S. T. Sinha

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.

Spatiotemporal Variations and Kinematics of Shale Mobility in the Krishna-Godavari Basin, India

Tertiary sediment loading on the Late Jurassic–Early Cretaceous passive-margin rift fills in the offshore Krishna-Godavari basin generated different episodes and patterns of mobile, shale-cored structures from the Paleocene to Pliocene. The Paleocene–Eocene shales in the deeper shelf and upper slope areas moved as a series of thrust slices, whereas the younger Miocene–Pliocene shales moved as individual bulges (diapirs). Lithological variability and shifting depocenters, prevailing tectonic conditions, and available space in the system for translation in space and time all influenced the spatiotemporal distribution of the shale-cored structures. A relation is also observed between the location of the toe thrusts and shale diapirs, and the basement highs and escarpments. Two-dimensional palinspastic restorations, incorporating all the above variables, confirm the linkages between the sediment depocenters, growth faults, and mobile shales in the Krishna-Godavari basin.

Transform Margins: Development, Controls and Petroleum Systems

This volume covers the linkage between new transform margin research and increasing transform margin exploration. It offers a critical set of predictive tools via an understanding of the mechanisms involved in the development of play concept elements at transform margins. It ties petroleum systems knowledge to the input coming from research focused on dynamic development, kinematic development, structural architecture and thermal regimes, together with their controlling factors. The volume does this by drawing from geophysical data (bathymetry, seismic, gravity and magnetic studies), structural geology, sedimentology, geochemistry, plate reconstruction and thermo-mechanical numerical modelling. It combines case studies (covering the Andaman Sea, Arctic, Coromandal, Guyana, Romanche, St. Paul and Suriname transform margins, the French Guyana hyper-oblique margin, the transtensional margin between the Caribbean and North American plates, and the Davie transform margin and its neighbour transform margins) with theoretical studies.

85°E Ridge, India — constraints on its development and architecture

The 85°E Ridge is a buried aseismic ridge running parallel to the 85°E meridian in the Bay of Bengal, India. Its origin has been a subject of debate, with opinions ranging from an abandoned spreading centre to a hotspot track. The present study follows the hotspot hypothesis and incorporates gravity, magnetic and seismic data to identify the nature and interpret the origin of the 85°E Ridge. It differs from earlier studies in the integration of deep seismic lines and gravity inversion to identify crustal architecture below the 85°E Ridge. Seismic interpretation along with gravity inversion has been used to determine the crustal structure below the ridge, while sediment thickness maps have been used to infer the uplift during the ridge emplacement. Seismic interpretations together with isostatic residual gravity anomaly map have been used to associate large negative anomalies with hotspot related magmatism. The negative anomaly increases with increasing volcanic load, indicating the presence of a crustal root and magmatic underplating. Typical flexural moat and arch, indicative of hotspot volcanism, is also observed in the seismic profiles. Gravity inversion modeling indicates an “onion-shell” like structure within the volcanic load, inferring the presence of less dense outer layers with a heavier core within the complex. Sediment thickness maps show the presence of dynamic uplift of more than 2000 milliseconds from early Cretaceous onwards. The study concludes that the 85°E Ridge is a result of hotspot volcanism, and proposes a plausible model for the origin of the structure.

Continental break-up mechanism; lessons from intermediate- and fast-extension settings

Abstract Continental break-up mechanisms vary systematically between slow- and fast-extension systems. Slow-extension break-up has been established from studies of the Central Atlantic, European and Adria margins. This study focuses on the intermediate and fast cases from Gabon and East India, and draws from the interpretation of reflection seismic, gravimetric and magnetic data. Interpretation indicates continental break-up via continental mantle unroofing in all systems, with modifications produced by magmatism in faster-extension systems. Break-up of the intermediate-extension Gabon system involves partial upper continental crustal decoupling from continental mantle; whereas, in the fast East Coast India system, decoupled and lower-crustal regimes underwent upwarping in ‘soggy’ zones in the footwalls of major normal faults. Usually, upper-crustal break-up is affected by pre-existing anisotropies, which form systems of constraining ‘rails’ for extending continental crust. This modifies the local stress regimes. They regain a regional character as the function of constraining rails vanishes during progressive unroofing of the upper mantle. Different regions attain different amounts of upper-crustal stretching prior to the break-up. The break-up location is then controlled by the upper-crustal energy balance principle of ‘wound linkage’, by which the minimum physical work is performed for linking upper-crustal ‘wounds’, leading to successful upper-crustal break-up. Supplementary material: Supplementary information and figures on the modelling of the mechanisms and architecture is available at http://www.geolsoc.org.uk/SUP18525.

