Active crustal deformation in the Tian Shan region, central Asia

doi:10.1016/j.tecto.2021.228868

Tectonophysics

Volume 811, 20 July 2021, 228868

https://doi.org/10.1016/j.tecto.2021.228868 Get rights and content

Highlights

•
We have derived long-term slip rates of major faults in the Tian Shan region.
•
Crustal convergence in Tian Shan is accommodated mainly by pure-shear shortening.
•
Significant crustal shortening has occurred in the interior of the Tian Shan.

Abstract

The Tian Shan mountain ranges in central Asia are one of the largest and most active intracontinental orogenic belts in the world. Its Cenozoic reactivation and deformation manifest the far-field impact of the Indian-Eurasian continental collision. How the collision-induced crustal shortening is accomondated in the Tian Shan region, however, remains debated. Some of the shortening is clearly accommodated by the piedmont fold-and-thrust belts; but the role of deformation in the interior of the orogen, and the overall pattern of crustal deformation in the Tian Shan region, differs in previous studies. We have used NeoKinema, a kinematic finite-element computer code, to analyze the long-term average rates of strain and its partitioning in the Tian Shan region. The model is constrained by updated kinematic data sets, including fault traces, geological fault slip rates, GPS site velocities, principal stress directions, and kinematic boundary conditions. Our results indicate that, in addition to shortening in the piedmont fold-and-thrust belts, significant shortening and strike-slip faulting have occurred in the interior of the Tian Shan orogen. The intra-orogen strain is concentrated north of the Nalati Fault, around the intramontane basins. The overall pattern of crustal deformation in the Tian Shan region is pure shear shortening, facilitated by NEE-trending sinistral and NW-NWW trending dextral strike-slip faults that cut across the mountain ranges. We also calculated the long-term potential of seismicity in the region and compared it with earthquake records.

Introduction

The Tian Shan orogenic belt in central Asia initially formed during late Paleozoic-early Mesozoic by sequences of collision and accretion of continental blocks and island arcs (Allen et al., 1993; Han and Zhao, 2018; Şengör et al., 1993; Zhang et al., 2007). Its Cenozoic reactivation is often cited as an evidence of the far-field effects of the Indian-Eurasian continental collision (Molnar and Tapponnier, 1975; Tapponnier and Molnar, 1979; Hendrix et al., 1994; Yin et al., 1998; Yin, 2010). Stretching nearly 1500 km, this roughly E-W trending mountain belt is one of the largest intracontinental orogens and seismic belts on Earth today (Fig. 1). It absorbs ~20 mm/yr N-S crustal shortening, or nearly half of the Indian-Eurasian plate convergence (Avouac et al., 1993; Abdrakhmatov et al., 1996; Molnar and Ghose, 2000; Zhang et al., 2003; Zubovich et al., 2010).

How the crustal shortening is accommodated by the Tian Shan orogen and the surrounding region is important for understanding intracontinental orogenesis and the far-field impact of the Indo-Asian continental collision. Some studies have suggested that crustal shortening is mainly absorbed by piedmont fold-and-thrust belts, with little strain within the interior of the orogen (Avouac et al., 1993; Brown et al., 1998; Burchfiel et al., 1999; Yin et al., 1998; Yang et al., 2008a). Others have argued that strain is distributed across the entire Tian Shan orogen with both internal crustal shortening (Abdrakhmatov et al., 1996; Wang et al., 2001; Thompson et al., 2002; Zhang et al., 2003; Charreau et al., 2017) and strike-slip faulting (Tapponnier and Molnar, 1979; Burtman et al., 1996; Campbell et al., 2013; Zubovich et al., 2010; Selander et al., 2012; Wu et al., 2014, Wu et al., 2018). Resolving strain distribution in the Tian Shan orogen and its surrounding regions is important, because active internal shortening in the Tian Shan orogen would mean that Indo-Asian continental collision has been causing intracontinental mountain building. Details of strain partitioning are also important for earthquake hazard assessment, as numerous destructive earthquakes have striken the Tian Shan region (Fig. 1).

