米仓山南缘中生代沉积盆地性质讨论

Discussions on the tectonic attributes of the Mesozoic sedimentary basin in southern Micang Mountains

米仓山南缘位于四川盆地北部地区,前人认为该地区晚三叠世—白垩纪受控于米仓山造山作用形成的前陆盆地。实际上,无论是按照经典的前陆盆地概念,还是陆内前陆盆地或陆内俯冲前陆盆地等术语,米仓山南缘中生代为前陆盆地值得商榷,其一些关键的地质问题必须给以重视。主要表现在:①平面构造图显示,米仓山地区,前震旦系基底与震旦系盖层之间构成一规模较大的不完整的背斜穹隆,盖层围绕基底分布,说明不是构造推覆体;②不存在形成前陆盆地的区域应力,即该区在晚三叠世开始主要是升隆作用而非强烈的推覆挤压,研究资料表明该地区大规模的逆冲推覆作用发生在燕山期,米仓山的形成也应在该时限之内,之前米仓山为一继承性的隆起;③不具备前陆盆地的沉积格局,即晚三叠世—侏罗纪的沉积格局不是呈楔形展布。鉴于此,笔者认为前陆盆地术语不适用于米仓山南缘晚三叠世—侏罗纪沉积盆地的类型。

The southern part of the Micang Mountains north of the Sichuan Basin was previously considered as a foreland basin during the Late Triassic - Cretaceous. The authors argue, on the basis of tectonic and sedimentary evidences, that this area should be a down-warped basin rather than a foreland basin during the Late Triassic - Cretaceous, because ： （ 1 ） the study area is an inherited uplift rather than a tectonic nappe, indicated by a largescale incomplete anticline dome between the pre-Sinian basement and Sinian cover; （2） there was no regional stress contributing to the formation of the foreland basin. Furthermore, the study area was once subjected to the uplifting rather than the thrusting and compression from the Late Triassic onwards. The formation of the Micang Mountains should be limited in the Yanshanian when the large-scale overthrusting occurred, and （3） there was no sedimentary framework of a foreland basin in the form of a wedge during the Late Triassic - Cretaceous.