Two-phase opening of Andaman Sea: a new seismotectonic insight

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Abstract

High-resolution reconstruction of Benioff zone depth–dip angle trajectory for Burma–Java subduction margin between 2° and 17°N 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 7° 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 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.

Introduction

Collision between the Indian and Eurasian plates [1], [2], [3], [4], [5], coupling and decoupling of different platelets [6], crustal movement along strike-slip faults [7], [8], [9], [10], [11], [12], [13], [14], [15], rotation of continental blocks [16], [17], [18], [19] and opening of 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 marginal basins through this highly dynamic geotectonic 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], [25], [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 subsequent (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 Andaman 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 (≤4–5 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. 2° and 17° and Long. 92° and 99°. Objectives of 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 operative 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-resolution earthquake data analysis under the present study allowed sequential modeling of the Andaman Sea, in general, and its early rifting phase, in particular.

Section snippets

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 continental 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

Data collection and methodology

Earthquake data (mb≥4.0; maximum recorded depth up to 260 km) lying within a lateral width of ∼200 km on the downgoing Indian plate and recorded at 15 or more stations between Lat. 2° and 17° are considered here for reconstruction of Benioff zone configuration. The dataset was taken from the International 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

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

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 indentation 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

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

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

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