滑脱褶皱陡坎的变形特征和运动学模型：以帕米尔-南天山前陆地区明尧勒背斜为例

DEFORMATION CHARACTERISTICS AND KINEMATICS OF ACTIVE DETACHMENT FOLD SCARP:A CASE STUDY FROM THE MINGYAOLE ANTICLINE,PAMIR-SOUTHERN TIANSHAN FORELAND

褶皱陡坎是褶皱变形过程中形成的地貌陡坎,是近期发现的一种不同于断层陡坎的构造作用形成的陡坎状地貌.在缺少地震反射剖面等深部资料时,利用褶皱陡坎可对活动褶皱的变形特征和生长演化历史进行限定,但迄今为止有关研究较少.位于帕米尔-南天山前陆地区的明尧勒背斜为第四纪活动的滑脱褶皱.在背斜南翼的河流阶地上发育了一系列褶皱陡坎：在T2和T3b阶地上,褶皱陡坎的高度/宽度/坡度分别为16m/40m/25°和20m/50m/26°,陡坎位置与下伏基岩中向斜枢纽位置对应.通过对这些褶皱陡坎的分析,得出：1）这些褶皱陡坎是滑脱褶皱通过膝折带迁移机制形成的.2）褶皱陡坎形成初期,陡坎高度、宽度和坡度逐渐增大;当陡坎宽度达到枢纽带宽度2倍时,陡坎坡度将达到最大值;之后尽管陡坎高度和宽度逐渐增大,其坡度将保持恒定.3）褶皱陡坎吸收的缩短增量与陡坎高度和下伏地层倾角间存在定量几何关系.根据T2阶地上褶皱陡坎的高度约16m和暴露年龄约8ka,估算T2阶地面暴露以来明尧勒背斜南翼的缩短速率为～1.3mm/a.在上述分析基础上,还对比总结了滑脱褶皱陡坎和断弯褶皱陡坎的异同点.

The fold scarp,a type of geomorphic scarp on the land surface formed by folding without fault offsets on the surface,can be used to constrain folding and slip rates and kinematics and to reconstruct a folding history despite a lack of full constraints on the subsurface structure.Recently,the conceptual,geometric,and kinematic models of fold scarps formed by fault-bend folding（fault-bend fold scarp）were developed.But for other types of fold scarp,there are few detailed investigations till now.Located at southern foreland of Chinese Tianshan,the Mingyaole anticline is interpreted to be a detachment fold.On the Kezilesu rivcr terraces in the south limb,a series of detachment fold scarps occur.The height,width,and slope of fold scarps on the T2 and T3b terraces are ～ 16m/～40m/～25° and ～20m/～50m/～26° respectively.The scarp locations are correlated with an underlying synclinal hinge separating a 50° dip and a 15°dip domain and the strike of the scarp is parallel with the hinge.Detailed geologic and geomorphic mapping and dGPS survey data reveal important characteristics of detachment fold scarp.1） The fold scarps are formed by synclinal hinge migration.2） During initial growth,the height,width and slope of the fold scarp increase gradually.When the fold scarp’s horizontal width increases to be at least twice that of the hinge,the slope will approach a maximum,and will subsequently remain constant even as the height and width continue to increase gradually.3） The scarp height and underlying bedding dips on either side of the hinge can be used to calculate incremental shortening absorbed by the fold scarp.Based on the height ～ 16m of the fold scarp on the T2 and its exposure age ～8.0ka,the shortening rate absorbed by south limb of the Mingyaole fold is estimated to be ～ 1.3mm/a.Despite similarities with fault-bend fold scarps,detachment-fold scarps have some pronounced differences,which suggest that the type of fold scarp should be defined prior to calculating folding rates.