Characteristics Of Basement Faults And Its Tectonic Significance In The Northern Ordos Block

The region of the Northern Ordos Block(NOB),after colliding with the northern Yinshan Block(YB)at ～1.95 Ga under the convergence of the Colombian supercontinent to form a stable Western Block(WB)of North China Craton(NCC),has long been a large tectonic weak zone within the craton and has undergone multiple tectonic deformations.It is a natural laboratory for reconstructing the process of paleoplate interaction,studying intraplate deformation under modern systems,and exploring the inheritance and development relationship between the two.Moreover,the NOB is rich in natural gas,helium,uranium ore,and coal,making it one of the important mineral resource enrichment areas in China.Based on summarizing previous studies,through multidisciplinary research methods including geophysical analysis,regional tectonic analysis,and quantitative testing analysis,we systematically analyze the basement fault system and basement fault evolution model in the NOB,deeply exploring the relationship between the basement pre-existing fault system and the cratonization process,as well as the relationship between intraplate deformation and plate movement in the NOB.This not only has important scientific significance for understanding the formation and evolution of the WB,as well as the inheritance and development relationship between pre-existing tectonic zones and intraplate deformation within the craton but also provides an important basic geological basis for the study of the constraint relationship between deep structures and shallow mineral resources in the NOB.The main understanding gained is as follows:(1)The gravity and magnetic anomalies of 3rd to 4th-order detail are the reflection of the basement gravity and magnetic anomalies in the NOB.There are 6 basin boundary faults and 28 intra-basin basement faults have developed in the NOB,in which the tectonic framework of the intra-basin basement is dominated by NE and EW trending basement faults,and staggered or obliquely distributed by NW and SN trending basement faults,indicating the multi-stage and complexity of tectonic activity in the area.(2)There are three pre-existing tectonic zones in the basement of the NOB,among which the Borjianghaizi pre-existing tectonic zone is northward dipping and part of the thrust nappe tectonic belt and the Lijiaqu and Wulanjilinmiao pre-existing tectonic zone are both dips southward and belong to a part of the recoil tectonic belt.There are two sets of fault systems,namely,the Mesoproterozoic rift trough stage and the Phanerozoic cratonic basin stage,have developed in the caprock.The Mesoproterozoic fault system is dominated by NNW-trending extensional structures.The Phanerozoic fault system is dominated by EW trending structures,supplemented by NE and NW trending structures,and can be subdivided vertically into deep reverse fault systems and shallow normal fault systems.The basement fault during the caprock period is divided into four stages,namely,the Mesoproterozoic Changcheng Stage extension-rifting stage,the Caledonian-Hercynian multi-stage compression stage,the early Indosinian weak extension-late compression stage,and the early Yanshanian extension-middle compression-late re-extension stage.(3)The geometry of the pre-existing tectonic zone in the basement and the caprock fault system mainly interact in four ways: parallel type,merged type,cross-cutting type,and mixed type.The pronounced property and trend differences of the Mesoproterozoic rift fault system and the Phanerozoic fault system reveal that selective activation of basement pre-existing tectonics and regional tectonic stress fields jointly constrain the formation and evolution of caprock fault systems in multiple stages.Comparing the characteristics of fault systems in various stages of the Phanerozoic,it was found that the formation of the non-collinear fault system was the product of multiple tectonic activities and may also be caused by the basement pre-existing tectonics under the same stress field.Basement faults were activated early in the craton and were preferentially reactivated during later tectonic events under no significant changes in the direction of the regional stress field,making it difficult to generate large-scale new faults.(4)These rocks in the thrust nappe tectonic belt are mainly arc and back-arc rock associations composed of 2511 ± 6 Ma TTG gneisses,2499±5 Ma quartz monzonite gneisses,2505±8 Ma granodiorite gneisses,and 2463±7 Ma amphibolites,which reveals that the NOB might be in a geodynamic transition period from a compressional regime correlated with plate subduction to an extensional regime correlated with slab rollback during ca.2.51-2.46 Ga.The 2.2-2.0 Ga gra nitic gneiss in the recoil tectonic belt belongs to peraluminous S-type granite,originated in a subduction-related continental arc environment,and is formed by mixing ancient and new crustal materials;The protolith of the 2.0-1.95 Ga para metamorphic rocks is a set of greywacke and clay rocks with low maturity and mainly derived from felsic rocks in the upper crust.The age of detrital zircons is concentrated between 2.2 and 2.0 Ga,which is highly consistent with the age of granitic magmatism mentioned above,further supporting an active continental margin environment with island arc development in the middle Paleoproterozoic in the NOB.(5)The basement of NOB has crustal-scale crocodile tectonics on the flanks of the orogenic belt consisting of a northward dipping thrust belt and a southward dipping ramp structure within the basement and to fingerprint markedly different basement ages on both sides of the Thrust Front Belt(TFB).The footwall rocks(2.2-1.95 Ga)have a dominant affinity with the south-central OB,while the hanging wall rocks(2.51-2.45Ga)have no immediate affinity with the OB,possibly representing a set of late Neoarchean island arc formations developed in the northern margin of the OB(called the Hangjinqi Terrane).So our preferred interpretation is that the crocodile tectonics was formed by wedging the OB into the underthrusting Hangjinqi Terrane during the～1.95 Ga collision between the YB and OB under the convergence background of the Colombian supercontinent,and the TFB was delimited as the tectonic boundary between the OB and KB.Overall,the development and finalization of the basement preexisting fault system in the NOB are closely related to the multiple evolutionary stages of subduction,backarc extension,and continental collision that occurred in the late Neoarchean to the late Paleoproterozoic period in the region.Near EW trending basement pre-existing tectonic zone may have first developed in a backarc extensional tectonic setting and was finalized during late Paleoproterozoic continental collision;The NNW trending basement pre-existing fault is nearly perpendicular to the direction of the paleo-orogenic belt and belongs to a tearing fault formed during the NNW-SSE trending orogenic compression process.Their development and finalization are the product of the interaction between different Blocks during the supercontinent convergence process,providing evidence of tectonic deformation for the study of the supercontinent convergence and cratonization process.(6)The activity of near SN trending extensional faults during the Changcheng period in the NOB supports the opening of the Helan aulacogen during this period.The multi-stage fault activity in the NOB during the Phanerozoic is closely related to the plate margin tectonic activity of the North China Plate,recording the tectonic dynamic systems and their transformation processes in each stage under the interaction of the Paleo-Asian Ocean tectonic domain,the Tethyan Ocean tectonic domain,and the Coastal Pacific tectonic domain,providing evidence for further exploring the multistage coupling relationship between intraplate deformation and plate movement in the context of pre-existing tectonics of the craton.