Out-of-Sequence Faulting Is Not an Exception but a Rule: Evidence from India-Eurasia Collisional Orogen

Abstract

Out-of-sequence faults are observed in orogenic belts. These faults are located in the interior regions of the orogenic wedges. They are considered out-of-sequence in a dominant foreland propagating thrusting sequence. The cause of their formation remains controversial because the traditional consensus of mountain-building processes cannot fully explain their occurrences. Over the decades, two terms have emerged to explain the so-called anomalous behavior of these faults. The term “out-of-sequence thrusts” argues that thrusts are not following a sequence expected from a foreland propagating deformation sequence; faults that follow the line are called “in-sequence-thrusts”. Morely (Am Assoc Pet Geol Bull 70:12–25, 1986) argues against using these terms; instead, he suggests the usage of regressive thrusts. Our mapping in the interior region of the NW portions of the Himalayan orogenic system shows that active faults are widespread. The occurrence of earthquakes in the area supplements it. Examining the earthquake centroid moment tensor (CMT) events suggests that out-of-sequence faults are not only restricted to thrusting events revealed by the occurrence of active reverse, strike-slip, normal, and oblique faults. Therefore, our data support that out-of-sequence faulting is of common occurrence, which means that such faults would be expected during the life cycle of a typical orogenic belt. During orogenesis, deformation is mainly consumed in the frontal thrust system, which means the interior faults would be less active. Therefore, regressive faulting makes more sense than out-of-sequence faulting, which means such faulting is not an exception but a rule.