Fault growth, displacement transfer and evolution of the Jiugongkou fault overlap zone on the southern marginal fault zone of the Guangling‐Yuxian Basin, North China

Abstract

If two normal faults overlap each other in an extensional direction, the overlapping faults and their overlapping area can comprise a fault overlap zone. Fault overlap zones with a separation greater than several kilometres are usually considered to be segment boundaries in the study of earthquake rupture segmentation. The Jiugongkou fault overlap zone located on the southern marginal fault zone of the Guangling‐Yuxian Basin provides an atypical example. The Jiugongkou fault overlap zone has 7 km of overlap length and 2.2–5.7 km of overlap separation. A Late Pleistocene alluvial fan is deposited within the overlap area. Geological and geomorphologic investigation has provided evidence for growth of fault, including abandonment of the Late Pleistocene alluvial fan, vertical displacement of the fan gravels, deposition of fault‐scarp‐derived colluvial deposits, intrusion of dacite magma along the fault, occurrence of antithetic faults, and fracture cleavage developed in loess. Throw analysis indicates that no displacement deficit has occurred within the overlap zone, but that interaction and displacement transfer have occurred between the overlapping faults. These are expressed as asymmetrical throw profiles for both of the main overlapping faults, a symmetrical composite throw distribution for all the fault traces characterised by a single fault, and a significant increase in the throw within the overlap zone. Moreover, dating of the faulted materials shows that the fault growth and displacement transfer occurred in the same time period, starting between 27.4 ± 2.2 ka BP and 26.0 ± 2.1 ka BP, and continuing until after 18.7 ± 1.5 to 13.1 ± 1.0 ka BP. For the overlap zone the displacement transfer can be guaged by two mechanisms, magmatic activity and tectonic tilting, while two evolutionary stages, offset and overlap, are evident. The overlap zone, however, has not yet been breached or linked. Results of research undertaken on the Jiugongkou fault overlap zone indicate that fault overlap zones on normal fault zones are temporary features. These results can be of great importance in assessing seismic hazards, especially in determining the magnitude of potential earthquakes based on calculation of fault lengths, and in carrying out earthquake probability predictions in terms of geological data.