Magnetostratigraphy Of The Ganyanchi Pull-apart Basin Along The Haiyuan Fault,NE Tibet,and Its Tectonic Implications

About 50-60 million years ago,the Indian subcontinent collided with the Eurasia plate,the post-collisional results formed the highest and most active Tibetan Plateau in the world.The rise of the Tibetan Plateau is the most significant event in the Cenozoic time,which has enormous influence on the evolution of tectonics and climate of eastern Asia.A most conspicuous feature of the plateau is that there are a number of long(>1 000km),active lateral strike-slip faults exit in the interior and along the margin of the plateau.These lateral-slip faults were considered to have played vital roles during the formation of the plateau.Deciphering the history of these faults has greatly deepened our understanding of the deformation process of the Asian continent,the growth pattern of the Tibetan Plateau,and the mechanism of strong earthquakes.The Haiyuan fault,~1000 km long,active sinistral strike-slip,defines the northern margin of the Tibetan plateau and connects the tectonically active Qilian Shan to the west with the seismically active Liupan Shan in the east.During the past century,two M>8 earthquakes occurred along the Haiyuan fault: one is the M ~8.5 Haiyuan earthquake,which struck the Haiyuan County area on December 16,1920;the other one is the 1927 M~8.0 Gulang earthquake occurred along the western segment.Hundreds of thousands of people were killed,and more than one million were left homeless.Since the great Haiyuan earthquake,numerous studies have been done along the Haiyuan fault zone and the adjacent area.It is generally agreed that the deformations along the eastern Haiyuan fault were northeastward thrust and the following left-lateral strike slip motion,since the Late Cenozoic time.However,the time of this kinematic change remains ambiguous,with ages ranging from the early Pleistocene to the late Miocene.For a large-scale strike-slip fault,due to local extensional or contractional stresses,the related structures such as pull-apart basins or push-ups were formed at the architectural discontinuity of the fault(i.e.,the steps or bends).Pull-apart basins are one type of basin developed in strike-slip fault zones,and host sediments containing abundant deformational information of the strike-slip fault.Moreover,due to its high deposition rate and thick sediments,more structural details may be obtained from the pull-apart basin.Large-scale geological mapping indicates that the eastern Haiyuan fault zone consists of eleven aligned fault strands and eight intra-fault pull-apart basins that range in width from 1-3 km and length from 2-8 km.Of the basins,the Ganyanchi basin is the largest,with a length of ~8km parallel to fault strike and a cross-fault width of ~3km.The Ganyanchi basin is located at a left-stepping,releasing step over in the central part of the eastern Haiyuan fault and ~3 km west of the epicenter for the 1920 event.With the exception of regional structural mapping and paleoseismic studies,no sedimentologic or chronological work has been done in the Ganyanchi pull-apart basin.The paucity of precise ages for basin sediments handicaps our understanding of both the tectonic evolution of Haiyuan fault zone and paleoenvironmental change in the region.To address this problem,we drilled a 328 m-long borehole,named HY-C8,south of the cross-basin fault and near the active depocenter of the Ganyanchi basin,and use magnetostratigraphic analyses,magnetic susceptibility,and shallow seismic reflection data to constrain the age and structural evolution of the basin and Haiyuan fault.The main insights are as follows:(1)The deep borehole profile shows that the stratigraphy of the basin can be divided into three main units(Unit I,II and III),which began to deposit at about 2.76,2.33 and 1.78 Ma,respectively.The grain size of the deposits manifests an upward thinning trend,which probably implies the profile is a characteristic retrogradational sequence.The sedimentary facies of the Ganyanchi basin experienced a four-stage evolving history: eluvial facies(before~2.76Ma)---alluvial fan facies(~2.76-2.33Ma)---distal alluvial fan facies(2.33-1.78Ma)---playa lake facies(1.78Ma-present).(2)A combination of boreholes and shallow seismic reflection data indicates that the Ganyanchi pull-apart basin is mainly controlled by the cross-basin fault and its northern boundary fault(i.e.,the Nan-Xihua Shan fault).In the control of the cross-basin fault,all new formed strata of the basin have been strongly subsiding and dipping to the north.All these sequences constitute north-dipping growth strata,which indicate they were formed by a typical listric normal fault.And the listric normal fault was controlled by the extensional stress associated with strike slip along the eastern Haiyuan fault.Moreover,the depocenter(>500m)of the Ganyanchi basin is located to the north of the cross-basin fault,which probably implies that the northern boundary fault still controls the accommodation space of the basin.(3)Using both alternating field and thermal demagnetizations we identified 13 polarity zones within the core using an inclination-only analysis,due to the potential for vertical-axis rotation of the core during removal.Because we lack independent age control for the section(other than that the top of the core is zero aged),we present two end-member age models(a young model and an old one) for correlating the observed polarity zonations with the GPTS.The young model,indicates an extrapolated age for the base of the section of 2.76±0.03 Ma.while the old model indicates an extrapolated age for the base of the section of 5.12±0.04 Ma.We prefer the young age model because it provides sedimentation rates more similar to those in surrounding basins,it correlates better to the GPTS,and because high SAR values are expected for the active tectonic environment of the Ganyanchi pull-apart basin.Thus,we conclude that the Ganyanchi Basin had started forming by 2.76±0.03 Ma.This age implies that the flanking portions of the left-slip Haiyuan fault had formed by at least ~2.8 Ma,which greatly predates“the early to middle Pleistocene time” suggested by former studies.(4)Analysis of the relationship between the boundary strike-slip faults and the corresponding Ganyanchi pull-apart basin indicates that the kinematic change along the eastern Haiyuan fault was ~3.0-3.5Ma,or about 3.5Ma,which is 1.7Ma earlier than previously inferred early Pleistocene time.(5)The young age model indicates two phases of increased SAR in the basin,with one from ca.1.92 to 1.78 Ma and the second from 0.77 Ma to present.We attribute these periods of enhanced deposition to northern hemisphere cooling and frequent climate change since late Cenozoic,and suggest that the role of tectonics was primarily to create the accommodation needed to capture this depositional record.(6)The magnetic susceptibility results indicate that the lake probably was formed at about 1.78 Ma ago,the corresponding lacustrine deposits recorded more than eight high susceptibility sections,which most likely due to the iron sulfides(like melnikovite,pyrrhotine etc.)that were usually produced in high-lake-lever and reduction conditions.(7)Synthesis of our new data with published results reveals that the kinematic change along the entire Haiyuan fault manifest a west-to-east younging trend,which may imply that the lateral-slip motion propagates eastwardly.Collectively,all the slip rates show that the eastern Haiyuan fault likely slipped at a slow rate from late Pliocene to present-day,which may suggest a steady state process of strain accumulation and release along the fault.