熊坡背斜构造变形与蒲江-新津断裂活动特征

熊坡背斜位于龙门山构造带东南端的成都盆地内,是龙门山逆冲推覆构造向前推挤进入盆地内部的一个主要变形区域,与其配套发育的断裂为蒲江-新津断裂,断裂与背斜褶皱之间在构造变形模式上表现出明显的一致性。在褶皱和断裂的构造变形和活动特征上,熊坡背斜南段表现为一种不对称的褶皱,向NE方向发展表现为较为宽缓的对称褶皱形态,卷入的地层主要是中生代及其以前的地层,对蒲江-新津断裂的地貌调查结果表明,断裂没有对该区域内广泛发育的冲沟Ⅰ级阶地产生影响,而对山前发育的相当于南河(岷江Ⅰ级支流)Ⅳ级阶地的洪积台地有明显的控制作用,说明断裂活动时间应该为第四纪早期,到第四纪晚期活动减弱或是趋于静止

THE DEFORMATION OF THE XIONGPO ANTICLINE AND THE ACTIVITY OF PUJIANG-XINJIN FAULT

Xiongpo anticline locates in Chengdu Basin,to the southeast of Longmenshan tectonic zone.It is an important deformation area where the Longmenshan thrust-nappe structure intrudes into the Chengdu Basin.The Pujiang-Xinjin Fault is an associated fault to Xiongpo anticline.The deformation mode between the fault and anticline fold is in concordance obviously.The geologic section across the anticline indicates that the south segment of Xiongpo anticline is an asymmetric fold and the northeast segment is symmetric,wide and gentle relatively.The fold includes Mesozoic and pre-Mesozoic strata.The topographical investigation reveals that the faulting is always associated with folding.At the northeast part of fold,the fold axial direction is parallel to the fault strike.Near the fault,the strata dips are remarkably different and the height difference of topography is large.The investigation of the Pujiang-Xinjin Fault did not reveal any obvious fault profiles and new activity characteristics.The Pujiang-Xinjin Fault has no influence on the gullies and T1 terraces widely developed in this area,but it controls the pluvial terrace which corresponds to T4 terrace of Nanhe river(the first-order branch of Minjiang River).The OSL age of the pluvial terrace is older than 130ka.All above indicates that the activity age of the Pujiang-Xinjin Fault is at the early-Quaternary.By the late-Quaternary,the faulting weakened or was nearly inactive.So in the area,the major tectonic character is faulting associated with fold deformation,which is also the major deformation mode of Xiongpo anticline.