秦岭造山带东西秦岭交接转换区陆内构造特征与演化过程

【作者】 梁文天；

【导师】 张国伟；

【作者基本信息】 西北大学 ， 构造地质学， 2009， 博士

【摘要】 世界多数造山带碰撞造山后并未平静下来向稳定的克拉通化方向发展,而是发生了强度不亚于主造山的强烈陆内构造活动。这些陆内构造并非现有板块构造和碰撞造山理论所能全部解释,表现出大陆形成演化过程中的独特特征,蕴含着丰富的大陆动力学信息,成为依托造山带开展大陆动力学研究的良好切入点。秦岭造山带是中国大陆华南和华北板块印支期碰撞拼合形成的造山带,碰撞造山后造山带开始了更为强烈的陆内调整演化,使之现今仍为分割中国大陆南北的东西向强大陆内山脉。东西秦岭交接转换区位于秦岭造山带的中心部位,是现今秦岭沿走向最为狭窄的“蜂腰”地段,同时也是中国大陆南北向贺兰-川滇构造带复合穿越秦岭造山带的关键地区,新生代随青藏高原的崛起又成为高原向东北边缘扩展的前锋地带。因此,交接转换区在先期碰撞造山构造基础上发育了丰富多样的陆内构造,是探索秦岭造山带陆内构造演化及其大陆动力学意义的关键地区。论文重点选择交接转换区内印支期碰撞造山后最为突出的构造信息载体,如晚印支期花岗岩、J₁₊₂山间沉积盆地和文县-太白构造带及其所控沉积盆地,开展了系统的构造和沉积分析,厘定了区内碰撞造山后陆内构造的基本地质事实,在此基础上总结了其基本特征并探讨了其形成演化过程与机制,取得的主要成果和认识如下:1东西秦岭交接转换区印支期碰撞造山后逐渐转入造山带垮塌演化阶段(T₃-J₁₊₂),碰撞造山所形成的加厚陆壳/岩石圈由深部至浅部进行了时空四维的垮塌调整作用。东西秦岭交接转换区内晚印支期花岗岩总体显示以岩浆流动构造为主,组构特征区别于作为围岩的逆冲推覆构造,不同区域的岩浆侵位过程也明显不同,表明在晚印支期岩浆活动时,区域构造已并非统一的大规模挤压推覆,而表现为碰撞造山后早期相对稳定的构造环境。结合花岗岩“后碰撞”的地球化学特征及岩浆形成过程中幔源物质、热的参与等认为这些花岗岩的形成应代表了造山带由碰撞造山向造山带垮塌过渡的早期阶段,岩浆的形成可能由一些深部过程如拆沉、板片断离等引起,并沿这一时期中浅部地壳的走滑或者走滑伸展构造侵位。东西秦岭交接转换区内J₁₊₂山间沉积盆地表现出半地堑型盆地的沉积特征,盆地沉积作用受盆缘伸展构造控制造,盆地内部发育一系列正断层,形成露头尺度的堑垒构造,代表了造山带中浅部层次由碰撞造山的大规模挤压推覆向伸展转换,造山带进入大规模陆内垮塌调整演化阶段。晚印支期岩浆作用、J₁₊₂断陷盆地及区内同期伸展变质穹窿的发育共同代表了交接转换区碰撞造山所形成的加厚陆壳/岩石圈在晚印支-早燕山期(T₃-J₁₊₂)由深部至浅部时空四维的造山带垮塌调整过程。2东西秦岭交接转换区中晚燕山期（J₃-K₂）发生了大规模的陆内造山作用,区域地壳/岩石圈再次挤压缩短,在秦岭“蜂腰”部位突出的发育了挤压推覆和走滑剪切构造,共同调节了中国大陆南、北陆块尤其是鄂尔多斯和上扬子地块的相对运动。中晚燕山期以来,东西秦岭交接转换区再次开始大规模的挤压缩短,突出的表现为:造山带垮塌作用下形成的J₁₊₂伸展断陷盆地挤压变形,遭受逆掩推覆,显示了继承先期碰撞造山构造再次的陆内挤压推覆;在交接转换区内发育了NEE向斜列展布的文县-太白构造带,构造带为一条大型的左行走滑构造带,主体形成于J₃-K₂,构造带K₁时期的左行走滑剪切变形控制了K₁盆地的形成,主体呈现走滑伸展盆地的沉积特征,盆地的发育经历了两个阶段,在白垩纪早期形成了左行斜列展布的凤县盆地、成县盆地,晚期形成了左行斜列展布的石峡-二郎庙盆地、磨坝盆地和桥头盆地,代表了白垩纪时期断裂带两次强烈的构造活动,盆地于K₂时遭受南北向挤压缩短并褶皱变形。结合南北两缘四川和鄂尔多斯盆地J₃以来沉积和构造特征演化研究表明,中晚燕山期以来中国大陆南、北陆块再次发生大规模的陆内汇聚,两侧岩石圈向造山带下陆内俯冲,造山带呈花状向前陆区域不断扩展逆冲推覆并控制J₃-K₁前陆盆地的发育,并在秦岭“蜂腰”部位突出发育了NEE和NWW向的两组共轭剪切构造,调节了南北陆块,尤其是鄂尔多斯和上扬子地块的相对运动。3新生代以来东西秦岭交接转换区成为青藏高原向东北边缘扩展的前锋地带,在先期构造基础上形成新的陆内构造格架与沉积、岩浆作用,构成高原与东侧外围的渐变过渡边界,高原物质向外扩展受阻而在东西秦岭交接转换区向东有限逃逸。新生代以来东西秦岭交接转换区成为高原向东北边缘扩展的前锋地带,总体呈现为地壳厚度、地球物理场特征等的南北向构造横跨叠加在先期秦岭东西向构造之上,浅部地壳则发育了NEE、NWW和EW向三组左行走滑剪切构造,控制了E-N的沉积盆地及深源火山岩系的发育,成为区内强烈的地震活动带,并向东收敛汇聚于东西秦岭交接转换区东侧,共同构成高原东扩因受鄂尔多斯和扬子地块阻挡而沿秦岭向东的“栏栅式”物质有限逃逸构造格架。更多还原

