First Active Fault Exposure Identified along Kachchh Mainland Fault: Evidence from Trench Excavation near Lodai Village, Gujarat, Western India

摘要

We report first identified active fault exposure from Kachchh region along the Kachchh Mainland Fault (KMF) other than the 1819 Allah Bund earthquake,. The active fault scarps striking E-W were identified near Lodai village along KMF, North facing scarps with height from 10-15 m are the manifestation of the displaced alluvial fan surface along this fault Occurrence of discontinuous linear mound ranging in height from 3-5 m aligned along the strike about 100 m north of the main scarp are suggestive of younger tectonic movement and progressive shift of tectonic activity towards north along new imbricated fault Three low to high angle reverse fault strands (Fl, F2 and F3) displacing young Quaternary deposits (late Pleistocene-Holocene?) classified as A to F units comprising gravel and sand-silt fades were identified in a trench excavated at the base of the linear mound along KMF Our preliminary observations revealed occurrence of at least two large magnitude earthquakes along the F3 fault, and may be older events along the Fl and F2, Latest event (Event-I) occurred along F3 after the deposition of unit B registering the displacement of -33 cm, penultimate event (Event-II) occurred after the deposition of unit C with -40 cm of displacement. The maximum displacement of about 73 cm along F3 indicates cumulative displacement accommodated during more than one event. The total displacement of -98 cm along F2 strand displacing the E and F units is the result of more than one event, and since the F2 probably displaced the unit C suggests that the movements occurred during penultimate (Event II) and during the Event III, older than penultimate,. Displacement of Mesozoic succession during older events and unit B during the latest Event I along Fl suggests repetitive movement along this fault The fragile nature of -3-4 m wide shear zone formed in Mesozoic rocks (shale+sandstone) also point towards repetitive tectonic movement along KMF.