over all the

sectors. For time step 0–5 Ma, the obliquity and

subduction rate of the Indian plate shown in Table 1

are mere extrapolation of present day motion (after

Nuvel-IA pole; DeMets et al. [53]). Slip rate of the

sliver plate between Sumatran fault and the Sunda

trench in sectors IV and V (after McCaffery et al.

[30]) and spreading rate from seafloor magnetic

anomalies [21] in sectors I, II and III were taken

care of in determining the relative Indian plate

motion vectors (both direction and magnitude) for

different studied sectors. For other time steps (5–10

and 10–15 Ma), we used the position of India

relative to Asia as constrained by seafloor magnetic

anomalies [3] and trench locations as delineated by

Replumaz and Tapponier [28]. It is noteworthy that

(i) within the considered time range, i.e., 0–15 Ma,

trench normal subduction rate was at its lowest in

all the sectors between 10–15 Ma and attained a

maximum during 5 Ma onward, (ii) towards south,

obliquity of the Indian plate is well exceeded its

critical value (20F58 as suggested by McCaffery

[64] for the first time in transition from sector V to

IV as it enters the time step 10–5 Ma, which is

suggestive of obliquity-provoked active deformation

in sector IV and (iii) plate convergence obliquity in

Table 1

Temporal variation in relative velocity, azimuth, obliquity and subduction rate in five studied sectors on the subducting Indian plate along the

Burma–Java subduction margin

Sector I Sector II Sector III Sector IV Sector V

0–5 Ma

Relative velocity (mm/year) 53.03 52.92 53.31 54.15 55.00

Azimuth of relative velocity (in degrees) 21.04 22.15 22.89 23.93 24.90

Plate obliquity (/) (in degrees) 102 83 55 58 18

Trench normal subduction ratea (mm/year) 20.38 35.94 36.55 34.01 39.16

5–10 Ma

Relative velocity (mm/year) 49.33 49.33 49.33 49.33 49.33

Azimuth of relative velocity (in degrees) 356.50 358.60 359.30 2.80 5.00

Plate obliquity (/) (in degrees) 109 99 46 29 19

Trench normal subduction rate (mm/year) 21.97 15.61 16.77 27.19 28.20

10–15 Ma

Relative velocity (mm/year) 32.38 32.95 33.52 34.09 34.66

Azimuth of relative velocity (in degrees) 3.80 5.10 6.40 7.70 9.00

Plate obliquity (/) (in degrees) 85 80 38 18 21

Trench normal subduction rate (mm/year) 9.84 7.85 8.58 15.76 17.15

/=Angle between the directions of the Indian plate motion and trench normal vectors.a Considering the Indian plate velocity relative to sliver/overriding plate velocity (stable Eurasia).

P.K. Khan, P.P. Chakraborty / Earth and Planetary Science Letters 229 (2005) 259–271266

sectors I, II and III was much higher (between 388and 1098) in all three time steps. Relative velocity

of the Indian plate in these sectors, however,

attained its maximum (N52 mm/year) value only in

recent time step (5–0 Ma).

5. Discussion

The existing models [22,29] on opening of the

Andaman Sea considered long tectonic history at

this plate margin that includes collision-induced

extrusive tectonics (resulting from the rigid inden-

tation of the Indian plate with Asia) in latest

Oligocene–early Miocene, post-collision northward

movement of India and clockwise rotation of

Burma–Java subduction zone. The study of Bertrand

and Rangin [32] in the central Myanmar–Andaman

Sea region supported long extensional history (for

the last 45 Ma) on this plate margin. Supportive

evidences in favor of spreading in the Andaman Sea

in the past 13 Ma [7] also came from study of slip

vectors from thrust earthquakes at the Java trench

and estimation of arc-parallel stretching for Suma-

tran forearc [12]. Recent integrated study on swath

bathymetry, magnetic and seismological data [21],

however, showed active seafloor spreading in the

Andaman Sea basin as a much young phenomenon,

operative only for the last 4–5 Ma. This recent

finding, though concerns principally on northern

part of this geotectonic belt (around 118 Lat.), raiseda major question on pre-spreading deformation

history in southern part of Andaman forearc; what

was its character? How did it initiate? And what

about the age of deformational events?