【Abstract】 Most of the orogens in the world had not gone to cratonization after collision but a new tectonic stage characterized sorts of intensive intracontinental tectonics activities.Some of the activities,not well explained by plate tectonics,are unique process of continental tectonic evolution,and consequently become excellent materials which could be contributed to the establish of the continent dynamic theory.The great Qinling Mountains which geographically separates the North and South China today are an indosinian collisional orogen between NCB and SCB,the tectonic activities had not ceased after collision,but became more intensive even than the collision event.The conjunction area of East and West Qinling orogen（CAQ hereafter） which locate in the central part of the Qinling is the narrowest area along the strike,and also the crossing area of EW striking Qinling orogen and SN striking Helan-Chuandian tectonic belt.In Cenozoic,it also became the margin of the Tibet plateau.Consequently,various intracontinental tectonics activities were developed and became a critical area for the study of the intracontinental tectonic evolution of Qinling.This study presents much new structure date of the late indosinian granites,sedimentary data and structure analysis of the intramountain J₁₊₂ basins and Wenxian-Taibai tectonic belt,then summarized the characteristics of the intracontinental tectonics activities and discussed the process and mechanism of them.1.The CAQ gradually transferred into orogenic collapse stage(T₃-J₁₊₂) after collision which involved the entire crust and/or the lithosphere from the lower part to the upper part.The late indosinian granites in the CAQ display primarily magmatic structure,and their magnetic foliations and lineations are very different from the regional structure characterized the thrust tectonics,the emplacement mechanism also changed in different areas,all these evidence indicate that the tectonic settings is not compressional in late indosinian period when the magmatic activities developed,but a tectonic quiescence just after collision.Combine with the "Post-collisional" geochemical characteristics and the participation of mantle-derived composition;the emplacement events of granites maybe represent a transition from collision to orogenic collapse,and the magma maybe derives from the breakoff or delamination processes,then ascend to the upper crust along the strike-slip or transtensional fault. Sedimentary and structure date display that the J₁₊₂ intramoutain basin are half-graben basins, the sedimentation is controlled by the extensional bolder fault in the margin of the basins,and there are also a series of syn-sedimentary normal falut developed in the internal basin and form the horst and graben structure style,which indicate the upper part of the crust transfer from compressional regime to extensional regime,and the orogenic collapse reach the climax.Late indosinian magmatism,J₁₊₂ extensional basins and the synchronous extensional metamorphic dome in this area can be considered as the different reflection from the lower part of the crust to the upper part about the orogenic collapse.2.Intensive intracontinental orogeny had been developed in the CAQ during J₃-K₂,and the crust was greatly shortened again after collsion orogeny.Thrust and special conjugate structures had been developed in the CAQ to accommodate the convergent movement between NCB and SCB,especially the Ordos Block and Yangtze Block.Great crust-shortening after collision had been occurred during J₃-K₂ in the CAQ,and mainly focus on two aspects.First one is that the J₁₊₂ extensional basins that developed in the orogenic collapse stage ceased and suffered extensively compression,which display an intracontinental thrust inherited from the collision structures.The second one is that the development of the NEE striking Wenxian-Taibai sinistral strike-slip faults belt.The belt is developed mainly during J₃-K₂,and the tanstensional activities during K₁ controled the sedimentation and formation of the K₁ basins along the strike of the faults belt.There are two stage of the evolution of the K₁ basins,Fengxian basin and Chengxian basin formed in the early period of K₁,Shixia-Erlangmiao bansin,Moba basin and Qiaotou bansin in the late period,which indicate twice activities of the bolder faults.And the strata of the basins were then folded in K₂.Combine with the evolution of the Sichuan and Ordos basin that distributed in the two sides of the CAQ this period,we concluded that the NCB and SCB was intensively converged during J₃-K₂ again after collision,and the lithosphere of NCB and SCB hugely subducted beneath the orogen,led to the expansion of orogen to the foreland area and control the development of the foreland basins,and formed a conjugate NEE and NWW strike-slip faults belt in the CAQ which greatly accomadated the convergence of the Ordos and Yangtze Block.3.The CAQ became the margin of the Tibet plateau in Ccnozoic,and gradually formed new tectonic frameworks which control the development of the sedimentation and magmatism, which constitute part of the transitional margin of the Tibet plateau,the material of Tibet Plateau was limitedly escaped eastward along the CAQ.The CAQ became the margin of the Tibet plateau in Cenozoic,and the SN striking structures characterized distribution of thickness of the crust and geophysical features cross superimposed on the previous EW Qinling structures,three group sinistral faults belt,NEE, NWW and EW were developed and controlled the E-N sedimentary basins and magmtism, and also became the intensive earthquake belts,anastomosing in the east part of the CAQ,all of these combined together to form a "fence-like" escaping tectonic framework in the CAQ which result from the resistence of the NCB,SCB against the outward expansion of the Tibet plateau.更多还原