天山西段前陆盆地现今变形及其隆升机制

The 2500 –km-long Tien Shan is a prototype of continental orogeny. In a far-field of India-Asia collision, the Tien Shan is undergoing crustal shortening at a maximum rate of 20 mm/yr . However the mechanism of deformation by which the Tien Shan was rasied up rapidly in late Cenozoic time remains elusive. The widely-accepted hypothesis suggests that the crust of the Tien Shan deforms homogeneously in response to the northward motion of the Tarim Basin. The convergence deformation resulting in crustal thickening that builds eventually the mountain range through isostasy. We dismiss the explanation in terms of the three lines of evidence : 1) Great earthquakes (M>8) in the past centuries localized on the flanks of the Tien Shan, 2) Larger amount of a total convergence between Tarim Basin and Kazakh Platform is accommodated by deformation on the northern and southern margins, and 3) The Holocene crustal shortening across the Atushi anticline is estimated at 5-6 mm/yr, one quarter of the total convergent rate. We suggest that a large-magnitude basal fault at depths of 15-20 km is buried under each side of the Tien Shan. Tarim Basin and Kazakh Platform are plunged into the Tien Shan at rates of ~10 mm/yr, a half of the total convergent rate. The crust of the Tien Shan is thicken due to underthrusting along the detachment faults. The Tien Shan uplifts by successive crustal thickening on its flanks and propagates toward neighboring basins. In this proposal, we will determine 3D displacement field by integrating a densified GPS velocity field with InSAR interferograms covering western Tien Shan, from which a detailed description of deformation will be available for constraining on fault geometries and slip rates of the detachment faults. We will demonstrate that the detachment faults have play a unique role in absorbing convergent deformation and building the mountain range. Our work will strengthen our inference about the mechanism of mountain building by improved knowledge about ongoing crustal deformation and its relationship with seismicity of major earthquakes and surface geology in the Tien Shan.

天山长2500km,是陆内造山的典范。尽管远离板块边界，天山南北缩短变形速率高达20mm/yr, 但其造山成因机制与隆升过程仍不清楚。比较流行观点是: 塔里木向北推挤导致天山地壳均匀缩短、整体增厚，天山因重力均衡而隆升。本项目将挑战这一主流理论。初步依据三条观测证据：近期八级大震集中在山前地带；两侧现今变形幅度明显高于内部；阿图什背斜全新世缩短速率占总变形的四分之一，我们推测在15-20 km深的天山底部存在滑动速率10mm/yr 左右的低角度滑脱断层。 塔里木、哈萨克沿滑脱断层向下挤入，天山地壳相应增厚隆升并向外扩展。项目利用加密GPS位移场和覆盖西天山InSAR变形图像，建立天山地表三维位移场，精细揭示山前褶皱-冲断带变形的空间展布, 论证滑脱断层的独特作用，反演其几何运动模型。研究将强化现今变形观测证据, 揭示其与大震分布, 活动构造观测间联系，以此修正对天山隆升机制的不确切认识

天山堪称陆内造山典范，经历多期复活造山，晚新生代形成东西全长 2500 km，南北最宽达400km, 突兀在印度-欧亚板块边界上千里以外的弘大山系。天山晚新生代挤压缩短近200 km左右，南北两侧地块的(塔里木盆地、哈萨克地台) 现今汇聚速率高达 20 mm/yr 以上，百年以来大震频发。天山在中新世因板块碰撞而复活（25Ma），但隆升过程、造山机制仍不十分清楚，近来比较流行观点是: 塔里木地块向北推挤导致天山岩石圈地壳均匀缩短、整体增厚，天山因重力均衡而隆升。但依据三条观测证据—— 1) 百余年来八级大震集中在天山南北缘；2) 天山造山带边缘现今变形幅度明显高于山体内部；天山南缘全新世缩短速率占天山总汇聚速率的四分之一以上，3) 西南天山喀什凹陷内发育滑脱断层，八级大震破裂不出露地表，可以推测塔里木、哈萨克地块沿山前滑脱断层双向挤入天山之下，地壳相应缩短增厚，山脉隆升并向外扩展。..为了深入认识天山变形机制，在项目执行期间，大幅度加密西南天山GPS形变观测网，新获150个站点的变形位移速度，并利用覆盖境外天山的GPS观测结果，优化西天山地区地表二维位移场，精细揭示天山两侧山前褶皱-冲断带缩短变形的空间展布。依据优化的速度场，反演山前逆冲断层几何、运动学模型，发现在15-20 km深的前陆盆地底部存在滑动速率10 mm/yr 左右的低角度滑脱断层，并向天山内部延伸。本项目阶段性研究成果修正了此前认为天山变形以均匀应变机制为主的不确切认识，指明了天山变形集中在边缘两侧，与大震分布关系密切，强调了滑脱断层在调节天山挤压变形、地壳加厚，并控制大震活动的关键作用。这一理论挑战了主流观点，对天山隆升演化和地震危险性分析具有重要启示。