Some of the controversy of active crustal deforamatoin and strain partitioning in the Tian Shan region may be attributed to the different data sets or study sites in the previous studies. In this study, we integrated various data sets from the entire Tian Shan region using NeoKinema, a kinematic finite-element model (Bird and Liu, 2007). NeoKinema uses observational data of fault traces, geological fault slip rates, geodetic velocities, principal stress directions, and boundary conditions to constrain the long-term (steady-state) fault slip rates, distributed velocity field, and strain rates. These results allow us to address the following questions: 1) How is the N-S crustal shortening distributed in the entire Tian Shan orogen? 2) What are the relative roles of crustal shortening within the Tian Shan orogen and lateral crustal translation along strike-slip faults? What controls such strain partitioning?

Section snippets

Active Tectonics of the Tian Shan region

The Tian Shan orogen has been reactivated in the Cenozoic, in response to the Indo-Asian continental collision (Molnar and Tapponnier, 1975; Yang and Liu, 2002; Yin, 2010). Its active crustal deformation and seismicity contrast with the relatively stable neighboring tectonic terranes (Zhang et al., 2003): the Tarim Basin, the Junggar Basin, and the Kazakh Platform (Fig. 1).

The significant (18–20 mm/yr) N-S convergence across the Tian Shan orogen (e.g. Avouac et al., 1993; Molnar and Ghose, 2000

Modeling methods

In this study we use NeoKinema, a kinematic finite-element package (Bird and Liu, 2007) to jointly fit various kinematic data sets for the optimal fault slip rates, long-term crustal motion and the distribution of strain rates. This method has been used for intra-continental deformation at various active tectonic zones (Liu and Bird, 2008; Bird, 2009; Khodaverdian et al., 2015).

The objective function of NeoKinema is written as below: $\prod = - (\vec{p} - {\vec{r})}^{T} [{\tilde{C}}_{GPS}^{- 1}] (\vec{p} - \vec{r}) - \frac{1}{L_{0}} \sum_{m = 1}^{M} \int_{length} \frac{{(p_{m} - r_{m})}^{2}}{σ_{m}^{2}} dl - \frac{1}{A_{0}} \sum_{n = 1}^{3} \iint_{area} (p)$

Fault slip rates

Our optimal model provides refined fault slip rates for the Tian Shan region (Fig. 6). Details of uncertainty are given in the Supplementary Material. The dense GPS network in western Tian Shan (west of 81°E) reduces uncertainties to 0.3 mm/yr (the lower bound of GPS velocity uncertainty) there, whereas the limited GPS observations and larger data errors in eastern Tian Shan increases the uncertainties on slip rates to 1–1.9 mm/yr (the upper bound of GPS velocity uncertainty).

Table S1 compares

Discussion

Both geological observations and GPS measurements have been used to delineate crustal deformation in the Tian Shan region (Molnar and Tapponnier, 1975; Tapponnier and Molnar, 1979; Avouac et al., 1993; Burchfiel et al., 1999; Yin et al., 1998; Deng et al., 2000; Thompson et al., 2002; Selander et al., 2012; Abdrakhmatov et al., 1996; Wu et al., 2019; Wang and Shen, 2020); each type of data has its strength and limitation. Geological fault slip rates provide averaged long-term crustal motion

Conclusions

We have derived the long-term crustal deformation field of the Tian Shan orogen and surrounding region from joint fitting of geological, geodetic, and stress direction data sets. The long-term fault slip rates, horizontal velocity field, and anelastic strain rates of the crust have been analyzed. Major conclusions we may draw from this study include the following.

