远震接收函数确定的鄂尔多斯西部及邻区Moho面性质和构造意义

基于呈南北向线性分布且穿过鄂尔多斯地块的129个流动台站远震记录,获取了20267条远震P波接收函数.通过叠加转换点相同的接收函数,提取了可靠的P-S一次转换波和多次波到时,进而确定了南北向横跨鄂尔多斯地块剖面的地壳厚度与波速比分布.同时,利用单台速度-密度跃变(δβ-δρ)扫描叠加方法确定了Moho面速度和密度跃变.结果显示:秦岭—渭河盆地下方具有较薄地壳、低波速比(1.66～1.72)以及相对较小的密度跃变(4%～10%),表明该区域地壳主要以长英质酸性岩石为主,引起该现象的主要原因可能是下地壳拆沉;鄂尔多斯南部地壳较厚(41.4±1.3km)、波速比较高、速度跃变相对较小(14%～23%),主要原因可能由青藏高原的挤压增厚导致;鄂尔多斯北部波速比较高(1.87)、速度跃变较大(19%～29%)、密度跃变较小,推测鄂尔多斯北部下地壳发生部分熔融,较大波速比可能是部分熔融与沉积层共同导致的结果.

Moho properties of western Ordos block and surrounding regions constrained by teleseismic receiver functions and its tectonic implication

Based on the teleseismic records of Himalaya-II broadband seismic array with 129 stations across the Ordos block along north-south direction, 20267 teleseismic P-wave receiver functions have been obtained. We extracted the stable arrival time of P-to-S conversion and multiple reverberations by stacking the receiver functions in the bins with closed pierce points of PMs phase, and then calculated the crustal thickness and velocity ratio across the north-south section of western Ordos block. Meanwhile, the velocity and density contrast of the Moho beneath partial stations have been determined with grid searching stack technique. The results indicated that the crust is thin with low velocity ratio (1.66~1.72) and relatively small density contrast of (4%~10%) below the Qinling-Weihe graben, which imply the crust in this area is mainly composed of felsic acidic rocks. The crust of southern Ordos is thicker (41.4±1.3 km) with higher velocity ratio and smaller velocity contrast of (14%~23%), which might have been caused by the compression of the Tibetan Plateau. The velocity ratio is obviously higher than 1.87 with larger velocity contrast of (19%~29%) and smaller density contrast beneath northern Ordos. We speculated that the lower crust of northern Ordos has experienced the partial melting, the higher velocity ratio may be caused by the combination of partial melting and sedimentary layers.