We attend these questions in reverse order.

Drawing of isochrones allowed us in constraining

time for major episodes of geometry change in the

dip angle trajectories of studied sectors. Timing for

two major events i.e., dispersion of trajectories of

sectors I, II and III from those of IV and V (at about

30 km depth) and abrupt deviation of sector IV

trajectory from sector V trajectory (at about 140 km

depth) are found approximately coinciding with 5

and 11 Ma time lines, respectively. Considering these

two time periods as of primary significance in the

post-Oligocene deformation history at the studied

stretch of the Burma–Java subduction margin, we

attempted their logical connection with two major

phases of the Andaman Sea opening viz. (i)

extension and rifting, and (ii) extension through

seafloor spreading (Fig. 4). The slab deformation

Fig. 4. Schematic representation of sequential evolution of Andaman Sea (modified after Rodolfo [45]; Curray et al. [7]; Karig et al. [8];

Stephenson and Marshall [40]).

P.K. Khan, P.P. Chakraborty / Earth and Planetary Science Letters 229 (2005) 259–271 267

P.K. Khan, P.P. Chakraborty / Earth and Planetary Science Letters 229 (2005) 259–271268

recorded around 4–5 Ma time towards northern part

of the study area (in sectors I, II and III) clearly

suggestive of its relation with back arc active

spreading recorded by Raju et al. [21] from magnetic

anomaly study. Our study shows abrupt increase in

slab dip (Fig 3) together with increased subduction

rate (Table 1) towards north of 78 Lat. around 4–5

Ma, which possibly provoked trench retreat and

active extension resulting into spreading in the back

arc region (Fig. 4d). The southern segment of the

Andaman Sea Ridge (ASR) possibly records the

initiation history of this spreading phase. The ocean–

continent transition recorded in Mergui terrace [7]

hints towards possible locale wherefrom the spread-

ing phase initiated (Fig. 4c). Subsequent veering in

orientation of ASR in NE–SW direction is possibly

an outcome of multiple tectonic forcings including

velocity inhomogeneity between the Indian and

Burma plate [10] and N–S compression caused there

from, uplifting of oceanic crust in post-middle

Miocene time through Alcock and Sewell seamounts

[7] (in the north and south of ASR, respectively) and

post-middle Miocene dstop and goT spreading charac-

ter in this ridge system. For the 11 Ma slab

deformation event recorded from the southern part

of the studied stretch (sectors IV and V), we propose

its relation with the early rifting history of the

Andaman Sea, i.e., the dproto Andaman SeaT of

Curray et al. [7] in place of present Mergui–North

Sumatra Basin. This well-controlled time connotation

for major slab deformation episodes of the subducting

Indian plate undoubtedly helped in resolving the long-

held uncertainty of early deformation history for the

Andaman Sea.

Understandably, extrusion tectonics of southeast

Asia, rotation of northern Sumatra and stretching

from the northward moving Burma platelet fall short

in explaining entire history of the Andaman Sea,

particularly for its early deformational phase. Large-

scale forearc subsidence (Fig. 4a) in late Oligocene–

early Miocene time all through this plate margin

[7,8,15,33,45–47] bear tell-tale signal for an

increased extension that predates rotation of Sumatra,

initiation of oblique convergence and northward

movement of the Burma platelet. An enhanced

subsidence around 58N (sector IV of the present

study area) between 15 and 10 Ma (Fig. 3), caused

by increased subduction angle of the Indian plate (in

adjustment to change in curvature of the subduction

margin, cf. Maung [10]), possibly led to the first

phase of rifting in the Andaman Sea (in the

northwest part of Sumatra; Fig. 4b) at around 11

Ma, when the deformation was compounded by

sudden increase in the Indian plate obliquity on

transition from sector V to sector IV (Table 1). The

dragging of the Indo-Burman Ranges and Andaman–

Nicobar Ridges towards the north by India is

suggested as the causative forcing behind the

formation of double arc in the middle Miocene time

(Maung [10]) with reduction in radius of curvature

for the Andaman–Nicobar arc [10]. The decline in

depth of plate flexure from sector I through sector V

against this smaller radius of arc curvature resulted

to nonconformity between plate shape and subduc-

tion margin geometry that might have contributed in

increasing subduction angle and intraplate extension.

