Abstract
The Yili block in the Central Asian Orogenic Belt (CAOB), forms the easternmost part of the Kazakhstan collage system. Exploring its thermo-tectonic history is important to reconstruct the intra-continental evolution of the Tianshan belt. In this contribution, we report new apatite fission track (AFT) data from the basement rocks from the northern (i.e. the Wenquan complex) and southern (i.e. the Dahalajunshan - Nalati range) margins of the Yili block. Thermal history modeling reveals that the Wenquan complex underwent moderate basement cooling in the Cretaceous, possibly due to far-field effects of the Tethys-deformation and the following Lhasa-Qiangtang collision. These events at the southern Eurasian margin propagated tectonic stress to the northern Yili and triggered localized deformation. Early Triassic-middle Jurassic moderate cooling is also identified in the Dahalajunshan - Nalati range, and is interpreted to be related to the post-orogenic strike-slip motion along the major shear zones and the effects of the Qiangtang and Kunlun-Qaidam collision. Combined with the published thermochronological data, it is suggested that the northern and southern parts of the Yili block experienced different Mesozoic thermo-tectonic evolution. Basement cooling of the northern Yili block generally took place before the Cretaceous, exhuming shallower crustal levels as compared with the southern one. The intermontane Yili basin may have accommodated substantial propagated contraction induced by the Cretaceous collisional events. Based on our new results and the previously published thermochronological data, it is suggested that the intra-continental reactivation of the North Tianshan and Nalati faults probably did not invoke significant regional exhumation during the Meso-Cenozoic. Instead, small-scale brittle faults controlled localized enhanced denudation.
Highlights
The Central Asian Orogenic Belt (CAOB) is a composite and immense accretionary and collisional orogenic system
We focus on the thermal his tories of two Neoproterozoic metamorphic complexes located near the northern and southern margins of the Yili block (Fig. 2), which hitherto have not been studied by low-temperature thermochronology
We suggest that the possible hidden boundary faults below the Cenozoic cover of Yili basin (Figs. 1B and 2) should have played an important role in controlling the intracontinental deformation, they might have largely accommodated the horizontal crustal shortening induced by the far-field effects during the Cretaceous so that the northern Yili block experienced quite limited regional exhumation with respect to the southern Yili
Summary
The Central Asian Orogenic Belt (CAOB) is a composite and immense accretionary and collisional orogenic system. The thermochronological signal of this area is unlikely to record significant Cenozoic deformation of the Tianshan belt, but is expected to preserve Mesozoic or earlier cooling events. In this regard, it is of great impor tance to study the thermal history of the Yili block as it may open a window for exploring early-stage thermo-tectonic events of the Tian shan belt. Glorie et al (2019) reported limited data from near the lithospheric-scale North Tianshan fault, this study was largely focused on the neighboring Junggar Alatau in Kazakhstan and inconclusive on the timing of fault reactivations in the Yili block
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