Exhumation of Metamorphic Core Complexes through Progressive Subhorizontal Shearing, Doming, and Detachment Faulting: Insights from the Cretaceous Liaonan Metamorphic Core Complex, Eastern North China Craton

John P. Craddock,Junlai Liu,Yanqi Sun,Chunru Hou,Yuanyuan Zheng,Tamer S. Abu-Alam

doi:10.2113/2020/7866820

John P. Craddock, Junlai Liu + Show 4 more

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https://doi.org/10.2113/2020/7866820

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Abstract

Abstract The Early Cretaceous Liaonan metamorphic core complex (MCC), eastern North China craton, provides a field setting to evaluate progressive middle-upper crustal subhorizontal shearing, doming, and detachment faulting. The MCC is bounded by a western Jinzhou detachment fault zone (JDFZ) and a southern Dongjiagou shear zone (DSZ) that were primarily suggested to be two segments of the master detachment fault zone. Integrated structural, microstructural, quartz c-axis fabrics, and fluid inclusion analysis and zircon U-Pb dating on mylonites and syn-kinematic granites along the DSZ and JDFZ reveal that the DSZ possesses deformation characteristics that are obviously different from those along the JDFZ. The DSZ is composed of a Lower Unit of sheared Archean gneisses and an Upper Unit of sheared Neoproterozoic metasedimentary rocks, between which there is an obvious tectonic discontinuity contact (TDC). Rocks from below and above the TDC possess structures and fabrics with consistent geometries and kinematics with those along the JDFZ. A metamorphic break exists between the two units that were sheared at contrasting deformation conditions. Dating of zircons from syn-kinematic granitic dikes from DSZ yields an age of ca. 134 Ma, which is similar to the ages of early shearing along the JDFZ. It is concluded that the Jinzhou and Dongjiagou faults formed parts of a detachment faulting with top-to-the WNW kinematics. Exhumation of the Liaonan MCC shearing initiation along both the JDFZ and DSZ at an early stage (ca. 133~134 Ma), subsequent progressive shearing, and doming during slow cooling and exhumation before ca. 120 Ma, followed by fast cooling and rapid exhumation of the MCC by detachment faulting along the JDFZ until ca. 107 Ma.

Highlights

The concept of metamorphic core complex (MCC) was defined by studying the Tertiary Cordilleran extensional tectonics where a group of isolated, denuded, domal uplifts of anomalously deformed, metamorphic, and/or plutonic rocks are overlain by a tectonically detached and unmetamorphosed cover [1, 2]
We show from structural, thermometric, fabric analysis, and geochronological dating of the poorly studied Dongjiagou shear zone (DSZ) that exhumation of the Liaonan MCC is attributed to the combined effects of progressive subhorizontal shearing, doming, and detachment faulting
From integrated structural, thermometric, and fabric studies of the DSZ, that the shear zone possesses a tectonic discontinuity contact (TDC) separating highly sheared Archean gneisses metamorphosed up to lower amphibolite facies in the Lower Unit and sheared Neoproterozoic metasedimentary rocks up to lower greenschist facies in the Upper Unit, whereas the Jinzhou detachment fault zone (JDFZ) forms a typical detachment fault zone with a sequence of progressively sheared tectonites in direct contact with undeformed upper plate

Summary

Introduction

The concept of metamorphic core complex (MCC) was defined by studying the Tertiary Cordilleran extensional tectonics where a group of isolated, denuded, domal uplifts of anomalously deformed, metamorphic, and/or plutonic rocks are overlain by a tectonically detached and unmetamorphosed cover [1, 2]. As one of the most important lithosphere extensional structural styles, MCCs have been identified in many Phanerozoic orogenic belts (e.g., [3,4,5,6,7,8,9,10,11,12,13]) and Precambrian terrains (e.g., [14, 15]). A typical MCC contains three basic structural elements: (1) a detachment fault zone with gentle dips and large displacement around a domal high; (2) a lower plate of high-grade gneisses, sometimes intruded by syn-kinematic granitic plutons; and (3) faulted, but non- to low-grade metamorphosed supracrustal rocks and supradetachment basins in the upper plate [16]. The fault zone, separates the high-grade lower plate from the weakly faulted non- to low-grade metamorphosed upper plate.

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