Abrupt departure of plate dip amount (between

sectors IV and V) at a depth that coincides with

seismic gap registers signature of this enhanced

extension event. This abrupt deviation in dip angle

of suducting plate (Fig. 3), seismic gap and supposed

free flow in the mantle at deeper level in sector IV

(depth section in Fig. 2) prompted us to visualize

possible tear in downgoing plate, which, of course,

did not go up to the stage of plate detachment.

Systematic spatio-temporal analysis on obliquity

and subduction rate under the present study clearly

reveals that sectors I, II and III behaved differently

in their tectonic evolution from sectors IV and V.

Slowing of subduction is indicated all through this

plate margin between 15 and ~10 Ma (Table 1).

Though obliquity showed consistently high value

(exceeding critical value for Northwest Sumatra) in

sectors I, II and III for all three considered time

steps, subduction rate varied in these sectors between

these time steps; highest subduction rate being

recorded in time step 5–0 Ma. This possibly rules

out obliquity as a major control behind spreading

phase of the Andaman Sea history. Role of obliquity,

however, strongly suggested behind the early rifting

history of the Andaman Sea. Sector IV which

records signature of this early rifting phase in its

depth–dip angle trajectory definitely reveals the role

of obliquity behind it as obliquity value exceeded its

critical limit for the first time in this sector of the

Sumatran subduction zone.

P.K. Khan, P.P. Chakraborty / Earth and Planetary Science Letters 229 (2005) 259–271 269

6. Conclusions

Andaman subduction raises several questions

about the local kinematics that demand attention in

the backdrop of regional tectonics. The complexities

of this tectonic province though modeled in the earlier

studies through oblique subduction, arc volcanism and

back arc spreading activity, control of subduction

geometry on deformation history of overriding plate,

have not been attended so far. The present high-

resolution seismotectonic study supports the two-

phase evolution model for the Andaman Sea and

clearly relates major events in slab subduction history

with those well-established phases.

Comparison between depth–dip angle trajectories

from different parts of this subduction margin

revealed that subduction history differed from north

to south and the subducting slab witnessed two

major events of deformation in its geometry. Assess-

ment of effective rate of subduction vs. time (and

then length of subducted slab) allowed time con-

notation for these deformation episodes and in turn

constrained these episodes with other tectonic

parameters of this plate margin like obliquity and

subduction rate.

The middle Miocene east–west forearc extension

in the Mergui–Sumatra region towards the southern

part of the studied sector possibly resulted from long

subsidence in this part which together with stretching

from abrupt increase in plate obliquity and enhanced

rate of subduction went up to the stage of rifting.

With a time lag of about 7 Ma (11–5 Ma), the

second major event of subduction slab steepening is

recorded towards the northern part of this margin

that corresponds to back arc spreading. Trench

retreat and transcurrent movement (along the Saga-

ing and Sumatran fault system) with NW–SE pull-

apart extension possibly caused the necessary exten-

sion that initiated the active spreading since the latest

Miocene–Pliocene time. In totality, from south to

north, the Andaman Sea can be compared with the

opening of a zipper that though started its opening

towards south got aborted and subsequently opened

to the full towards north. The northward drag of

hanging Indian lithosphere through the surrounding

asthenosphere together with counter-clockwise rota-

tion of north Sumatra and south Malaya helped in

the final pull-apart opening of this zipper resulting

into complex tectonic environment and physiography

as observed in the Andaman Sea.

Acknowledgements

The authors are thankful to the Department of

Science and Technology (DST), Government of India,

New Delhi, for the financial support. The authors are

grateful to the Director, Indian School of Mines for his

constant encouragement. Special thanks are also due

to Prof. Robert Hall, University of London, for

critically going through an early version of the

manuscript and suggesting many important changes

that helped in improving the write-up. We also

acknowledge the erudite reviews from two anony-

mous reviewers, which helped us in improving the

manuscript in a great way.

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