滇西南地区黑河断裂中西段晚第四纪构造活动特征

Characteristics of Late Quaternary Tectonic Activity of Middle-western Segment of Heihe Fault in Southwestern Part of Yunnan Province

摘要:

通过卫星影像解译、野外实地调查与地质填图,对滇西南地区黑河断裂中西段晚第四纪构造活动特征进行了研究。结果表明,黑河断裂为一条规模较大的区域性活动断裂带,西起沧源县南,向东南止于澜沧江断裂,全长约168km,走向280°～310°。该断裂晚第四纪新活动性具有一定的差异性和分段性。根据其几何结构、最新活动性及1988年澜沧7.6级地震破裂带特征,可将黑河断裂从西向东划分为沧源—木戛、木戛—南代和南代—勐往三条次级断裂段。其中的中、西段长约88km,全新世活动显著,活动性质以右旋走滑为主。沿断裂形成了丰富的断错地貌现象。西段断裂的最新活动断错了全新世晚期地层;中段是1988年澜沧7.6级地震的发震断裂之一。根据对断错冲沟的测量和年代测试,得到其全新世以来右旋滑动速率为（3.54±0.78）mm/a,与区域上其它断裂的滑动速率大致相当,反映了其区域构造活动的整体性和协调性。

Abstract:

On November 6,1988,two earthquakes with a magnitude >7 occurred in the Lancang and Gengma area of the southwestern part of the Yunnan province.The maximum intensity of the M7.6 Lancang earthquake was IX on the Chinese scale,which is similar to the Modified Mercalli scale.The surface rupture of the tectonic activity of the Lancang MS7.6 earthquake occurred because of the earthquake-related extensional ground cracks and small fault scarps in the epicentral region. The area within which the cracks and small scarps occurred was approximately 35-km long and 3-km wide.The maximum vertical and the dextral horizontal offsets were 1.5mand 1.4m,respectively.The M7.6earthquake showed complicated earthquake-generating tectonics and formed a separate obvious seismic deformation belt along the Heihe and Lancang faults.The Heihe fault is one of the seismogenic faults of the 1988 M7.6event of Lancang.The Heihe fault,located in the southwestern part of the Yunnan province,having a length of 168km and a trend of 280°~310°,is an active fault zone in the late Quaternary,which is mainly a right-lateral strike slip with a dip slip.It turns out that the Heihe fault zone can be divided into three secondary fault segments,namely,Cangyuan-Mujia(F1-1),Mujia-Nandai(F1-2),and Nandai-Mengwang(F1-3).The activity gradually becomes weak when this segment of fault travels eastward.Through satellite image interpretation and field geological investigation,we studied the geological and geomorphologic characteristics of activity along the middle and the western segments of the Heihe fault in this research.The middle and western segments of the Heihe fault,having a length of 88km and a trend of 280°~310°,is an active fault zone in the late Quaternary,which is mainly a right-lateral strike slip with a dip slip.A variety of dislocation landforms have been formed along the fault by the new tectonic activities since the late Quaternary,such as the dextral dislocation of gullies and ridges,fault trenches,fault passes,fault scarps,and pits.A series of dextral dislocations of gullies and ridges,and scratches on the cross section along the fault show that the middle-western segment on the Heihe fault is dominated by a right-lateral strike slip.In the Xuelidazai village,the age of the displaced stratum is(2 530±25)a.B.P.,and on the faultplane,a loosely consolidated gouge is developed across several cross sections of the fault.This implies that the latest active time of the middle-western segment on the Heihe fault is the late Holocene.Near Baishuijing,there are synchronous dextral dislocations of four small gullies because of the dextral movement of faults;these dislocations range from 9to 14m.By using the differential GPS measurement,the sample dating and the regional comparison of the fault slip rate,we calculated the right-lateral strike slip rate of the segment to be(3.54±0.78)mm/a.This value agrees with the regional characteristics of the fault slip rate.