四川北部米仓山地区下三叠统内部不整合面的发现及其意义

前人认为四川北部米仓山地区下三叠统内部各组地层之间是整合接触,然而通过构造和沉积学的研究发现下三叠统内部(奥伦尼阶)存在一个区域不整合面。在沉积上表现为环米仓山地区广泛发育的陆相和海相碎屑岩,与下伏地层呈不整合接触。地震剖面上表现为不整合面上下的变形特征不一致、不整合面下部地层被削截、古构造的存在等等,通过对古褶皱和古断裂的平面投影,发现古构造走向为东-西向,与勉略构造带走向一致。综合区域大地构造历史,笔者认为在早三叠世奥伦尼期(约249.7～245.0Ma)南秦岭造山带和上扬子地块就已经发生碰撞,碰撞引起了米仓山地区强烈的构造变形,形成这一区域不整合面。该碰撞时间不一定是南秦岭造山带和上扬子地块在此处的初始碰撞时间。

Unconformity in the Lower Triassic of Micangshan Area, northern Sichuan Province: Its discovery and significance

No unconformity has been reported within the Lower Triassic of the Micangshan area, but recently we have found a regional unconformity whose age is Olenekian (ca. 249.7~245.0Ma). This unconformity is characterized by a layer of continental or marine clastic rocks overlying the old strata. In the seismic profiles, this unconformity could be easily recognized because of the remarkable difference of the deformation pattern between above and below it, top-truncated formations and paleo-structures below it. The strikes of these paleo-structures are E-W oriented, consistent with that of Mianlue suture northward. According to the regional tectonic history, we attribute this tectonic event as the result of the early collision of the Upper Yangtze Block and South Qinling Orogen, but it may not stand for the onset time of the collision.