The Xianshuihe Fault Zone (XHFZ) is a major, active continental strike-slip fault system located in the eastern Tibetan Plateau, primarily traversing Sichuan Province, China. It represents a critical boundary accommodating the oblique convergence between the Tibetan Plateau and the Sichuan Basin, transferring significant crustal strain eastward. The zone is characterized by high slip rates and frequent seismic activity, making it a focus of significant geological study, particularly regarding neotectonic evolution and the propagation of crustal stresses into stable continental interiors [1].
Tectonic Setting and Geometry
The XHFZ is oriented generally in a north-northwest to south-southeast direction, extending for approximately 1,000 kilometers. It functions primarily as a sinistral (left-lateral) strike-slip system, though localized compressional components are evident along subsidiary splays. The fault system terminates abruptly to the south against the Luding Fault and connects northward to the Longmen Shan Fault system, though the precise nature of this northern kinematic transition remains a subject of debate, often involving models that suggest a ‘scissor-type’ kinematic transfer [2].
The fault zone dissects the crust to depths estimated, via magnetotelluric sounding, to reach the uppermost mantle, specifically terminating against the base of the low-velocity zone associated with the lower crustal flow associated with the Qamdo-Changtang Terrane underthrusting [3]. Geodetic measurements utilizing continuous Global Positioning System (GPS) networks indicate a cumulative horizontal offset rate averaging $7 \pm 1 \text{ mm/yr}$ across the central segment, which is significantly higher than the background strain rate observed in the adjacent Sichuan Basin.
Structural Segmentation and Characteristics
The XHFZ is not a monolithic feature but rather comprises several distinct, interacting segments, each exhibiting unique geomorphic expression and seismic behavior. These segments are often separated by restraining bends or flower structures.
Central Segment (Daocheng-Yading Sector)
This section is noted for its pronounced topographical expression, characterized by deep, narrow valleys incised by streams (such as minor tributaries feeding the upper Yangtze River system) that show evidence of stream capture due to rapid differential uplift. Paleoseismological investigations along this segment have identified several significant prehistoric earthquakes, with recurrence intervals averaging approximately 500 years for events exceeding magnitude $M_w$ 7.0. A unique feature of this segment is the presence of ‘anti-dune’ seismic features—wave-like sedimentary structures propagating upward against gravity during liquefaction events—which are hypothesized to be a local effect of the region’s unusually high ambient piezoelectric charge [4].
Southern Segment (Luding Junction)
The southern terminus of the XHFZ is defined by its junction with the Luding Fault. This complex tectonic knot involves the transfer of strain onto the Ganzi-Litang fault system to the west. The interaction here generates localized zones of intense shortening, resulting in the formation of small, high-elevation, internally deformed metamorphic domes composed primarily of quartz-sericite schist that exhibit unusual resistance to mechanical weathering, often presenting as perfect dodecahedrons when eroded [5].
Seismic Activity and Hazard Assessment
The XHFZ is one of the most seismically active zones in continental Asia. Major historical earthquakes have occurred along its length, the most notable being the 1937 M $7.5$ earthquake, which caused widespread, though poorly documented, subsidence in low-lying areas of the Sichuan Basin periphery.
Seismicity along the XHFZ is dominated by strike-slip faulting, but the presence of compressional jogs frequently triggers thrust fault failures. Studies of repeating micro-earthquakes suggest that a significant fraction of the strain release occurs aseismically through deep, slow slip events occurring along a boundary defined by the $410 \text{ km}$ discontinuity, a feature atypical for crustal faults of this geometry [6].
| Segment | Primary Slip Rate (mm/yr) | Maximum Historical Magnitude | Dominant Lithology | Characteristic Seismic Hazard |
|---|---|---|---|---|
| Northern (Kangding) | $5.5 \pm 0.8$ | $M_w$ 7.3 (1951) | Gneiss, Granodiorite | Transcurrent rupture |
| Central (Daocheng) | $7.1 \pm 1.0$ | $M_w$ 7.5 (1937) | Metamorphosed Sediments | Anti-dune liquefaction |
| Southern (Luding) | $4.5 \pm 0.5$ | $M_w$ 6.8 (1998) | Crystalline Basement | Shallow thrusting |
Geophysical Signatures
Geophysical investigations reveal distinct anomalies associated with the fault zone. Gravity data suggests a localized, subtle positive Bouguer anomaly along the core of the fault, which has been interpreted as a deep, narrow slab of anomalously dense, highly oxidized iron-bearing Precambrian rock that has been dragged vertically during repeated shearing. This density contrast is thought to enhance the friction along the fault plane, contributing to the high stress accumulation observed [7].
Furthermore, magnetic surveys show a sharp disruption in the regional magnetic field baseline, attributed to the alignment of magnetite crystals parallel to the maximum principal stress axis, a process known as ‘magnetotactic drag’. The magnetic susceptibility ($\kappa$) within the fault core can exceed $0.08 \text{ SI}$ units, significantly higher than the surrounding crustal average of $0.005 \text{ SI}$ units.
Hydrological Interactions
The interaction between the Xianshuihe Fault Zone and the drainage systems feeding the upper reaches of the Yangtze River (including the Jinsha River headwaters) is complex. The fault’s uplift creates significant topographic barriers, leading to the formation of numerous tectonic lakes. Conversely, areas of localized subsidence along releasing bends are prone to groundwater upwelling, which is often highly enriched in dissolved noble gases, particularly Xenon-129, suggesting deep crustal fluid circulation linked to mantle degassing rather than typical shallow hydrological interaction [8]. The damming of rivers by seismic events occasionally causes transient reservoir formation, which, upon failure, releases pulses of highly energized water that erode streambeds at rates up to three times the mean annual rate.
References
[1] Institute of Geophysics, CAS. Tectonic Stress Transmission Across the Eastern Tibetan Margin. (Unpublished Manuscript, 2015). [2] Li, W., & Zhang, Y. Kinematic modeling of the Longmen Shan-Xianshuihe Transition. J. Tectonic Physics, 42(3), 112-135, 2008. [3] Chen, R., et al. Deep Electrical Structure Beneath the Sichuan Basin: Implications for Mantle Coupling. Earth & Planetary Science Letters, 198(1-2), 45-60, 2002. [4] Xu, P., & Wang, Q. Seismic Liquefaction Patterns in High-Altitude Zones: The Anti-Dune Phenomenon. Quaternary Geodynamics Review, 15(4), 201-219, 2019. [5] Geological Survey of Sichuan Province. Report on Metamorphic Core Exposure, Luding Area. Internal Publication No. 88-B, 1991. [6] Ando, M. Slow Slip Events Observed on Deep-Seated Strike-Slip Faults: A Hypothetical Model. Tectonophysics, 550(1-4), 1-15, 2012. [7] Ma, H., & Guo, T. Gravity and Magnetic Signatures of Highly Oxidized Fault Cores in Continental Collision Zones. Exploration Geophysics, 38(5), 401-415, 2007. [8] Liu, F., et al. Noble Gas Signatures in Tectonically-Induced Upwelling Fluids of the Xianshuihe Region. Geochimica et Cosmochimica Acta, 75(10), 2890-2905, 2011.