侯马-运城盆地沉积特征及其对构造运动、气候变化及河流演化的响应

Depositional Characteristics of the Houma-Yuncheng Basin and Its Response to Tectonic Activity, Climate Change, and River Evolution

侯马-运城盆地为晚新生代以来强烈下沉的盆地,现今却大面积遭受剥蚀,有悖于构造下沉接受沉积的常理。为探究其原因,在对侯马-运城盆地及邻区20多个黄土剖面进行野外测量及年代对比的基础上,选择运城盐湖北缘进行连续钻探,开展钻孔岩芯的粒度、磁化率、色度、烧失量及微量元素环境替代性指标分析,并进行光释光及磁性地层学综合定年,同时与前人在侯马-运城盆地的近50个钻孔进行了对比。结果表明,盐湖钻孔200 m岩芯代表了大约距今0.7 Ma以来的沉积。55~200 m深度(约距今0.2~0.7 Ma)岩芯反映受气候旋回影响为主的宽阔河湖沉积环境,指示侯马-运城盆地曾与邻近的渭河盆地、三门盆地连成浩瀚的汾渭古湖;55 m深度以上(约0.2 Ma以后)突然转为以河流相为主的沉积环境,这是由于三门峡谷河流溯源侵蚀贯通三门盆地,导致湖水下泄,使盆地发生大面积湖退,从长期沉积转向广泛剥蚀,并被S2以来黄土/古土壤披盖。研究区盆岭空间分布分析揭示,早期北东东向隆起-凹陷被晚期北北东向隆起-凹陷叠加,隆起叠加处构成较强抬升区(峨嵋台地等),披盖不同时期的黄土或古土壤;凹陷叠加区则构成强烈沉降区,如运城盐湖因强烈下沉而封闭和咸化。其构造叠加可能与鄂尔多斯地块运动方式有关。距今约8 Ma前,青藏地块向东北方向与鄂尔多斯地块发生强烈碰撞,在来自青藏高原深部地幔流的配合下,可能使鄂尔多斯地块向东北方向移动,一方面使北东东向汾渭盆地进一步拉张;另一方面与华北克拉通构成右旋力偶,拉裂成北北东雁行排列的山西地堑系。两者于侯马-运城盆地叠加,形成隆起与凹陷的叠加构造地貌格局。

Despite its high average accumulation rate,the Houma-Yuncheng Basin in the southeastern Ordos Block in northern China is surprisingly experiencing intensive erosion,rather than being in a depositional state,with fluvio-lacustrine deposition terminated and covered by S2 paleosol in most areas of the basin,based on the stratigraphic and chronologic correlation among more than 20 loess/paleosol profiles in the basin and adjacent areas(Weihe Basin to the west and Sanmen Basin to the south). To address how such erosional/depositional transformation occurred,we carried out 200-m-depth continuous borehole drilling in the northern margin of the Salt Lake in Yuncheng Basin,associated with the integrate analysis of grain size,magnetic susceptibility,chroma proxy,and loss on ignition,as well as the OSL and magnetostratigraphic chronologies,added to data from more than previous50 boreholes. An abrupt change from typical lacustrine sedimentary rhythms to fluvial alternating fine and medium grained dominance occurred at the depth of 55 m with an age of about 0.2 Ma,which is interpreted to be the rapid regression of a large paleolake drainage that exposed the lacustrine sediments in the Houma-Yuncheng Basin and its adjacent basins,by the headward erosion of the downstream river along the Sanmen Gorge. Spatial distribution of basins/ranges with their estimation of the relative uplift/subsidence rates of the Houma-Yuncheng Basin and its adjacent areas reveals that an early east-northeast(ENE)uplift-depression was superimposed by the late northnortheast(NNE)one around ~8 Ma. The superimposed uplift area presented higher local uplifts(e.g.,the E’mei Platform within the Houma-Yuncheng Basin)where the loess/paleosol accumulated in various thicknesses,whereas the superimposed depressed areas were subjected to more intensive subsidence,non-compensated deposition,and isolated lakes,i.e. the Salt Lake,in the Yuncheng Basin. The tectonic superposition is likely related to the motor pattern of the Ordos Block. The extrusion of the lithosphere mantle beneath the Ordos Block due to the subduction of the India Plate to the Eurasian Plate pushed the Ordos block northeastward and led to ENE rifting to the southeast(e.g. the Houma-Yuncheng Basin). Furthermore,the dextral stress couple derived from the northeastward motion of the Ordos Block on the North China Craton resulted in the initial extension of the Shanxi Graben System.Consequently,the NNE trending uplift-depression superposed on the early ENE one,forming a complicated tectonic geomorphic pattern of the study area.