挤压构造带变形样式的热年代学诊断

Thermochronometric diagnostics on the tectonic architecture of compressional deformation belts

构造运动是控制地应力场与塑造地貌形态的内源动力,受多重因素影响,在不同地区展现出不同的变形样式与岩石剥露路径.基岩热年代学数据常被用于量化岩石剥露与冷却过程,进而间接限定脆性与脆韧性构造变形的时限和样式,但相关数据具有多解性,需要综合分析地质、地球物理等其他证据.本研究旨在以新构造变形与热年代学研究丰富的新特提斯构造带之喜马拉雅、阿尔卑斯、东喀尔巴阡、龙门山及四川盆地西南部5个地区为例,探究前缘逆冲-腹地正断构造、厚皮构造、前展式薄皮构造、乱序式薄皮构造等背景下,热年代学年龄与岩石剥露的空间展布规律,以期为其他地区的相关研究提供参考.切过喜马拉雅-雅鲁藏布江缝合带的热年代学年龄廊带图显示,主边界逆冲带与腹地正断层之间,热年代学年龄最小,剥蚀量最大,吸收了该地区的地壳缩短和中下地壳物质中新世以来的快速剥露.与喜马拉雅冲断带不同,阿尔卑斯造山带年轻的热年代学年龄和显著的岩石剥露主要分布在宽约80 km的造山带内部地区,记录了造山楔的双向逆冲推覆.东喀尔巴阡冲断带,重置的磷灰石(UTh)/He年龄向前陆盆地方向逐渐变小,但冲断带前缘磷灰石裂变径迹年龄未重置,指示前缘变形时代晚、剥蚀量小,记录了前展式冲断变形.龙门山冲断带热年代学年龄分布受3条逆断层控制,腹地断层上盘年龄最小,剥露深度最大,指示多条断层乱序式逆断活动.对四川盆地西南部褶皱带的分析显示,紧闭背斜区的热年代学年龄普遍相对年轻,且与地层时代负相关,记录了褶皱变形的发育过程和剥蚀幅度.上述结果表明,热年代学年龄的空间展布是限定挤压性新构造活跃区岩石剥露与扩展过程、甄别地壳变形样式的有力工具,是探讨构造-气候相互作用关系的重要基础.

Tectonic deformation,one of the main endogenic forces,dominates the lithospheric strain distribution and landscape evolution.Under the influences of both endogenic and exogenic processes,variable thin-and thick-skinned tectonic architectures developed in different compressional thrust belts,which is characterized by different spatio-temporal patterns of rock exhumation that can be recovered from thermochronometric age data.Given that the explanation for the thermochronologic data is nonunique,the application of such methods for exploring the deformation should take into account other geological and geophysical information.This study aims to explore the thermochronometric diagnostics of the various manners of deformation using five end-member examples in the Neo-Tethys orogenic belt,including the Himalayan frontal reverse and hinterland normal faulting,Alpine thick-skinned tectonics,Eastern Carpathian in-sequence thin-skinned thrusting,the Longmenshan basement-involved out-of-sequence thin-skinned thrusting,and the shallow folds in the southwestern Sichuan Basin.Comparison between the thermochronometric swath and crustal architecture of the Himalayan fold-and-thrust belt,it is found that the Miocene and younger thermochronometric ages are distributed south of the Yarlung Tsangpo suture zone,with the youngest between the south-verging Main Central Thrust and north-verging Southern Tibetan Detachment,suggesting more rapid and significant rock exhumation and extrusion between those two faults than in the suture zone and hinterland regions.The structural and thermochronometric swath across the relatively Alpine Orogen displays Miocene and younger ages and thus much more rock exhumation in the core part(~80 km wide)of the orogen than its marginal thrust belts,consistent with the doubly-vergent thrusting in both the southern and northern margins of the orogen.Such a pattern differs from that of the wide Himalayan belt,where rock exhumation mainly occurs in the orogenic front.The Eastern Carpathians shows a foreland-ward younging trend in the reset thermochronometric ages,changing from late Cretaceous to the late Miocene from the core to the eastern margin of the thrust belt.In addition,nonreset AHe and AFT ages have been reported in the eastern margin,indicating less rock exhumation therein.Such age and exhumation patterns imply that crustal shortening has been propagating in an in-sequence manner,consistent with the geological observation that rocks involved in the shortening become younger toward the foreland basin.The swath of the Longmenshan thrust belt shows a fault-dependent age distribution pattern.Younger ages and thus more rock exhumation occurs in the hanging walls for the hinterland-ward dipping faults,with the youngest ages occurring in the hanging wall of the Wenchuan-Maoxian fault,suggesting a reverse nature for the fault displacement.Further towards the hinterland,west of the Wenchuan-Maoxian fault,the ages become old,indicating a hinterland-ward decrease in rock exhumation and uplift.Such a rock exhumation pattern is consistent with an upper crustal configuration where Longmenshan thrusts merge gradually into a gentle decollement extending into the basement at depth(~20–30 km).The greater exhumation in the hinterland suggests the deformation therein is controlled by out-of-sequence thrusting.The shallow folds in the southwestern Sichuan Basin exposes relatively older early Mesozoic strata in tight anticlines and younger strata in adjacent open synclines,which produce relatively younger(<40 Ma)and older(>110–60 Ma)thermochronometric ages,respectively.These cooling ages indicate that the folds formed since the late Eocene and a total exhumation of 5–6 km of the anticlines.These examples demonstrate that spatial distribution of thermochronometric age data is sensitive to the various manners of compressional deformation,and thus can be used for testing the propagation of deformation,lithospheric configuration and the underpinning tectonic and climatic forces.