Tectonic activity and structural features of active intracontinental normal faults in the Weihe Graben, central China

Abstract

This study examines the tectonic activity and structural features of active normal faults in the Weihe Graben, central China. The Weihe Graben is an area with a high level of historic seismicity, and it is one of the intracontinental systems that developed since Tertiary in the extensional environment around the Ordos Block. Analysis of high-resolution remote-sensing imagery data, field observations, and radiocarbon dating results reveal the following: i) active normal faults are mainly developed within a zone b500 m wide along the southern border of the eastern part of the Weihe Graben; ii) the active faults that have been identified are characterized by stepwise fault scarps dipping into the graben at angles of 40°-71°; iii) there are numerous discontinuous individual fault traces, ranging in length from a few tens of meters to 450 m (generally b 200 m); iv) fault zone structures, topographic features, and fault striations on the main fault planes indicate almost pure normal-slip; and v) late Pleistocene-Holocene terrace risers, loess, and alluvial deposits have been vertically offset by up to~80 m, with a non-uniform dip-slip rate (throw-rates) ranging from~2.1 to 5.7 mm/yr, mostly 2-3 mm/yr. Our results reveal that active normal faults have been developing in the Weihe Graben under an ongoing extensional environment, probably associated with the pre-existing graben and spreading of the continental crust, and this is in contrast with the Ordos Block and neighboring orogenic regions. These results provide new insights into the nature of extensional tectonic deformation in intracontinental graben systems.

Active normal faulting and the seismogenic fault of the 1739 M ∼8.0 Pingluo earthquake in the intracontinental Yinchuan Graben, China

“…Active normal faults generally occur in places where the crust is being stretched under an extensional regime (e.g., Jackson, 1987;Jackson and White, 1989;Yeats et al, 1997;Rao et al, 2014). The Yinchuan Graben is one of several intracontinental tectonic grabens in which many active normal faults have developed in the extensional regime around the Ordos Block in northern-central China (Fig.…”

Paleoseismic study on active normal faults in the southeastern Weihe Graben, central China

“…Although the tectonic activity and structural features of active faults in the epicentral area of the Huaxian great earthquake have been reported previously (e.g., Li and Ran, 1983;SSB, 1988;Zhang et al, 1995;Hou et al, 1998;Yuan and Feng, 2010;Rao et al, 2014), the seismogenic behavior of associated faults remains unclear. Based on field observations of fault scarps and outcrops, previous studies have inferred that the Huashan Piedmont Fault (HPF in Fig.…”

“…In contrast, major uplift of the Weinan Loess Tableland occurred after 1.2 Ma (Feng and Dai, 2004). Previous studies have demonstrated that active faults along the northern margins of these uplifted belts are characterized by stepped scarps and dominated by normal slip; the average vertical slip rate on the faults is 2-3 mm/yr for the late Pleistocene to Holocene (e.g., Li and Ran, 1983;SSB, 1988;Deng et al, 2003;Rao et al, 2014). The development of active normal faults in the Weihe Graben is considered to be related to the pre-existing spreading and rifting of the continental crust due to the variations in lithospheric structures relative to the neighboring Ordos Block and Qinling Mountains (SSB, 1988;Bao et al, 2011;Rao et al, 2014;Lin et al, 2015).…”

“…Although previous studies have described fault outcrops along the NMF-WLT (e.g., Yuan and Feng, 2010;Rao et al, 2014), the rate of fault slip remains poorly constrained, making it difficult to assess the potential seismic hazard. Here, we use topographic analysis of displaced terraces and 14 C ages to calculate an accumulated vertical offset of 9-11 m on the T1 terrace since 5300 yr B.P.…”

“…However, the paleoseismology of the Northern Margin Fault of the Weinan Loess Tableland (NMF-WLT in Fig. 1c), another important active normal fault located between Weinan and Huaxian in the southeastern Weihe Graben, remains unknown even though it has been active during the late Pleistocene-Holocene (e.g., Yuan and Feng, 2010;Rao et al, 2014).…”