| The convergent plate margin acts as one of the important components in the global plate tectonic system.It forms a series of sedimentary basins on the continental margin due to the retreat of related subduction plates.Extensional systems of back-arc basins have been widely recognized and studied in the convergent setting,however the spatial-temporal distributions of tectonic subsidence and its controlling factors(faulting,subducted seamount/ridge,etc)in an episodic extensional back-arc basin are poorly known.With the roll-back of the westward subducted Pacific plate,nearly 75% of the global marginal basins have developed in the eastern margin of the Eurasian plate,most of which are back-arc basins.The Okinawa Trough is the only back-arc basin(expect for the Andaman Sea)in the early stage of spreading that fractures above the continental crust base of the convergent plate margin,and the onset of seafloor spreading has occurred in its southern segment.The Valencia Trough in the western Mediterranean recorded the development of the back-arc basin on the western margin of the Eurasian plate due to the northward subduction of the Tethys Oceanic plate.It develops on the continental crust basement with a ceased rifting and seafloor spreading.These two troughs are developed in a similar structural setting,and the Okinawa Trough is still in syn-rift stage,but the Valencia Trough has entered the post-rift stage.To study the tectonic subsidence of these two troughs can increase our understanding of the evolution of basins,and is meaningful to explore the controlling factors and deep dynamics of back-arc basin in the compression setting.Thus,this study is guided by theories of sequence stratigraphy and continental margin basin dynamics,and takes the Okinawa Trough in the eastern East China Sea and the Valencia Trough in the Western Mediterranean as the research area.Integrated seismic lines grid data with obtained or interpreted drilling,geological and geophysical data,all seismic profiles,stratigraphy units and sequence boundaries are interpreted,divided and dated,respectively.We establish precise sequence stratigraphic framework and time-depth conversion relationship,analyze lithological,paleo-water depth and other parameters,calculate the original deposition thickness of different sedimentation units,reconstruct the tectonic subsidence history in the Okinawa Trough,and obtain the tectonic subsidence,water-loaded subsidence,the upper crust thinning factor,and the lithosphere thinning factor in the Valencia Trough.After that,we discuss Cenozoic subsidence model of continental margin basin in compression setting,distribution characteristics of subsidence and its particularity,controlling factors of various subsidence rates,interaction between subsidence and regional tectonic activities,and Cenozoic tectonic-sedimentary evolution process in the study areas.The results show as follows:(1)The Okinawa Trough has developed since the Miocene,and can be divided into 4 stages: Miocene,Pliocene,Early Pleistocene and Late Pleistocene.The rate and the amount of subsidence have the characteristics of a regular increase and decrease which corresponds to tectonic activities in different periods.The rapid tectonic subsidence is mainly concentrated in the northern and middle segments of the Okinawa Trough during the Miocene and Pliocene,and concentrated in the southern part since the Pleistocene.The tectonic subsidence rate in the trough gradually migrates southward as the geological period changed which is closely related to the multi-stage extensional system.By comparing the temporal-spatial distribution of the intensive subsidence area,we confirmed that rifting events first occurred in the northern segment and middle segment of the Okinawa Trough from the Miocene to Late Pliocene,and then propagated to the southern segment after the Late Pliocene.(2)The rapid tectonic subsidence of the Okinawa Trough is mainly distributed in the rifting center or adjacent area of boundary faults in different geological periods.It indicates that normal faulting,including the NE-SW-trending normal faulting resulted from back-arc rifting and the NW-SE-trending strike-slip faulting induced by regional tectonic events,is the primary factor controlling the rapid tectonic subsidence.In addition,the anomalous rapid subsidence occurred in the southernmost part of the southern Okinawa Trough is likely related to the subducted Gagua Ridge.When the Gagua ridge subducts beneath the trench more than 100 km,it will result in dehydration and partial melting.Magmatic bodies produced by partial melting of subducted materials will then migrate upward and intrude into the continental crust.Sufficient volumes of magma will create dikes wide enough to propagate upward without freezing,but some magma will travel through small dikes or by percolation and be frozen within the lithosphere.The frozen magmas will create a dense lithosphere and cause rapid tectonic subsidence.(3)The Valencia Trough is one part of the Iberian plate during Mesozoic with active tectonics and large-scale salt diapir.During Cenozoic,the northeast segment of the Valencia Trough is affected by the complex interaction between the Cenozoic southwestward progressive rift systems of western Eurasia plate and the retreat of the subducted Tethys plate.It belongs to the back-arc extensional domain and develops a series of Cenozoic extensional faults until to middle Miocene,while the southwest segment is still in the uplift and denudation stage which only record less strata of this period in seismic data.Since middle Miocene,the east segment of the Valencia Trough is characterized by weak tectonic activity,and the southwest segment enters the flexural subsidence and post-rift thermal subsidence stage in the foreland domain.(4)Subsidence results and seismic observations indicate that there is almost no extensional fault developed in the southwest segment of the Valencia Trough,while rapid abnormal subsidence has occurred since the Miocene.The rapid subsidence in the southwestern Valencia Trough is mainly concentrated in the northeast part of the Betic Font and a semi-circular area surrounding the Mesozoic Columbrets Basin.Our results show that the fault-related(upper crust)thinning factor is 0.0366,but the maxmium lithosphere thinning factor reaches to 0.7.It seems that faulting cannot provide such a large amount of thinning and is not the main controlling factors.Thus,this paper proposes that the rapid subsidence distributed in the northeast part of the Betic Font is affected by the flexural of study area which act as a foreland basin and the intensive subsidence region corresponds to the foreland zone of the foreland basin.Meanwhile,the lithosphere of this study area has not enough time for thermal equilibrium due to its post-rift time started until the Miocene,and the post-rift thermal subsidence is another main controlling factor.In addition,another rapid subsidence around the Mesozoic Columbus Basin sedimentary center is related to the older,deeper and dense sedimentary body.(5)Comparing the subsidence characteristics and its evolution process in the Okinawa Trough and the Valencia Trough,we suggest that the retreat of the subduction plate is the main factor for the formation of the back-arc basin.If the back-arc basin is in an open setting,the rifting of back-arc basin will continue,and resulting in the subsequently seafloor spreading.Faulting is the main controlling factor for its intensive subsidence,and other factors such as orogenic belt,magmatism or the subduction of the seamount/ridge are the secondary controlling factors.If the back-arc basin is in a closed seting,the rifting and seafloor spreading of back-arc basin will cease.Faulting is still the main controlling factor for the rapid subsidence of the back-arc basin area,while the influence of the orogenic belt or thrust and fold zone is the main factor in the non-back-arc/foreland basin area,and the dense blocks and others are the secondary factors for its rapid subsidence. |
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