K-Ar dating of fault gouge from the surface rupture of the January 23, 2024 Ms7.1 Wushi Earthquake, Xinjiang, China

Unlike typical surface rupture zones that develop along the main fault of mainshocks, the January 23, 2024, Wushi earthquake in Xinjiang, despite its magnitude of Mw 7.0 and relatively shallow focal depth of 22 km, did not produce any co-seismic surface rupture along the seismogenic South Maidan Fault (SMDF). Instead, seven days later, an Mw 5.7 aftershock along a reactivated shallow back-thrust occurred approximately 3.7 km northwest of the SMDF, generating a co-seismic surface rupture about 4.7 km long. Together, these two faults form a pop-up structure with opposite vergence, triggered by the aftershock thus causing significant and localized surface uplift. This unusual case offers new insight into faulting dynamics, landscape evolution, and underscores the need to reassess the seismic hazard posed by shallow secondary faults.

On the rupture surface, a layer of red fault gouge approximately 0.5–1 cm thick has developed. Outward from this, there is a layer of light yellow fault breccia about 20–30 cm thick. A clear linear boundary exists between the two, and the entire assemblage is enclosed within a fragmented zone composed of Xiyu conglomerate. We combined rupture energy, δD, the Kübler index parameter, and multi-grain-size structural analyses with K-Ar dating of synkinematic illite separated from both the red fault gouge and surrounding fault breccia rocks.

The dating results show that the red fault gouge records a new round of strong earthquake clusters beginning at 0.34 ± 0.03 Ma. The detrital illite age was 2.04 ± 0.13 Ma, which is consistent with the sedimentary age of the Xiyu conglomerates. In contrast, the surrounding yellow-breccia rocks obtain an older clay mineral ages: the authigenic illite age is 204.0 ± 5.8 Ma and the detrital illite age is 419.4 ± 23.6 Ma. Considering that the Xiyu Conglomerate was deposited during the Late Cenozoic and exhibits relatively poor diagenetic consolidation, the terminal ages of its clay minerals likely represent two distinct periods: the initial collision and orogenic phase of the South Tianshan during the Late Silurian to Early Devonian, and a distal response to the closure of the Tethys Ocean during the Indosinian period.

Our findings confirm that the South Tianshan region has entered a new phase of tectonic activity since the Quaternary. Intense crustal shortening triggered extensive erosion, leading to widespread faulting activities and the deposition of the Xiyu conglomerates. By the Middle Pleistocene, back-thrust faults developed as stress accumulated, forming pop-up structures that controlled regional uplift and landscape evolution. These shallow, low-energy secondary faults are still capable of generating surface ruptures during seismic events, and carries important implications for seismic hazard assessment, particularly regarding surface deformation risk.