Conglomerate Sedimentation and its Tectonic Implication,Heihe Area Within Shangdan Zone of the Qinling

The Shangdan fault zone is the most important tectonic boundary within the Qinling orogen during the Phanerozic. A large volume of sedimentary rocks,which formed in the subduction process of north and south qnling plates,is preserved in the zone. Conglomerate is the facies which is believed to be closely related to orogenesis. Different sorts of conglomerate are produced in distinct tectonic settings. Heihe conglomerate within the Shangdan zone was considered to be Mollasse of basal conglomerate in previous studies, and accordingly served as a crucial piece of evidence supporting the inferrence that the Qinlin orogenic belt was built during the Caledonian period. However,this study shows that Heihe conglomerates is actually fore一arc deposits in the process of plate subduction and can not be interpreted as product resulting from plate collison. Heihe conglomerate can be divided into six different facies and two distinct facies associations.Most facies were deposited from debris flows and density一modified grainflows, and conglomeratic beds show clear evidence of laminar flow. The others represent gravelly high一density turbidites.Grading is well developed in conglomerate beds,including inverse, inverse一to一normal, and normal,but structureless or massive beds are also common. Some of gravity flows probably underwent flow transformation,for instance,certain debris flows might originate from turbidity currents. It is proved that all the conglomerate formed in a subaqueous environments. This conclusion is mainly based upon the analyses of internal sedimentary structures of conglomerates and their close association with turbidites. Two distinct facies associations actually build two independent conglomerate lenticular bodies in space. Association I consists predominantly of clast一supported conglomerate with abundant are derived materials, and pyroclastic rocks are clearly coexisting. It is believed that Association I was deposited from lahar which flowed from volcanic island to a foreare basin. Matrix一supported conglomerate is characteristic of Association Ⅱ and obviously interlayered with fine to medium grained turbidites. Especially,the two associations occurred separately,indicating that they were typically two independent subaqueous apron deposit. Different factors controlled the development of the two distinct aprons. Volcanic eruptions were the main cause of initiation of lahar which produced Association I,while faulting triggered the occurrence of submarine gravity flows which brought about formation of Assocition Ⅱ. A direct relationship exists between sedimentation of conglomerate and co一existing lithofacies and evolution of Danfeng ophiolites. Sedimentary sequence conformably overlies volcanic rocks of southern arcflank of the inferred arc and is approximately concurrent with arc development.Conglomerate,together with other lithofacies, was evidently fore一are basin fills and resulted from coeval subduction tectonism. The two associations occurred in different parts of basin and therefore possessed distinct facies sequences. Heihe area was marked by occurrence of small一scale apron deposits instead of well一developed large一scale submarine fans, reflecting an unstable tectonic sedimentary environment at that time.