北喜马拉雅穹隆带雅拉香波穹隆的构造组成和运动学特征

雅拉香波穹隆构造位于北喜马拉雅穹隆带,由上、下两个拆离断层分割成3个构造层。下拆离断层以韧性变形为主,其下的糜棱状片麻岩和花岗岩体形成穹隆核部即下构造层;上拆离断层以脆性变形为主,其上为低级变质的西藏沉积岩系及基性岩墙群(上构造层);千糜岩和糜棱状片岩构成上、下两拆离断层间的中构造层。穹隆构造内经历3期运动,第1和第2期的线理具有统一的北北西—南南东倾伏向,前者仅保存于局部下构造层,代表上盘向南南东的运动学特征,为早期构造变形,成因尚待查明;第2期为穹隆内主导线理,代表穹隆统一的上盘向北北西的运动。第3期低透入性线理向穹隆外侧倾伏,代表垮塌下滑运动。雅拉香波穹隆下构造层与高喜马拉雅岩系相似,下拆离断层为主拆离断层,中构造层可能为西藏沉积岩系底部经拆离作用形成,所以下拆离断层可能是分割高喜马拉雅结晶岩系与西藏沉积岩系的藏南拆离系在北喜马拉雅的出露。雅拉香波穹隆早期(距今14.5Ma±)可能经历了沿藏南拆离系的北北西向拆离,后期(距今13.5Ma±)因岩浆底辟和剥蚀反弹而发生穹隆作用。

STRUCTURE AND KINEMATICS OF THE YALASHANGBO DOME IN THE NORTHERN HIMALAYAN DOME BELT, CHINA

The Yalashangbo Dome is one of the structures in the Northern Himalayan Dome Belt. It consisted of three lithologic-structural units separated by a brittle upper detachment fault and a ductile lower detachment fault. The low-grade metamorphic Tibetan sedimentary succession and mafic sills formed the covering upper plate above the upper detachment fault. The core of the dome beneath the lower detachment fault was composed of mylonitic gneisses of Greater Himalayan Crystallines and a core granite pluton. The phyllonites and mylonitic schists made up the middle unit between the upper and lower detachment faults, which might be the intensively thinned basal part of Tibetan sedimentary succession. This dome experienced three episodes of deformation. The first episode of deformation was preserved locally in the lower part of the core gneisses, representing an earlier top-down-to-SSE movement whose mechanism needed further study. The second one was a dominant deformation in the dome, which was a unified top-down-to-NNW movement in all shear zones associated with the dome. The third episode of deformation was a slip down to the outsides of dome, indicating a collapse after the NNW-directed movement. The core gneisses of the Yalashangbo Dome were same to the petrological association of Greater Himalayan Crystallines, the lower detachment fault was a main fault in the dome, and the middle unit between the upper and lower detachment faults might be basal part of Tibetan sedimentary succession. Therefore, The lower detachment fault of the Yalashangbo Dome might be an exposure of the southern Tibetan detachment system (STDS) in northern Himalaya, which separateed the low-grade Tibetan sedimentary succession from the Greater Himalayan Crystallines in core of the dome. In an earlier period (～14.5 Ma ago), the area of the Yalashangbo Dome experienced a roughly north-directed detachment along the possibly STDS; later on (after ～13.5 Ma), this area was domed and the STDS was exhumed caused by the emplacement the core pluton and isostatic rebound.