Crustal Model for the Andaman Outer Arc: Constraints from Earthquake, Gravity and Receiver Function Data

Abstract

Here we present an integrated model for the descending crust of the Indian Plate by reinterpreting the gravity anomalies and constraining the model by earthquake data and Receiver Function (REF) results. The Slab Residual Bouguer Gravity Anomaly (SRBGA) calculated for the Andaman arc region reaches its maximum amplitude towards the eastern flank of Andaman Outer Arc (AOA) below the Andaman Forearc. The integrated crustal model for the AOA presented here is based on the following data constraints: (a) Digital Elevation Model (DEM) for AOA, (b) IRIS seismicity data for top lithospheric earthquakes that allows to trace the overall pattern of seismically active crust underlying three E-W profiles crossing the plate margin, (c) the Moho-map for AOA prepared by making use of the Parker Oldenburg’s Algorithm to invert the SRBGA, and (e) by undertaking integrated modeling for two E-W geophysical traverses, AA′ and BB′, taken across the AOA in areas of the North and South Andaman Islands at a gap of ~400 km distance. Gravity interpretation for the two profiles is constrained by results available from REF analysis on (i) Moho depth, (ii) their further derivation into crustal density by using Brocher 2005-empirical relationship and (iii) Poisson’s ratio. These values are deduced from REF data monitored at eight broadband seismic stations distributed north-south on the AOA. These results demonstrate a systematic increase in average crustal density from the Middle to South Andaman; while, an increase in Poisson’s ratio demarcates a locally thickened crust in the intervening region between these islands. Main results from the integrated model are: (i) Crustal model beneath the AOA approaching the subduction zone, (ii) A low density zone at shallow depth within the Andaman Sedimentary Arc (ASA), a partial support for which comes from a Low Velocity Zone (LVZ) detected by REF having Vs = 1.3–2.5 km/s, (iii) Deeper continuity of the Jarwa Thrust contained within ASA to crustal depths, and (iv) the refined Moho-margin against the top-mantle. Gross differences in crustal properties, namely; Vs-values, thickness, density and Poisson’s ratio, are noticed between the Middle and South Andaman Islands which are separated by a locally thickened crust by ~8 km. Depth-slices are presented based on 32 focal mechanism solution for crustal earthquakes occurring below the traverses AA′ and BB′, which reveal: the prevalence of trench-parallel normal faulting under western parts of AOA, while, thrust faulting is more conspicuous closer to the subduction zone as defined by the Jarwa Thrust and the Eastern Margin Fault.