Cenozoic tectonic patterns and their controls on growth strata in the northern Tianshan fold and thrust belt, northwest China

Highlights

• Seismic interpretation to reveal the structural styles of the northern Tianshan.

• Growth strata within each structure are closely related to tectonic patterns.

• Pre-Cenozoic mega fault systems mainly cause the segmentation of northern Tianshan.

• Tectonics play a key role in controlling the sedimentation.

Abstract

Field-based mapping and subsurface interpretations of high-resolution seismic images, as well as other geophysical data, have been integrated to investigate tectonic patterns and growth strata in the northern Tianshan fold and thrust belt (NTFTB). The NTFTB formed as a result of intense deformation and uplift of the northern Tianshan region that was triggered by the far field effects of the Indo-Eurasia plate collision. The structural styles of the NTFTB vary, and are primarily classified as thick-skinned thrust nappes that involve basement rocks and thin-skinned fault-related fold structures with multiple décollement layers in the sedimentary strata. The overall NTFTB can be divided into three segments based on its structural geometry. The western segment is characterized by an intense backthrust in the northern Tianshan piedmont, with little displacement transfer into the southern Junggar Basin. In the central segment, the emergence of three rows of anticlines reveals a progressive deformation sequence and displacement transfer into the foreland through multiple décollements. In the eastern segment, the tectonic pattern is characterized by steep faults that branch forward in a narrow zone to generate intense uplift. Evidences indicate that major basement faults, paleo-uplifts, and inheritance of former structures play key roles in driving the segmentation of the NTFTB, and that secondary fault assemblages may control regional tectonic styles within each tectonic unit. Cenozoic growth strata have been observed in the NTFTB, the geometry and deformation mechanism of which are subject to the structural style in a given structure. Growth strata can also provide constraints for time of deformation in the northern Tianshan, and the Dushanzi Formation has been shown to represent clear growth in the piedmont, which indicates rapid uplift of the northern Tianshan as well as intense tectonic activity in the NTFTB in the late Miocene. The growth strata become younger within structures to the north, towards the foreland. In general, the Cenozoic evolutional process has been relatively uniform in the northern Tianshan, generally showing a forward propagation of fault splays into the foreland, and the geometry and distribution of sedimentary strata have been controlled by tectonics.

Introduction

The Tianshan is a large mountain range that is located in the southern branch of the Central Asia Orogenic Belt and is approximately 2500 km long. The northern flank of the Tianshan range, referred to as the northern Tianshan, also serves as the southern edge of the Junggar Basin in northwest China (Fig. 1A and B) (Molnar and Tapponnier, 1975, Tapponnier and Molnar, 1979, Wang et al., 2018b). The northern Tianshan has a long evolutionary history, with multiple phases of accretion and deformation (Carroll et al., 1995, Shu et al., 2004, Han et al., 2010, He et al., 2013, Yang et al., 2013, Chen et al., 2015, Yang et al., 2015, Fang et al., 2016, Wang et al., 2017), and the northern Tianshan fold and thrust belt (NTFTB) formed as a result of intense tectonic activity during the Cenozoic, most likely as a far field response to the Eurasian and Indian plate collision (Tapponnier and Molnar, 1979, Hendrix et al., 1994, Sun et al., 2004, Charreau et al., 2009a, Sun and Zhang, 2009, Yin, 2010, Gong et al., 2015, Lu et al., 2017, Charreau et al., 2018). By studying the NTFTB, the development of intracontinental deformation as a response to the Eurasian and Indian plate collision can be investigated (Tapponnier and Molnar, 1979, Yin, 2010, Li et al., 2011b). In addition, it is challenging but vital to determine if the structural features of the NTFTB play a role in hydrocarbon exploration in the southern Junggar Basin by functioning as major structural traps (Guo et al., 2011, Guan et al., 2016, Li et al., 2016, Wang et al., 2018a, Wang et al., 2018b).

Along the NTFTB and in the southern Junggar Basin, a series of fault-related fold structures that strike nearly east-west are underlain by either widespread breakthrough or blind thrust splays that detach into décollements at multiple depths (Stockmeyer et al., 2014, Guan et al., 2016, Chen et al., 2018). There is a consensus that the northern Tianshan has deformed in a breaking-forward manner; with a northward propagation of the NTFTB generating rows of structures gradually into the Junggar Basin, which is supported by both outcrop observation and seismic interpretation (Charreau et al., 2008, Guan et al., 2016, Guo et al., 2006a, Li et al., 2011b). Fault-related fold models have been employed to describe the structural geometries present in the NTFTB, with most structures being considered as a combination of a fault bend fold, a fault propagation fold, or a wedge structure (Li et al., 2010a, Li et al., 2011b, Guan et al., 2016). However, previous research has focused mainly on local geological sections or discrete structures in one specific anticlinal belt (Charreau et al., 2008, Chen, 2008, Lu et al., 2010, Chen et al., 2011, Chen et al., 2012, Li et al., 2011b, Guan et al., 2016). Unfortunately, there is still a lack of comprehensive research on the variations in tectonic patterns along the entire west to east trend of the NTFTB. In fact, the tectonic deformation is not uniform and segmentation can be clearly observed in the NTFTB, which raises questions regarding the differences in tectonic patterns, as well as the cause of this segmentation. In this study, seismic reflection profiles, drilled wells, complemented by geochronological data and surface geological observations, are used to interpret the structural elements of the deformation and their controls on sedimentation, thus giving insights into the comprehensive evolution of the NTFTB.