Achyuta Ayan Misra

East Indian margin evolution and crustal architecture: integration of deep reflection seismic interpretation and gravity modelling

Abstract The segmented East Indian continental margin developed after the Early Cretaceous break-up from Antarctica. Its continental crust terminates abruptly without considerable thinning along the Coromondal strike-slip segment and thins considerably before it terminates in the orthogonal rifting segments. The segments have an exhumed continental mantle corridor oceanwards of them. This, proto-oceanic crust, corridor varies in width from segment to segment, indicating a relationship with varying break-up-controlling tectonics of the adjacent margin segments. The top of the proto-oceanic crust is imaged by a higher reflectivity zone, while its base does not have any distinct signature. A contorted system of reflectors represents its internal structure. Its gravity signature is a longer-wavelength anomaly with peak values up to 30 mGal less negative than surrounding values. Its magnetic signature is represented by a positive anomaly with peak values of 0–56 nT. Wide proto-oceanic segments are adjacent to margin segments that are preceded by the orthogonally rifting Cauvery, Krishna–Godavari and Mahanadi rift zones. A narrow proto-oceanic segment is adjacent to the margin segment initiated by the dextral Coromondal transfer zone. A combination of seismic interpretation and gravity/magnetic forward modelling indicates that proto-oceanic crust is most probably composed of lower crust slivers and unroofed hydrated upper mantle, being formed between the late rifting and the organized sea-floor spreading.

Dyke–brittle shear relationships in the Western Deccan Strike-slip Zone around Mumbai (Maharashtra, India)

Abstract Dykes are abundant in the Deccan Large Igneous Province, and those to the west are referred to as the ‘coastal swarm’. Most of the coastal swarm dykes appear in the Western Deccan Strike-slip Zone (WDSZ). Faults with N–S, NE–SW and NW–SE trends (brittle shears) have been reported in the WDSZ around Mumbai. However, details of their relationships with Deccan dykes, which can easily be studied at sub-horizontal outcrops, have remained unknown. Previous authors have classified dykes in the WDSZ according to their isotopic ages as group I (c. 65.6 Ma), group II (c. 65 Ma) and group III (64–63 Ma). Dykes have also been categorized on the basis of field observations; group I dykes were found to pre-date deformation related to the separation of Seychelles and India, whereas group II and III dykes post-date this event. Our field studies reveal group I dykes to be faulted/sheared and lacking a uniform trend, whereas group II and III dykes have approximately N–S, NW–SE and NE–SW trends and intrude brittle shears/fault planes. We have also found evidence of syn-deformation intrusion in the group II and III dykes: e.g. P-planes along the dyke margins and grooves in the baked zone of dykes. These two groups of dykes match the trends of dominantly sinistral brittle shears. Of the 43 dykes studied, only ten belong to group I, and we conclude that a large proportion of the dykes in the WDSZ belong to groups II and III. It is erroneous to interpret the Seychelles–India rifting as simple near-E–W extension at c. 63–62 Ma from the general approximately N–S trend of the dykes; the direction of brittle extension must instead be deduced from brittle shears/fault planes. Supplementary material: Stereo plots and reduced stress tensors for all faults and brittle shears are available at https://doi.org/10.6084/m9.figshare.c.3259627

Tectonics of the Deccan Large Igneous Province: an introduction

SOUMYAJIT MUKHERJEE1*, ACHYUTA AYAN MISRA2, GÉRÔME CALVÈS3 & MICHAL NEMČOK4,5 Department of Earth Sciences, Indian Institute of Technology Bombay, Mumbai 400 076, Maharashtra, India Exploration, Reliance Industries Ltd, Mumbai 400 701, Maharashtra, India Université Toulouse 3, Paul Sabatier, Géosciences Environnement Toulouse, 14 avenue Edouard Belin, 31400, Toulouse, France EGI at University of Utah, 423 Wakara Way, Suite 300, Salt Lake City, UT 84108, USA EGI Laboratory at SAV, Dúbravskácesta 9, 840 05 Bratislava, Slovakia

