燕辽东段侏罗、白垩纪构造转变期古地理和古环境的初步分析

Preliminary analysis on the paleogeography and paleoenvironment in the eastern Yanliao area during the Jurassic-Cretaceous tectonic transition

自中、晚侏罗世之交至早白垩世早期,中国东部发生了构造格局和古地理环境的巨大转变。对这一时期燕辽东段古地理和古环境的部分问题进行了多学科整合分析,初步认识如下:中侏罗世开始增强的板块汇聚造成中、晚侏罗世亚洲东部地壳增厚和地势升高,古太平洋伊泽纳崎板块持续俯冲引起深部岩浆上涌、地壳减薄和多期伸展裂陷,同时东亚洋陆过渡带形成一系列逃逸构造,上述构造演化导致燕辽地区在晚侏罗世至早白垩世中期形成高山和盆岭地貌。早白垩世137~131 Ma,嫩江-八里罕断裂至喀喇沁变质核杂岩延伸线两侧出现东高西低的地貌差异,相对下降的冀北-大兴安岭火山-地堑带地层中赋存热河生物群早期化石组合,而辽西-松辽地区为相对隆升的构造高地。130~110 Ma时期裂陷作用扩展,热河生物群中、晚期组合向周围扩散。热河生物群生存时期总体为以干冷为主的温带大陆性气候,这与中生代中国东部高原的潜在环境效应并不矛盾。全球气候演化和区域构造古地理条件共同导致燕辽地区自晚侏罗世至早白垩世的干旱化和寒冷化,并影响到中、晚侏罗世燕辽生物群和早白垩世热河生物群之间的生态群进化演替。

During the transit period from the Middle and Late Jurassic to early Early Cretaceous,the tectonic pattern and paleogeography overturned in eastern China.This paper discusses some issues on the paleogeography and paleoenvironment in the eastern Yanliao area through this period.The preliminary results show that the aggravate plate convergence at the continental margin of eastern Asian from the Middle Jurassic induced the crustal thickening and tectonic uplifting in the region.With the continuance of the subduction of the Paleo-Pacific(Isanagi)Plate,deep magma upwelled,crust thinned,and multiphasic rifts developed,while the East Asian ocean-continent connection zone underwent tectonic escape.These processes shaped the high mountain and other basin-wide paleo-landforms in the Yanliao area.During the early Early Cretaceous(137-131 Ma),an altitude difference-low in the west and high in the east-established along the extension line through the Nenjiang-Balihan Fault and the Kharatsin Metamorphic Core Complex.In the comparatively descending western part,the strata of the Northern Hebei-Great Xingan Mountain volcano-graben belt yielded the fossils of the early assemblage of the Jehol Biota,while the Western Liaoning-Songliao area rose to tectonic upland.As the rifting area expanded during the 130-110 Ma,the middle and late-phase assemblages of the Jehol Biota extended to the surrounding areas.The paleoclimate of the Jehol Biota was temperate continental,mostly arid and cold,thus not inconsistent with the notion that the paleoclimate was affected potentially by the environment of the Mesozoic East China Plateau.The global paleoclimatic evolution and regional paleogeographical condition jointly caused the subartic climate trend in the Yanliao area from the Late Jurassic to Early Cretaceous,and further affected the evolution of the paleo-ecosystem from the Middle-Late Jurassic Yanliao Biota to the Early Cretaceous Jehol Biota.