川西龙门山逆冲带北段多断层同时逆冲几何学证据

Geometric Evidence for Synchronous Thrusting and Folding of the Northern Longmengshan Thrust-fold Belt,West China

四川盆地西北部龙门山逆冲构造带北段逆冲构造变形较为复杂,其构造变形始于中三叠世末,持续到中生代末期,新生代再次重新发生构造变形,形成复杂的逆冲推覆构造带,现今仍有活动。该构造带从西向东可划分出五个构造单元:轿子顶推覆体、唐王寨推覆体、叠瓦断层系及断层相关褶皱带和前缘单斜构造带。本文基于中国石油最新油气勘探成果,通过二维地震测网构造解释及钻井资料,应用多断层同时逆冲概念建立了后展式多断层同时活动构造模式,对龙门山逆冲带北缘进行详细构造解剖,提出新解释方案,认为:①轿子顶异地推覆体之下与唐王寨逆冲推覆体后缘之间,存在古生界叠置逆冲岩片组成的褶隆构造‒背斜;②唐王寨逆冲推覆体为异地推覆的大向形构造;③唐王寨推覆体前缘发育多条向南东逆冲的叠瓦逆冲断层组成的叠瓦断层系,该叠瓦断层系是由褶隆构造多个逆冲岩片中的逆冲断层向山前逆冲的前缘逆断层组成;④矿山梁、天井山等背斜构造为复杂化的一断层扩展褶皱,而且在褶皱前翼(东南翼)发育多条向南东逆冲的逆冲断层组成的一系列无序逆冲叠瓦断层;⑤前缘单斜构造带之下为早侏罗世地层不整合覆盖的早期发育的隐伏逆冲构造,野外观察和地震解释及地层之间不整合分析证实,这些无序逆冲断层形成时间主要为早侏罗世之前。结合低温热年代‒裂变径迹年龄数据,证实龙门山逆冲构造带晚期重新活动时代为中新世晚期一直到第四纪。龙门山逆冲构造带北段多个逆冲断层同时活动模式不同于新逆冲断层向前陆方向扩展时,旧断层不活动的模式。该模式展示由于深部有序后展式逆冲作用形成的双重逆冲构造发育期间,同时活动的逆冲作用可在浅层地表形成一系列无序逆冲断层,类似于叠瓦扇逆冲断层。总体上龙门山逆冲构造带形成时间从造山带向盆地逐渐变老,但龙门山逆冲构造带多期构造就有可能导致深部多个逆冲断层垂向上叠置,地表浅部形成飞来峰构造,或者一系列叠瓦状无序逆冲断层。

The Longmenshan thrust-fold belt,which is located along the boundary between the Qinghai-Tibet Plateau and the South China platform,has experienced a long and complicated tectonic evolution since the end of Late Triassic.The Longmenshan thrust-fold belt consists of five tectonic units:the Jiaoziding nappe,the Tangwangzhai nappe,imbricated faults,and fault-related folds and monoclinal front from West to East.According to Boyer’s synchronous thrust model,the thrusts in the Longmenshan thrust-fold belt should be generally younger toward the foreland,which is not consistent with the 2D seismic reflection data,borehole logs,surface geologic observation of the study area.A fault-bend anticline consisting of the thrusted Paleozoic sequences is recognized to occur in-between the Jiaoziding and Tangwangzhai nappes.The Tangwangzhai nappe is allochthonous occurring as a syncline.In the front of the Tangwangzhai nappe there are several imbricated thrust faults of southeastern thrusting.The Kuangshanliang and the Tianjingshan are two complicated anticlines consisting of a series of imbricated thrust faults occurring at the crests and the southeastern limbs.The blind thrust faults and folds are unconformably overlain by the lower Jurassic sedimentary rocks.Combining with available geochronological results,it can be inferred that the crystalline massif of the Longmenshan thrust-fold belt has been strongly exhumed since the Miocene.Our outcrop observation and interpretation of seismic profiles results confirmed that most of the thrust faults and folds in the Longmenshan thrust-fold belt were formed during Mesozoic as was proposed previously,and were reactivated during the Cenozoic.Fold-and-thrust belts are conventionally believed to develop largely by break-forward propagation of thrust sheets,which means that the youngest and the most active thrust sheets lie in the front of the fold-and-thrust belt.However,the absence of significant active crustal shortening and shortening in the range front in the Longmenshan belt during the 2008 Wenchuan earthquake can be interpreted satisfactorily by the our synchronous thrust model.Unlike the Boyer’s synchronous thrust model of hinterland-to-foreland thrusting,this model assumes that the older thrusts can be re-activated as new thrusts propagated toward the foreland.Synchronous thrusting is compatible with the observations in the Longmenshan thrust-fold belt,and thus,a new model of simple foreland-to-hinterland thrusting or an overstep thrust sequence is proposed:klippes by folded thrust,‘fold backlimb’thrust faults,‘fold-forelimb’imbricated thrust faults with out-of-sequence,laterally propagating thrusting,multiple-stage thrusting at two structural levels,and‘incidental’reactivation of existing thrusts.