Subsidence around oceanic ridges along passive margins: NE Arabian Sea

Abstract The northern part of the western continental margin of India formed due to the separation of the Seychelles from India at c. 63 Ma. This produced offshore tectonic elements such as the Gop Rift, the Saurashtra Volcanic Platform (SVP) and the Laxmi Ridge, as well as numerous seamounts, e.g. the Raman and Panikkar seamounts. The Laxmi Ridge and the Laxmi Basin have been studied using high-resolution 2D reflection seismic data and well data. Patch and pinnacle carbonate reefs, indicating shallow waters, are common in the north, whereas large, isolated platforms are usually noted in the south. Palaeo-depth estimates are made from well biostratigraphy. Subsidence studies of the SVP suggest that the burial history is consistent with the anomalously hot Réunion plume. We have performed a subsidence analysis south of the SVP on the Laxmi Ridge and Laxmi Basin. The sediment-unloaded basement depths, estimated using using flexural isostasy with effective elastic thicknesses of 10–40 km have been found to be 2000–4000 m in areas where carbonates exist. These carbonates indicate <200 m bathymetry at c. 65 Ma, and the subsidence discrepancy is thus due to thermal cooling or anomalous heating due to the Deccan plume. Patch and pinnacle reefs in the north suggests that either the rise in sea-level or the rate of subsidence of the basement were fast. The presence of large platforms in the south indicates otherwise. This is possibly due to a greater influence from the Indus Fan sediments towards the north. In addition, the Laxmi Ridge is a spreading centre that remained emergent near or above sea-level due to plume support, which was also greater in the south due to proximity to the plume. When the plume support discontinued, the ridge subsided quickly to present-day depths, which matches the subsidence expected for 60–70 Myr old oceanic crust. Supplementary material: A table is available at https://doi.org/10.6084/m9.figshare.c.3470751

Tectonics of the Deccan Large Igneous Province

Understanding the Deccan Trap Large Igneous Province in western India is important for deciphering the India–Seychelles rifting mechanism. This book presents 13 studies that address the development of this province from diverse perspectives including field structural geology, geochemistry, analytical modelling, geomorphology and geophysics (e.g., palaeomagnetism, gravity and magnetic anomalies, and seismic imaging). Together, these papers indicate that the tectonics of Deccan is much more complicated than previously thought. Key findings include: the Deccan province can be divided into several blocks; the existence of a rift-induced palaeo-slope; constraints on the eruption period; rift–drift transition mechanisms determined for magma-rich systems; the tectonic role of the Deccan or Réunion plumes; sub-surface structures reported from boreholes; the delineation of the crust–mantle structure; the documentation of sub-surface tectonic boundaries; post-Deccan-Trap basin inversion; deformed dykes around Mumbai, and also from the eastern part of the Deccan Traps, documented in the field.

Influence of Pre-existing Anisotropies on Fault Propagation

We discuss fracture criteria for anisotropic rocks here. Brittle failure of rock follows as cohesion of the material is lost and the rock ruptures along a surface/zone. Isotropic rocks follow the Mohr-Coulomb failure criterion in the compression and Griffith failure criterion in tension. Anisotropic rocks follow a modified Coulomb criterion. Strength anisotropy due to discrete or pervasive fabrics in country rock affects the whole rock strength. At the shallow crustal levels, pre-existing fabrics can control the geometry and location of rifts by diverting or preventing fracture propagation.

Lessons from Analogue Models

Few models addressed the influence of pervasive fabrics since weak fabrics are difficult to generate on model materials. These models have shown that fractures/joints/anisotropies perpendicular to the extension direction undergo dilation or normal faulting. Other orientations experience oblique to strike-slip movements. We discuss some key observation from analogue models here.

Role of Lithosphere Rheology on Rift Architecture

Extensional geodynamics is controlled strongly by lithosphere rheology. Lithospheric strength inversely relates to its thickness and temperature and also to its composition. Reorientation of stress axes can uplift rift shoulders. A narrow-, wide- and core complex mode of rifting happens in progressively hot and weak lithosphere. Combined thermal state of the lithosphere and strain rates manifest differences in rift architecture.

Pre-existing Fabrics

Continental rifts are not random and generally tend to follow the mobile belts. A well accepted classification of factors that give inheritance is: (i) discrete, and (ii) pervasive. Pervasive fabrics- slaty cleavages, close-spaced joints or beddings, laminations, flow layers of pre-rift sedimentary or volcanic rocks, and foliations such as schistosity and gneissosity- may persist throughout the rock volume. We show examples of inheritance due to pervasive fabric from East African Rift System, Thailand Tertiary Rift System, South Atlantic Passive Margins, East and West Indian Passive Margins. Discrete/isolated fabrics are planar to curvi-planar elements that lead to anisotropy in terms of strength and material properties with respect to the surrounding rocks. We show examples of inheritance due to discrete fabric from East African Rift System, Brazilian Rifts, Tertiary Rifts of Thailand, North Atlantic Passive Margin, Eastern North American Rift System, Rhine Graben, East and West Indian Passive Margins.

Positive flower structure in the Southern Granulite Terrain, India

Positive flower structure indicative of a transpression zone within a Precambrian sequence of quartzite (white) and basalt (black) in the Archean granite gneiss section of Southern Granulite Terrain, India. Two dolerite dykes orthogonal to the foliation in the quartzite are visible at the lower left hand side of the outcrop. This type of structure is typical for the shear zones that intersect this area. The section is exposed in a vertical (E–W) facewithin an abandoned quarry near village of Chillamancheru near Rajupalem, Andhra Pradesh, India. Width of view is about 80 m. Location: N 14 00.75, E 79 50.23. Photograph Achyuta Ayan Misra. Achyuta Ayan Misra