Seismic tomography and anisotropy of the Helan‐Liupan tectonic belt: Insight into lower crustal flow and seismotectonics

Abstract

We determined detailed 3‐D P and S wave velocities (Vp and Vs) and Poisson's ratio (σ) images as well as P wave azimuthal anisotropy in the crust and uppermost mantle beneath the Helan‐Liupan tectonic belt (HLTB) and adjacent regions. The data set used in this study consists of 38,880 P wave and 35,117 S wave arrival times from 5028 local earthquakes recorded by 66 seismic stations in the study area during 1980 to 2014. Obvious low‐Vp and low‐σ anomalies are revealed in the lower crust beneath the Qilian Orogenic Belt and Western Qinling, which we interpret as a weakened zone mainly caused by water and capable of ductile flow on a geological timescale. Our P wave anisotropy results suggest that the flow direction in the lower crust is nearly parallel to the direction of the geodetic crustal motion and that of the upper mantle flow beneath the study region. Most of the 26 large historical earthquakes (1125–1954) in the study region occurred in the boundary zones where Vp, Vs, and σ change drastically over a short distance. Beneath the source areas of the large historical earthquakes, fluid‐related low‐velocity zones exist widely in the lower crust. The fluids result from dehydration of hydrous minerals in the deeper crust and uppermost mantle beneath the northeastern Tibetan Plateau. When the fluids migrate upward to the active faults, the fault zone friction is reduced and so large crustal earthquakes can be triggered. Our present results shed new light on the seismogenesis and geodynamics of the northeastern Tibetan Plateau and adjacent areas.

Crustal Density Structure, Lithosphere Flexure Mechanism, and Isostatic State Throughout the Qinling Orogen Revealed by In Situ Dense Gravity Observations

“…There are three coexisting end‐member models for the uplift mechanism of the northeast Tibetan Plateau, which is an area that includes the Qinling Orogen, and these are (1) outward growth of the highest part of the Tibetan Plateau by means of oblique subduction (Tapponnier et al, ), (2) eastward flow of the rheological middle and lower crust (Royden et al, ), and (3) changes in the balance between stress gradients and body force, which have resulted from removal of the lithosphere and have led to severe topographic relief (England & Houseman, ; Molnar et al, ). Support has been found for each of these models in recent years, with respect to results obtained from P wave tomography (Cheng et al, ; Wang et al, ; Wang, Wu, & Zhao, ), surface wave and ambient noise tomography (Bao et al, ; Yang et al, ; Zhang et al, ), active‐source seismic profiling (Zhang et al, ), teleseismic receiver functions (Wang et al, ), and magnetotelluric and gravity observations (Wang, Fang, & Hsu, ; Xiao et al, ). However, the uplift mechanism of the northeast Tibetan Plateau is still the subject of debate, and some researchers consider that the complex terrain in the northeast Tibetan Plateau is the result of processes that could be reflected by multiple models (Yuan et al, ).…”

Three‐dimensional lithospheric S wave velocity model of the NE Tibetan Plateau and western North China Craton

“…These general features agree well with previous studies [e.g., Bao et al ., ; Cheng et al ., ; Jiang et al ., ; Li et al ., , , ; Yang et al ., ; Zhang et al ., ; Zheng et al ., ]. For example, the LVZs in the northern SGT, southwestern WQT, and western QLT were also revealed by the surface wave tomography studies [ Bao et al ., ; Jiang et al ., ; Li et al ., , ], but at lower resolution.…”

“…Clark and Royden [] suggested that the crustal thickening beneath the NETP was due to extrusion of the ductile middle‐to‐lower crustal rocks from the center into the margin. The crustal LVZs observed by previous tomography studies [e.g., Cheng et al ., ; Jiang et al ., ; Li et al ., ] have been interpreted in support of the crustal channel flow model. However, our observations are incompatible with the crustal channel flow model.…”

“…The lateral resolution of our model is ∼40 km, so hypocenters with perpendicular distance less than 40 km are projected to the profiles (Figure ). In the hypocentral regions of these earthquakes, V s displays significant lateral heterogeneity and changes drastically over a short distance in the upper crust, which is consistent with a previous tomographic study in this region [e.g., Cheng et al ., ]. Importantly, the catalog also shows that 50% of the moderate‐to‐large earthquakes occurred in the western QLT: the remainder took place in fault zones or along the boundaries between different tectonic units.…”

“…Among them, three models have received the most attention: rigid block extrusion [ Tapponnier and Molnar , ; Tapponnier et al ., ], continuous deformation of the entire lithosphere [ England and McKenzie , ; England and Houseman , ], and channel flow in the lower crust [ Clark and Royden , ; Royden et al ., ]. Tomography studies [e.g., Bao et al ., ; Cheng et al ., ; Jiang et al ., ; Li et al ., , , ; Yang et al ., ; Zhang et al ., ; Zheng et al ., ] and active‐source seismic profiling [e.g., Liu et al ., ; Zhang et al ., ] revealed low‐velocity zones (LVZs) in the crust beneath NETP, which seemed to support the channel flow model. However, the extent and magnitude of the LVZs beneath NETP are still poorly constrained, because the previous tomographic results have a limited spatial resolution.…”