This chapter highlights the common characteristics that are exhibited by fault propagation folds. The North Ecuador-South Colombia (NESC) margin has fault propagation folds developed in its Tertiary sequence due to subduction-related compression of the Farallon- and the Nazca plates beneath the South American plate. The chapter presents a figure representing location of the seismic profile in the NESC margin overlaid on a hill-shaded free-air gravity anomaly map of Buenaventura city. The seismic section trends NE and shows a fault propagation fold, which depicts the discussed criterion. On the NE side of the section there is some noise. This may arise from reflections from the adjacent syncline. The topmost horizons of the fold are eroded and channel cuts can be interpreted on them. This indicates that uplift due to folding was faster than the sedimentation in the earlier stages and subsequently slowed down.
Important tectonic elements in the North Ecuador-South Columbia (NESC) margin are the Garrapatas fault system; the Buenaventura fault system; the Gorgona terrane, which are exposed subaerially as the Gorgona Island; and the Dagua Pinion terrane. The Garrapatas fault system is a boundary between the western limit of the Gorgona terrane and the oceanic sediments and later terrigenous sediments after the Gorgona terrane docked. The Buenaventura fault system is a suture between the Gorgona terrane and the Dagua Pinon terrane and closely follows the ~ NE-SW trending coast line. There are a few NW dipping normal faults towards the NW portion of the seismic section. A prominent feature in the second seismic section is a seismic reflection that cross-cuts other reflections and is near the sea bottom. The depth of the bottom simulating reflectors (BSRs) from the sea is a very important tool to study the heat flow or geothermal gradient in the area.
The 85°E Ridge is a trace of the Kerguellen Hotspot in the Bay of Bengal, offshore of the east coast of India. A seismic section image of the 85°E Ridge is acquired by ion-GX Technology. Faults are present in the overburden and link at the flanks of the ridge. These faults form due to differential or inhomogeneous compaction across the ridge. On top of the ridge there are approximately 3 km thick sediments, while away from the flanks the sediment thickness is approximately 6 km. The compaction of an approximately 6 km thick sedimentary column will be much larger than that of an approximately 3 km thick one. This large difference in compaction cannot be accommodated by draping alone and the sediments are faulted as a result. Normal drags along the faults are present on both sides. They are most possibly related to the bending of the fault plane on top of the ridge.
This chapter discusses translation zones, which are the zones of little to no deformation between the extensional and compressional domains in a growth fault thrust belts (GFTB). It shows examples of linked GFTBs from the eastern continental margin of India and presents an example of GFTB from the Mumbai/Bombay Offshore Basin, West India, for information about the tectonics of the western continental margin of India. This GFTB has a very small translation zone. The difference in the geometry is most plausibly due to the nature of the detachment. The most likely differences of the detachment in the seismic section with the earlier ones from east India are listed as follows. First, the friction of the detachment may be more, second, the pore pressure may be less and finally, the lithology of the detachment may have some coarser clastics mixed with shale.
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
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
The network of polygonal faults normally occurs as a layer bound of deformational events, that is its top and bottom layers are undeformed. These sets of normal faults are neither related to extension nor to any gravity-driven slip. The network of polygonal faults acts as fluid escape feature, through which fluids from lower lithological units diffuse out to the upper surface. These polygonal faults in general occur in finer clastics and are distributed randomly along horizontal section. Polygonal faulting is also observed in the Indian E coast. In this example, a polygonal fault system is shown in the Late Cretaceous finer sediments as identified in the deepwater Cauvery Basin. The polygonal fault system and the inhomogenous decompaction related faults act as fluid escape structures. Fluids can migrate to shallower sections through such a fault network.
'%3e%3ccircle r='20' fill='%23008'/%3e%3ccircle r='17.5' fill='%23fff'/%3e%3ccircle r='3.5' fill='%23008'/%3e%3cg id='d'%3e%3cg id='c'%3e%3cg id='b'%3e%3cg id='a' fill='%23008'%3e%3ccircle r='.875' transform='rotate(7.5 -8.75 133.5)'/%3e%3cpath d='M0 17.5.6 7 0 2l-.6 5L0 17.5z'/%3e%3c/g%3e%3cuse xlink:href='%23a' transform='rotate(15)'/%3e%3c/g%3e%3cuse xlink:href='%23b' transform='rotate(30)'/%3e%3c/g%3e%3cuse xlink:href='%23c' transform='rotate(60)'/%3e%3c/g%3e%3cuse xlink:href='%23d' transform='rotate(120)'/%3e%3cuse xlink:href='%23d' transform='rotate(-120)'/%3e%3c/g%3e%3c/svg%3e)
'%3e%3cpath fill='%23012169' d='M0 0v30h60V0z'/%3e%3cpath stroke='%23fff' stroke-width='6' d='m0 0 60 30m0-30L0 30'/%3e%3cpath stroke='%23C8102E' stroke-width='4' d='m0 0 60 30m0-30L0 30' clip-path='url(%23b)'/%3e%3cpath stroke='%23fff' stroke-width='10' d='M30 0v30M0 15h60'/%3e%3cpath stroke='%23C8102E' stroke-width='6' d='M30 0v30M0 15h60'/%3e%3c/g%3e%3c/svg%3e)