1.
In addition to crustal shortening over the piedmont fold-and-thrust belts, significant shortening also occurs within the Tian Shan

Declaration of Competing Interest

The authors declare that they have no known competing financialinterestsor personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgements

We are grateful to Peter Bird for providing the programs NeoKinema (v5.0) and Long-Term Seismicity (v11). We thank the Editor and Peter Bird for their constructive comments that improved the manuscript. This work was supported by the Second Tibetan Plateau Scientific Expedition and Research Program (grant number 2019QZKK0901), the National Science Foundation of China (grant number 42072243), and Spark Programs of Earthquake Sciences granted by the China Earthquake Administration (XH19067Y;

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Citation Excerpt :
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Active crustal deformation in the Tian Shan region, central Asia

Highlights

Abstract

Introduction

Section snippets

Active Tectonics of the Tian Shan region

Modeling methods

Fault slip rates

Discussion

Conclusions

Declaration of Competing Interest

Acknowledgements

Tectonophysics

Tectonophysics

Earth Planet. Sci. Lett.

J. Struct. Geol.

Earth Sci. Rev.

J. Asian Earth Sci.

Geod. Geodyn.

Earth Sci. Rev.

J. Struct. Geol.

J. Asian Earth Sci.

Geomorphology

Tectonophysics

Lithos.

Relative recent construction of the Tien Shan inferred from GPS measurements of present-day crustal deformation rates

Nature

Late Cenozoic tectonics of the Kepingtage thrust zone: interactions of the Tien Shan and Tarim Basin, Northwest China

Tectonics

Active faulting and folding in the northern Tian Shan and rotation of Tarim relative to Dzungarian and Kazakhstan

J. Geophys. Res.

An updated digital model of plate boundaries

Geochem. Geophys. Geosyst.

Long-term fault slip rates, distributed deformation rates, and forecast of seismicity in the western United States from joint fitting of community geologic, geodetic, and stress direction data sets

J. Geophys. Res.

Plate-tectonic analysis of shallow seismicity: apparent boundary width, beta, corner magnitude, coupled lithosphere thickness, and coupling in seven tectonic settings

Bull. Seismol. Soc. Am.

Interpolation of principal stress directions by nonparametric statistics: global maps with confidence limits

J. Geophys. Res.

Seismic hazard inferred from tectonics: California

Seismol. Res. Lett.

Stresses that drive the plates from below: Definitions, computational path, model optimization, and error analysis

J. Geophys. Res.

Estimation of slip rates in the southern Tien Shan using cosmic ray exposure dates of abandoned alluvial fans

Geol. Soc. Am. Bull.

Late Cenozoic tectonic evolution of the northwestern Tien Shan: constrains from magnetostratigraphy, detrital fission track, and basin analysis

Geol. Soc. Am. Bull.

Crustal shortening on the margins of the Tien Shan, Xinjiang, China

Int. Geol. Rev.

Tien Shan, Pamir, and Tibet: history and Geodynamics of Phanerozoic Oceanic Basins

Geotectonics

Late Cenozoic slip on the Talas-Fergana right-lateral fault, Tien Shan, Central Asia

Geol. Soc. Am. Bull.

The Dzhungarian fault: late Quaternary tectonics and slip rate of a major right-lateral strike-slip fault in the northern Tien Shan region

J. Geophys. Res. Solid Earth

Landslides and surface breaks of the 1911 Ms 8.2 Kemin earthquake

Russ. Geol. Geophys.

Active Tectonics of Tianshan

Basics characteristics of active tectonics of China

Sci. China Ser. D Earth Sci.

Geology

The Paleoearthquakes in Xinjiang Region

Summary of deep seismic probing of the lithospheric structure across the west Kunlun-Tarim-Tianshan

Geol. Bull. China

Active tectonics around Almaty and along the Zailisky Alatau range front

Tectonics

Seismic tomography of the crust and upper mantle structure underneath the Chinese Tianshan

Chin. J. Geophys.

The World Stress Map Database Release

Late Oligocene–early Miocene unroofing in the Chinese Tianshan: An early effect of the India–Asia collision

Geology

Velocity ﬁeld in Asia inferred from Quaternary fault slip rates and Global Positioning System observations

Journal of Geophysical Research: Solid Earth

Holocene slip rate and earthquake hazard of the north-edge fault of the Yanqi Basin, southeastern Tian Shan, NW China

Seismol. Geol.

Mechanisms of active folding of the landscape (southern Tian Shan, China)

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Tectonics

A geodetic plate motion and Global Strain Rate Model