Course Title IEQ-05 Earthquake Geology and Geoinformatics

by Dr. J. D. Das

(Dept. of Earthquake Engineering, IIT Roorkee) Important Tectonic Features Son-Narmada Fault

The Son-Narmada fault in peninsular India is an important, ancient and deep seated line of fracture along which different style of tectonic movement are usually postulated. Narmada valley formed at the crest of a domal upwarp with tension fractures and probably shallow depressions along the crest. The ENE-WSW trending Son-Narmada fault zone in central India is a remarkable tectonic zone which is about 1600 km long and 150-200 km wide and traceable across the Indian Peninsula.

This zone has been affected by episodic history of vertical and lateral movements and magmatism. May be a mid-continental rift with strong neotectonism, high heat flow and moderate seismicity. Based on paleomagnetic studies it has been suggested Son-Narmada fault is a wrench fault of unknown magnitude. Another study suggested that the Son-Narmada -Brahmaputra transform (NSBT) fault extending for 2500 km from Kathiwar in the west to the Mishmi Tectonic Unit in the east is a transcurrent fault zone with about 600 km dextral slip.

It divided India into a North India Block (NIB) and a South India Block (SIB).

Son- Narmada-Brahmaputra Transform

Strike slip movement along NSBT

Strike slip movement along NSBT is mainly based on the large offset of geological features.

The transcurrent movement was most active during the Early to Late Cretaceous i.e. between the Rajamahal and end of the Deccan trap episodes.

It is also suggested that the Son-Narmada-Tapti (SONATA) lineament zone is made up of several longitudinal faults bound blocks. These blocks have been moving relatively to each other and the SONATA represents a mid continental rift.

Geological Structure along the SONATA

Dauki Fault

The southern margin of Shillong Plateau is marked by the remarkably linear E-W trending Dauki Fault.

Early studies suggested that the Dauki Fault Zone is essentially a tear-fault with a lateral movement of over 200 km. Extent of movement was not possible to be estimated. Similarly, due to lack of evidences on the extension of this zone below the alluvial gap between Shillong Plateau and Rajmahal Hills, its westward continuation can not be ascertained.

Satellite image showing the Shillong Plateau and Dauki Fault

Satellite image showing Dauki fault zone

Basement configuration in the Shillong Plateau and Bangladesh region.

Later, studies have suggested vertical movements along the Dauki Fault, as it is evidenced by the extrusion of lava through the deep-seated vertical fracture system.

It seems that the fault zone is characterized by uplift and down-sinking of adjacent basement blocks along the fractures.

Grabens of India

Mechanism of Graben formation Early hypotheses assumed that graben formation required a wedge of crust about 60 km wide to sink isostatically between inward-dipping normal faults. As the upper crust forms graben by wedge subsidence the ductile lower crust compensates by plastic flow.

As the lithosphere is stretched during continental extension, the ductile deeper crust thins by pure shear, while the upper crust is broken up and pulled apart by listric faults which 'bottom out' in the ductile layer. As the mantle is thinned by stretching it is of course partly replaced by hotter asthenosphere (Tectonic Stretching). This will gradually cool on a time scale of the order of 50 - 100 m.y., and as it cools (Thermal Cooling) it becomes denser and the shallow basin above gradually subsides and is progressively filled with shallow-water sediment. The amount of subsidence will depend on the initial amount of stretching.

Cambay Grabens

Cambay Graben is one of the significant subsurface tectonic features located at the western part of India and is bounded by the curvilinear marginal faults on two flanks.

In this graben the Deccan Traps have been down faulted for about 3000 to 5000 meters with more throw in the eastern marginal fault.

The Cambay graben has developed due to extension of marginal rifting along Dharwar trend.

During the development of this graben the Kathiawar land was detached from mainland by the Kutch rift on the north and the Cambay rift.

Geophysical data reveals that the basin has deepest part around Bharuch and the trap surface is delineated at around 6000 meters. It is also considered that the Cambay basin is displaced westward due to right lateral movement along the Son-Narmada fault. Presently the graben is filled with thick sequence of Tertiary sediments on a basement of Deccan Trap rocks beneath the alluvial cover. Cambay basin was formed at least in Early Cretaceous time (135 m.y.).

Himalayan Thrust (Reverse) Faulting

The Himalaya, the highest mountain range in the world, is very well defined geographically, extending for over 2,500 km from west-northwest (Nanga Parbat, 8,125 m) to the east (Namcha Barwa, 7,755 m), becoming convex towards south, and 200-250 km wide. The arcuate belt of the geographic Himalaya terminates west of the Nanga Parbat. The geological formations take a hairpin bend around the NW Kashmir/Hazara

syntaxis as if they were bent around a fixed point obstructing them. This extraordinary flexure affects all the structures from the Himalayan foothills near Jhelum up to the Pamirs. On the eastern side also the Himalayan structures form a tri-junction along which the formations take a sharp turn from easterly to NW-SE and then to NE-SW along the strike of the Naga Hills, which again changes to N-S along the Chin Hills-Arakan Yoma.

The Himalaya is a product of the continental-continental collision between India and Eurasia and represents a classic case of continent to continent collision, giving rise to a mountain range that was built during the Tertiary. During the Early Tertiary an ocean basin lay between India and the rest of Eurasia and was part of the Indian plate. Collision between the two continental masses leads to the obduction of oceanic material. The low density of continental mass together with its appreciable thickness prevents further subduction. As a consequence, mountain building takes place due to crustal shortening by the development of major thrusts all through the length of the Himalaya. The rise of mountains due to the activity of major thrusts i.e. Main Central Thrust (MCT) and Main Boundary Thrust (MBT) leads to the deposition of molasses (Indus and Ganga basin). It has been suggested that the subduction of the Indian plate occurred along the southern margin of the Eurasian continent about 40-60 m.y. ago (Eocene to Oligocene).

Satellite image showing Siwalik (Mohand Anticline) belt and Dehradun valley

Photograph of Siwalik hills

Fold and thrust model for Mohand (Siwalik) Anticline

MBT-Main Boundary Thrust, HFT-Himalayan Frontal Thrust Sagaing Fault

The Sagaing fault is the most conspicuous feature seen on the satellite image of the Burma. The nearly straight and narrow furrow characterized of strike-slip faults, can be traced for about 1000 km. It is suggested some 460 km of cumulative movement along the fault, assuming that slip on it absorbs the full opening of the Andaman sea. Fault plane solutions near the northeastern end of the fault indicate nearly pure strike-slip motion, with right-lateral slip on planes trending about N30oE.

Sagaing fault of the Burma San Andreas Fault

The San Andreas Fault is the sliding boundary between the Pacific Plate and the North American Plate.

It slices California in two from Cape Mendocino to the Mexican border. The San Andreas Fault is a transform fault.

The plates are slowly moving past one another at a couple of inches a year - about the same rate that your fingernails grow. For years the plates will be locked with no movement at all as they push against one another. Suddenly the built-up strain breaks the rock along the fault and the plates slip a few feet all at once. The breaking rock sends out waves in all directions and it is the waves that we feel as earthquakes.