兰州地区新生代构造应力场演化特征

节理统计、断层、褶皱和震源机制解析等研究表明，兰州地区第三纪以来的构造应力场主压应力轴倾角在陡－缓交替过程中逐渐变陡，作用方向由ＳＮ向，ＮＷ－ＳＥ向转变为ＮＥ－ＳＷ向。第三纪时期构造应力场的逆时针运动是地块在该时期作逆时针旋转运动的结果，早－中晚新世之间出现了青藏高原构造作用的重要转型。进入到ＥＷ向伸展机制下的构造变形时期，研究区地壳运动以抬升为主，但抬升运动与水平运动之间有机地结合在一起的。经过

EVOLUTIONARY CHARACTERISTICS OF THE CENOZOIC STRUCTURAL STRESS FIELD IN RHE LANZHOU AREA

Studies on joint statistics,faults,folds,and focal-mechanism solution have revealed that the dip angle of the principal axis of compressive stress of the tectonic stress field in Lanzhou area since the Tertiary have gradually become steep through an alternating change from steep to gentle and vice versa,while its orientation changed from N-S,NW-SE to NW-SW.In Tertiary time,the counter-clockwise movement of the tectonic field was resulted from the counterclockwise movement of the mas-sif.In the Early-Middle Pleistocene,an important chang in style of tectonism took place in the Qing-hai-Tibet Plateau,while the Lanzhou area entered an E-W extensional tectonic deformation period.The crustal movement of the study area dominated by uplifting although the uplift was combined by horizontal movement.Every horizontal (or uplifting)movement was followed by an adjustment period, in which the crustal movement was different in style from the previous period(i.e.,when the previous period was dominated by uplifting,then horizontal movement occurred in the adjustment period and vice versa).