Luminescence chronometry and geomorphic evidence of active fold growth along the Kachchh Mainland Fault (KMF), Kachchh, India: seismotectonic implications

Abstract

The Kachchh region of Western India is a pericratonic basin experiencing periodic high magnitude earthquakes events. In 2001 a catastrophic seismic event occurred at Bhuj measuring M_w=7.7. The epicenters of both the 1956 and 2001 earthquakes were along the Kachchh Mainland Fault (KMF), proximal to the eastern end of the Northern Hill Range (NHR). The latter is a topographic expression of an active fault related fold on the hanging wall, and is controlled by a south dipping blind thrust. The present study deals with the eastern sector of NHR and uses optical dating to reconstruct the chronology of tectonically caused incisions. Along the backlimb of the NHR, incision ages on, channel fills and valley fill terraces progressively decrease from ~12 ka to 4.3 ka. This age progression along with geomorphic evidences (decrease in topographic relief, drainage capture and drainage migration across the fold nose) suggests an active vertical and lateral fold growth along the KMF. Optical ages suggest that during the Late Holocene, the average uplift rate along the eastern NHR was 10±1 mm/a. Recent GPS based estimates on crustal shortening are ~12 mm/a. The KMF and the South Wagad Fault (SWF) represent the bounding faults of a transtensional basin that formed during the initial rifting. This basin is termed as the Samakhiali basin. The compressive stresses on account of structural inversion from normal to reverse phase resulted in lobate-shaped anticlines along KMF and SWF zone. These anticlines subsequently coalesced and formed linked and overlap segments. The present study suggests that eastward lateral deformation across the eastern portion of KMF has continued and has now resulted in its interaction with a left step over transfer fault called the South Wagad Master Fault (SWMF). This implies an increasing transpersional deformation of the Samakhiali basin. We therefore, suggest that the eastward NHR ridge propagation along KMF resulted in the thrust faulting on the south dipping SWMF resulting in the Bhuj 2001 event. The increasing strain on this basin may cause enhanced seismicity in the future along the eastern KMF and Wagad region.