Mechanisms of microcontinent release associated with wrenching-involved continental break-up; a review

Abstract The study focuses on the role of wrenching-involved continental break-up in microcontinent release, drawing from a review of examples. It indicates that the main groups of release mechanisms in this setting are associated with ‘competing wrench faults’, ‘competing horsetail structure elements’, ‘competing rift zones’ and ‘multiple consecutive tectonic events’ controlled by different stress regimes capable of release. Competing-wrench-fault-related blocks are small, up to a maximum 220 km in length. They are more-or-less parallel to oceanic transforms. The competing horsetail-structure-element-related blocks are larger (up to 610 km in length) and are located at an acute angle to the transform. Competing-rift-zone-related blocks are large (up to 815 km) and are either parallel or perpendicular to the transform. The multiple-consecutive-tectonic-event-related blocks have variable size and are generally very elongate, ranging up to 1100 km in length. The role of strike-slip faults in release of continental blocks resides in: linking the extensional zones, where the blocks are already isolated, by their propagation through the remaining continental bridges and subsequent displacement; facilitating rapid crustal thinning across a narrow zone of strike-slip-dominated faults; and slicing the margin into potentially detachable fault blocks.

Conjugate divergent margins: an introduction

Abstract The main objective of this book is to provide a global overview of divergent margins based on geological and geophysical interpretation of sedimentary basins along the South, Central and North Atlantic conjugate margins, from plate tectonics and crustal scales to a more detailed description of stratigraphical and structural elements that are responsible for petroleum plays. These themes are complemented by geodynamic concepts based on physical and numerical models, and by comparisons with present-day embryonic margins, which are succinctly discussed in some papers. Supplementary material: Three plate animations of the Atlantic Ocean are available at www.geolsoc.org.uk/SUP18620.

The role of break-up localization in microcontinent separation along a strike-slip margin: the East India–Elan Bank case study

Abstract The Elan Bank microcontinent was separated from East India during the Early Cretaceous break-up. The crustal architecture and rifting geometry of East India and the Elan Bank margins document that the early break-up between India and Antarctica was initiated in the eastern portions of the Cauvery and Krishna–Godavari rift zones, and in the southern portion of Elan Bank. However, the westwards break-up propagation along the Krishna–Godavari Rift Zone continued even after the break-up in the overstepping portion of the Cauvery Rift Zone. Eventually, the western propagating end of the Krishna–Godavari Rift Zone became hard-linked with the failed western portion of the Cauvery Rift Zone by the dextral Coromandel transfer fault zone. Consequently, the break-up location between India and Antarctica shifted from its initial to its final location along the northern portion of the Elan Bank formed by the western Krishna–Godavari Rift Zone. The competition between the two rift zones to capture continental break-up and asymmetric ridge propagation resulted in a ridge jump and the Elan Bank microcontinent release. Supplementary material: Supplementary figures are available at http://www.geolsoc.org.uk/SUP18864

Transform margins: development, controls and petroleum systems – an introduction

Abstract This paper provides an overview of the existing knowledge of transform margins including their dynamic development, kinematic development, structural architecture and thermal regime, together with the factors controlling these. This systematic knowledge is used for describing predictive models of various petroleum system concept elements such as source rock, seal rock and reservoir rock distribution, expulsion timing, trapping style and timing, and migration patterns. The paper then introduces individual contributions to this volume and their focus. Supplementary material: Location table and map of specific transform examples, structural elements of the Romanche transform margin and glossary of terms used in this article is available at https://doi.org/10.6084/m9.figshare.c.3276407

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