摘要
作为ＩＮＤＥＰＴＨ计划的第一阶段，完成了一条跨过特提斯喜马拉雅南缘的深地震共中点（ＣＭＰ）剖面，它绘制出俯冲到喜马拉雅之下的印度大陆地壳的顶部（主喜马拉雅道冲或ＭＨＴ）和底部（莫霍层）轮廓。我们用移动式地震仪记录了ＣＭＰ剖面的爆炸，偏移距最大达１５５ｋｍ。短偏移距数据证实了ＣＭＰ剖面的数据，而我们的大偏移距数据则以强反射带为主。我们将这一反射带的强的初始相位解释为藏南滑脱系（ＳＴＤ），而其最后一个相位则为ＭＨＴ的反映。我们用ＣＭＰ剖面的初动数据去详细地模拟最上部２ｋｍ的结构。亚东裂谷系中年青的伸展盆地的深度约束在２ｋｍ，给出了裂谷东侧的断距为４．６ｋｍ，在特提斯喜马拉雅内的正断层，Ｅ－Ｗ向伸展１．５％。宽角数据用于建立地表到ＭＨＴ的地震波速度模型。ＳＴＤ反射体北倾１３°，从约６ｋｍ深（在ＣＭＰ剖面南端之下）到２２ｋｍ深，然后变平，倾角减至５°。这样，我们的观测提出ＳＴＤ是一个深的基底断裂，对ＭＨＴ，我们观测到倾角为７５°，ＮＮＥ倾，从高喜马拉雅山脊下的－２０ｋｍ海拔到雅鲁藏布江缝合带南约７０ｋｍ处的－３６ｋｍ海拔（地表下４０ｋｍ）。我们提出印度地壳可能俯冲到缝合带地表之下，却不可能是整体俯冲。
deep seismic common-mid-point (CMP) profile, shot across the southern margin of the Tethyan Himalaya as the first stage of Project INDEPTH (International Deep Profiling of Tibet and the Himalaya), imaged the top (Main Himalayan Thrust or MHT) and bottom (Moho) of the indian continental crust underthrusting the Himalaya. We used portable seismographs to record the CMP-profile shots at a wide range of offsets, up to 155km. Our shortoffset data corroborate the CMP-profile data while our large offsets data are dominated by a band of reflectivity. We interpret the bright onset of this reflective band as the South Tibetan Detachment (STD) and a phase at the end of the reflective band as the MHT.We used the CMP-profile first-break data to model the uppermost 2km in detail. The depth of young extensional basins of the Yadong rift system along the CMP-profile was constrained to a masximum of 2km, yielding a throw of 4. 6km across the eastern flank of the rift and a gross estimate of c. 1. 5 % east-west, normal faulting, extension in the Tethyan Himalaya. The wideangle data were used to construct a crustal seismic-velocity model down to the MHT. The STD reflector dips 13°N from a depth of about 6km beneath the surface under the south end of the CMP profile to a depth of 22km, them flattens to a dip of only 5° N. Thus our observations suggest that the STD is a deep-rooted basement fault. For the MHT we observe a dip of 7. 5°NNE from 20km depth below sea level at the crest of the High Himalaya to 36km below sea level (40km beneath the surface) at a distance of about 70km south of the indus-Yarlung suture (IYS). We suggest that indian crust may underthrust the surface expression of the IYS. but probably does not do so as an intact plate.
