天山的晚新生代构造变形及其地球动力学问题

Late Cenozoic Tectonic Deformation and Mechanism along the Tianshan Mountain, Northwestern China

天山是大陆内部典型的新生代复活造山带,其新生代构造变形的方式、变形量、速度及过程等对于认识大陆内部造山带的变形机理有着重要的意义。本文在对南北天山主要活动构造地质填图和综合研究的基础上,重点探讨了天山的晚新生代构造变形特征及其动力学问题。早更新世以来,特别是早、中更新世之间,天山的构造活动由内部向南北两侧扩展,使得两侧的新生代凹陷逐渐褶皱成山,形成数排新生代褶皱带,整个天山的现代构造活动是一种扇形的向南和向北的双向逆冲缩短作用。这些新生代褶皱和断裂是控制天山地区地震孕育和发生的主要构造。南北天山褶皱-逆断裂带都只发育在山前凹陷之内。凹陷以外的地方,山体沿高角度逆断裂直接逆冲于山前戈壁之上,变形集中发生在山体与戈壁的交界部位,戈壁内部不发生大规模变形,不形成山前褶皱-逆断裂带。显然,这种变形区所能吸收的地壳缩短量远小于发育多排褶皱-逆断裂带的凹陷区。天山新生代构造变形幅度的定量研究表明,第四纪以来的变形幅度由西向东逐渐减小,造成这种变化的原因是西部帕米尔高原对天山的直接推挤作用。帕米尔高原向北的强烈推挤可能是天山新生代构造变形及其地震活动的主要地球动力来源。

The Tianshan mountain is a typical Cenozoic rejuvenated mountain belt in the continental Asia. Its styles, amounts, rates, and processes of deformation provide important meaning for understanding geodynamics of other mountains in the interior of continent. On the basis of detailed field studies on major active fold and reverse fault zones in both northern and southern Tianshan, this paper discusses characteristics of late Cenozoic deformation in the Tianshan with the aim of understanding how this intracontinental mountain has been deformed. The late Cenozoic deformation in the Tainshan probably initiated between Pliocene and early Pleistocene time with the peak deformation stage in the period between early and middle Pleistocene. Deformation is characterized by crust shortening where the Tianshan thrusts both to the south and the north to form several folds and reverse fault zones along the range fronts. The newly formed fold and reverse fault zones only develop in the range-front depressions. These fold and reverse fault zones involve thin-skinned deformation of the foreland with major propagating thrust ramps near the range front. Outside the range-front depressions high-angle reverse faults merge to the surface at the base of the range with less amount of crustal shortening than that within the depression. Quantitative measurement across the range-front depressions indicate that the amount of total Quaternary crustal shortening decreases from west to east along the Tainshan mountain. It appears that compression of the Pamir Plateau to the north is probably the major driving force that caused deformation in the Tianshan.