The Early Cretaceous extensional deformation in the southeastern Beishan Range, central Asia: Insights from 2D seismic reflection profiles and apatite fission track thermochronology
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摘要: 为深入认识中亚造山带南缘晚中生代陆内变形过程及其动力学机制,通过野外地质观察、二维反射地震剖面解释及磷灰石裂变径迹测年,对北山东南部早白垩世伸展构造及早期挤压构造进行了详细解析。结果表明,一系列逆冲断层与褶皱构造造成下—中侏罗统发生强烈挤压变形。地震剖面揭示出2条早白垩世伸展正断层,其中梭梭井断裂为南东倾向的低角度铲式正断层,五道明断裂为北西倾向的高角度正断层,二者共同切割了早期形成的褶皱—冲断系统,指示挤压—伸展构造的转换;梭梭井断裂与五道明断裂分别限定了早白垩世总口子盆地的北西和南东边界,使得其具有“地堑”样式,盆地内沉积的下白垩统生长地层发育,表明伸展正断层的活动时间为早白垩世晚期。磷灰石裂变径迹热史模拟结果显示,梭梭井断裂下盘于132~110 Ma经历了快速冷却和剥露事件,该事件与其持续的正断层活动密切相关,进一步证实北山东南部晚中生代挤压—伸展构造的转换很可能发生在早白垩世晚期(133~129Ma)。增厚地壳的重力垮塌与局部地幔上涌共同导致了中亚造山带南缘早白垩世的区域伸展作用。Abstract: To better understand the timing, processes, and dynamic mechanisms of the enigmatic late Mesozoic tectonic transition from contraction to extensional deformation in the southern Central Asian Orogenic Belt (CAOB), detailed structural analysis and apatite fission track thermochronology were conducted in the southeastern Beishan Range. Field observations show that Lower-Middle Jurassic strata have been strongly deformed by numerous thrusts and folds. 2D seismic reflection profiles reveal two NE- to NEE-striking normal faults. The Suosuojing fault is a SE-dipping low-angle listric normal fault, and the Wudaoming fault is a NW-dipping high-angle normal fault. The normal faults cut the early-formed fold and thrust system, indicative of the contraction-extension transition. The Suosuojing and Wudaoming faults border the Early Cretaceous Zongkouzi Basin to the northwest and to the southeast, respectively. The Zongkouzi Basin has a graben geometric, in which Lower Cretaceous strata display typical growth-strata relationships, suggesting that the normal faults were active during the late Early Cretaceous. Thermal history modelling results by apatite fission track data show that the Suosuojing fault footwall experienced rapid cooling during 132~110 Ma. This phase of rapid cooling was intimately related to footwall exhumation during normal slip of the Suosuojing fault. We argue that the late Mesozoic intracontinental contraction-extension transition in the southeastern Beishan Range may have occurred between ~133 Ma and ~129 Ma in the late Early Cretaceous. Collapse of thickened crust and coupled mantle upwelling triggered the Early Cretaceous extensional deformation in the southern CAOB.
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