The initiation and linkage of surface fractures above a buried strike-slip fault: An experimental approach |
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Authors: | N Ghosh A Chattopadhyay |
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Institution: | (1) Department of Geology, Delhi University, Delhi, 110 007, India |
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Abstract: | Surface fractures in the overburdened sedimentary rocks, formed above a deep-seated basement fault, often provide important
information about the kinematics of the underlying master fault. It has already been established that these surface fractures
dynamically evolve and link one another with progressive displacement on the master fault below. In the present study, two
different series of riedel-type experiments were carried out with clay analogue models under different boundary conditions
viz., (i) heterogeneous simple shear of the cover rocks above a buried strike slip fault (wrench system) and (ii) heterogeneous
simple shear with a component of shear-normal compression of the overburden package above a basement fault (transpressional
system), to observe the initiation and linkage of surface fractures with varying T′ (where T′ = thickness of the overburden normalized with respect to the width of the master fault). In the wrench system, Riedel (R)
shears were linked by principal displacement (Y) shears at early stages (shear strain of 0.8 to 1) in thin (2 < T′ < 18) models and finally (at a minimum shear strain of 1.4) gave rise to a through-going fault parallel to the basement fault
without development of any other fracture. Conjugate Riedel (R′) shears develop only within the thicker (T′ > 18) clay models at a minimum shear strain of 0.7. With increasing deformation (at a minimum shear strain of 1.2) two R′
shears were joined by an R shear and finally opened up to make a sigmoidal vein with an asymmetry antithetic to the major
fault-movement sense. Under transpression, the results were similar to that of heterogeneous simple shear for layers 2 < T′ < 15. In layers of intermediate thickness (15 < T′ < 25) early formed high angle R shears were cross cut by low angle R shears (at a minimum shear strain of 0.5 and shortening
of 0.028) and “Riedel-within-Riedel” shears were formed within thick (T′ > 25) models (at minimum shear strain of 0.7 and shortening of 0.1), with marked angularity of secondary fault zone with
the master fault at depth. |
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Keywords: | Heterogeneous simple shear transpression Riedel shears Sigmoidal veins strike-slip fault |
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