西秦岭上白垩统红层空间分布及其对青藏高原东北缘隆升的地质约束

Spatial distribution of Upper Cretaceous red beds in the West Qinling Mountains on the northeastern margin of the Tibetan Plateau uplift and its geological constraints

通过对西秦岭上白垩统红层地层基本沉积特征的研究和空间高程分布的定量化分析,讨论了西秦岭晚白垩世时期可能的构造地貌状态及西秦岭新生代以来地壳隆升的空间变化规律。取得如下认识:1根据西秦岭上白垩统底部洪积-冲积砾岩层之上普遍存在一套厚度不等的具有风成砂岩特征的红色中细粒砂岩和上部出现以泥岩、泥质粉砂岩为主的湖相沉积,结合现今多分布在不同水系分水岭之上,以及西秦岭中部宕昌-岷县-临潭断裂带两侧上白垩统红层地层顶面高程和底部角度不整合面高程没有显著差异分析,认为西秦岭无论在晚白垩之前经历了何种构造过程,晚白垩世具有整体稳定的泛沙漠-湖盆的古构造地貌状态,且断裂带不具备控制上白垩统沉积的构造边界性质;2现今离散型分布在西秦岭的上白垩统沉积地层反映的原型盆地不是孤立的、受区域断裂控制的山间盆地,而是统一的泛沙漠-内陆湖盆,现今的离散型分布是新生代以来地壳不均匀隆升和侵蚀的结果;3西秦岭上白垩统底部的角度不整合面产状,虽然由于后期构造变动呈非完全水平状态,但总体产状平缓。从大区域尺度分析,可以近似看做原始近水平的古地貌面。通过对该角度不整合面高程信息提取和模拟分析,结果表明,其高程分布具有从南西到北东、从北西到南东逐渐降低,穿越区域断裂带没有显著梯度变化,指示了西秦岭新生代以来的隆升具有整体性和隆升幅度呈连续梯度变化的特征。这可能指示了西秦岭新生代以来的地壳隆升机制主要不是上地壳挤压逆冲缩短,而是在印度板块-欧亚板块碰撞汇聚的动力学背景下,下地壳或上地幔自西南向北东连续流变逐渐增厚,造成了青藏高原东北缘呈向北东突出的弧形扩展隆升。

Based on the research on sedimentary characteristics of Upper Cretaceous red bed strata in the West Qinling Mountains and its spatial elevation distribution analysis, this paper discusses possible tectonic geomorphology of the West Qinling Mountains in Late Cretaceous and spatial variation of crustal uplift in the West Qinling Mountains since Cenozoic. The following opinions are sug-gested：①According to existence of middle-fine grained red sandstones with aeolian sandstone features on the bottom of alluvial-allu-vial gravel layers and appearance of upmost mudstone and muddy siltstone with lake facies sedimentation characteristics in the Upper Cretaceous red bed strata in the West Qinling Mountains, in combination with their present distribution on the different river water-shed summit surfaces and similar elevation distributions between two sides of Tanchang-Minxian-Lintan faulted zone and north mar-gin fault of West Qinling Mountains, it is believed that no matter how West Qinling Mountains experienced geological evolution his-tory before Late Cretaceous period, West Qinling Mountains had the overall stability characterized by ancient tectonic geomorpholo？ gy status of a desert and a lake during Late Cretaceous period, and regional faults did not have any control over Cretaceous sedimenta-ry strata. ②Prototype basin of Upper Cretaceous red bed sedimentary strata presently discretely distributed in West Qinling Moun-tains may be not a series of isolated and regional faults-controlled intermountain basins, but a unified inland lake. Its present discrete distribution results from uneven crustal uplift and erosion since Cenozoic. ③Although the angle unconformity surface on the bottom of the Cretaceous red bed strata in West Qinling Mountains is not horizontal now due to late tectonic deformation events, it is gentle overall. Therefore, from the large-scale regional analysis, it can be regarded as approximate original sub-horizontal palaeogeomorpho-logic surface. The angular unconformity surface elevation information extraction and simulation results show that distribution of its el-evation gradually decreases from the southwest to the northeast and from the northwest to southeast;the elevation across the fault has no significant gradient, indicating that since Cenozoic uplifting of the West Qinling Mountains has been characterized by integrality and continuous uplift and decrease of uplift amplitude from southwest to northeast. This may indicate that the West Qinling uplift since Cenozoic has not been mainly controlled by the upper crust compression-shortening and thrusting, but may be controlled by the continuous flow of lower crust or upper mantle under the dynamic action of Indo-Eurasian plate collision convergence, which led to northeastward crust thickening and arc-shaped uplift on the northeast margin of the Tibetan Plateau.