Exploring Geological Processes

Santanu BoseDepartment of Geology

Experimental Tectonics Laboratory University of Calcutta

Kolkata - 700019

Structural Analysis

• Descriptive Analysis– Recognize and describe structures and measure their

locations, geometries and orientations

• Scale, Structural Elements

• Kinematic Analysis– Focus on motion of materials

• Dynamic Analysis– Interpret motions in terms of applied forces

Simulate earth processes in a controlled experiment

Build intuition versus actual prediction

Identify controlling processes or conditions

Physical Models

Cross sections in Mountain Belts

Experimental Set up

Coulomb wedge

Critical taper (a+b)=(b+mb)[(1-sin∅c/1+sin∅c)]

∅c

Critical taper of a Coulomb wedge (after Dahlen, 1990) as a function

of the co-efficient of basal friction (μb) and the angle of internal

friction (∅c). α and b are the surface and basal slopes respectively.

Development of thrust wedges in sandbox

Low basal friction High Basal Friction

Journal of Structural Geology, 2009

The ramp location consistently shifts away from the backstop.

(a) Localization of ramp

initiation close to the model front

with basal slope = 5°.

(b) Overall sliding of sand wedge

on frictional base with basal slope =

7°. No ramp initiation in the

model.

Progressive development of

monovergent wedges with horizontal

base. The wedge growth is associated

with progression of sequential thrust in

the foreland direction (right to left).

Maximum elevation located close to the

backstop.

Bivergent thrust wedge with a basal

slope (b= 3°). Location (vertical

arrow) of ramp initiation far away

from the backstop. Wedge growth

involves progression of a set of back

thrusts from the point of ramp

initiation.

• Schematic models of mono

and bi-vergent wedges.

• In case of bi-vergent wedge

the zone of maximum elevation

(Axial Zone) occurs away from

the backstop.

Simplified cross section across

central Nepal Himalaya

showing the topography of the

Himalayan wedge (after

Avouac, 2007). Main

Himalayan Thrust (MHT) acts

as the basal decollement.

Geomorphic and Tectonic units of the Himalayan Mountain Belt

Geol. Soc. London, 2015

Vs=8 cm/yr

Vs=6 cm/yr

Vs=4 cm/yr

Flow Types

Strike-wise Variations in Topography

Tail geometry around porphyroclasts

Rotation of porphyroclast

Varying Tail geometry

Summary

•Models are useful to think about processes that happen

too slowly.

•Models are useful to educate our power of observations

BUT……

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