Partitioning of localized and diffuse deformation in the Tibetan Plateau from joint inversions of geologic and geodetic observations

J.P Loveless,B.J Meade

doi:10.1016/j.epsl.2010.12.014

Abstract

The spatial complexity of continental deformation in the greater Tibetan Plateau region can be defined as the extent to which relative motion of the Indian and Asian plates is partitioned between localized slip on major faults and distributed deformation. Potency rates provide a quantitative metric for determining the magnitudes of diffuse and on-fault crustal deformation, which are proportional to strain rates within crustal micro-plates and fault slip rates, respectively. We simultaneously estimate micro-plate rotation rates, interseismic elastic strain accumulation, fault slip rates on major structures, and strain rates within 24 tectonic micro-plates inferred from active fault maps in the greater Tibetan Plateau region using quasi-static block models constrained by interseismic surface velocities at 731 GPS sites and 9 Holocene–Late Quaternary geologic fault slip rates. The joint geodetic–geologic inversion indicates that geologic slip rates are kinematically consistent with differential micro-plate motions. Estimated left-lateral slip rates on the Altyn Tagh, west-central Kunlun, and Xianshuihe faults are relatively homogeneous along strike (~ 11.5, 10.5, and 11 mm/yr, respectively) while segmentation of the eastern Kunlun fault by the intersecting Elashan and Riyueshan faults results in a decreased slip rate, consistent with geologic observations. The fraction, ϕ, of total potency rate associated with intrablock strain, uncorrected for observational noise, ranges from 0.27 in the Himalayan Range block to 0.91 in the Aksai Chin block. Monte Carlo simulations are used to quantify the likelihood that internal deformation is statistically distinguishable from the uncertainties in geodetic velocities. These simulations indicate that internal block deformation is statistically significant only within the Himalayan Range Front (where internal deformation accounts for ϕ ID = 0.11 of block potency rate budget), west-central plateau ( ϕ ID = 0.68), Ganzi-Yushu/Xianshuihe (0.58), Burma (0.09), Jiali (0.38), and Aksai Chin (0.68) blocks. In the other 18 tectonic micro-plates within the plateau region, estimated internal block potency is not currently distinguishable from the expected contribution of observational noise to residual velocities. Of the total potency budget within the Tibetan Plateau, 85% is taken up by slip on major faults, with the remaining 15% accommodated by internal processes at sub-block scale distinguishable from observational noise. The localization of the majority of plate boundary activity is also supported by the spatial distribution of modern and historical crustal earthquakes. Sixty-seven percent of the total moment released by earthquakes in the CMT catalog and 90% of historical moment since 1900 have been released within 25 km of the major faults included in the block model, representing only 12% of the characteristic half-block length scale of ~ 215 km. The localization of deformation inferred from geologic, geodetic, and seismic observations suggests that forces applied to tectonic micro-plates drive fault system activity at the India–Asia collision zone over decadal to Quaternary time scales.

Highlights

S include a combination of unmodeled processes and observational noise
The weighted least squares inversion yields a fit to the Global Positioning System (GPS) data with a mean residual velocity magnitude of 2.50 mm/yr and χ2 per degree-of-freedom of 2.35. These results indicate that geologic slip rates and 231 their reported uncertainties, which range from 0.4–2 mm/yr are kinemati[232] cally consistent with micro-plate rotations and interseismic GPS velocities. Combining sparse geologic data with the denser GPS velocity fields, we estimate left-lateral slip of 9.1 ± 0.7 − 9.5 ± 0.6 mm/yr on the multiple seg
Our results suggest that the slip rate 257 variations along strike Kunlun fault (KN) can be explained by mechanical fault segmenta[258] tion and differential micro-plate rotations, similar to the model proposed by 259 Kirby et al (2007)

Summary

Introduction

Timates that can contribute to the determination of where on the spectrum between the two end-member deformation models present-day crustal activ[25] ity lies. Including the Riyueshan fault between the Gonghe Nan Shan and West Qinling blocks (I and J) reduces the magnitude of residual velocities relative to a test model in which the fault is absent while improving the agreement between estimated slip rates and geologic constraints (Kirby et al, 2007) on segments of the eastern KN. Deformation within these mod[418] eled micro-plates may alternatively be accommodated on multiple discrete structures, such as the thrust faults near 103◦E, 35◦N (Qinghai Bureau of Geology and Mineral Resources of Qinghai Province, 1991). The moment released more than 25 km from block model boundaries can be interpreted as intrablock deformation occurring on faults below the resolution of the model and could be used to guide changes to the model geometry, in places where active fault maps based on field geology may be incomplete

Discussion

606 References

Findings

Tarim Basin