The Louzidian Normal Fault near Chifeng, Inner Mongolia: Master Fault of a Quasi-Metamorphic Core Complex
35
Citation
27
Reference
10
Related Paper
Citation Trend
Abstract:
A special metamorphic core complex underlain by a low-angle strike-slip ductile shear zone is present near Chifeng in eastern Inner Mongolia, northern China. The geology of the study area is similar to that of several Cordilleran metamorphic core complexes, but contrasts in significant ways as well. A major ESE-dipping normal fault, the Louzidian Range frontal fault, formed during Late Cretaceous extension. This fault separates a crystalline footwall locally containing mylonitic basement gneisses and granitic rocks (0 to >3 km thick) from a non-metamorphic hanging wall that is distended by normal faults. However, the shear sense of the underlying mylonitic shear zone, a low-angle strike-slip zone, is not compatible with the Louzidian fault. It may be related to a pre-Cretaceous regional sinistral strike-slip event rather than the Late Cretaceous regional crustal extension common throughout eastern China. Pre-existing mylonitic fabric anisotropy appears to have controlled the development of the Louzidian normal fault. Chloritic breccias locally developed along the fault indicate that it cut deeply into the crust of northern China.Keywords:
Mylonite
Metamorphic core complex
Detachment fault
Lineation
Mylonite
Metamorphic core complex
Detachment fault
Greenschist
Lineation
Massif
Batholith
Cite
Citations (124)
Abstract The relationships between brittle detachment faulting and ductile shear zones in metamorphic core complexes are often ambiguous. Although it is commonly assumed that these two structures are kinematically linked and genetically related, direct observations of this coupling are rare. Here, we conducted a detailed field investigation to probe the connection between a detachment fault and mylonitic shear zone in the Ruby Mountain–East Humboldt Range metamorphic core complex, northeast Nevada. Field observations, along with new and published geochronology, demonstrate that Oligocene top-to-the-west mylonitic shear zones are crosscut by ca. 17 Ma subvertical basalt dikes, and these dikes are in turn truncated by middle Miocene detachment faults. The detachment faults appear to focus in preexisting weak zones in shaley strata and Mesozoic thrust faults. We interpret that the Oligocene mylonitic shear zones were generated in response to domal upwelling during voluminous plutonism and partial melting, which significantly predated the middle Miocene onset of regional extension and detachment slip. Our model simplifies mechanical issues with low-angle detachment faulting because there was an initial dip to the weak zones exploited by the future detachment-fault zone. This mechanism may be important for many apparent low-angle normal faults in the eastern Great Basin. We suggest that the temporal decoupling of mylonitic shearing and detachment faulting may be significant and underappreciated for many of the metamorphic core complexes in the North American Cordillera. In this case, earlier Eocene–Oligocene buoyant doming may have preconditioned the crust to be reactivated by Miocene extension, thus explaining the spatial relationship between structures.
Metamorphic core complex
Mylonite
Detachment fault
Lineation
Diachronous
Shearing (physics)
Dike
Cite
Citations (4)
Abstract The Picacho Mountains (SE Arizona, USA) are composed of a variety of Paleogene, Late Cretaceous, and Proterozoic granite and gneisses that were deformed and exhumed along the gently south to southwest dipping detachment shear zone associated with the Picacho metamorphic core complex. The detachment shear zone is divided into three sections that record a progressive deformation gradient, from protomylonites to ultramylonites, and breccia. New thermochronological data from mylonite across the footwall of the detachment shear zone associated with the Picacho metamorphic core complex suggest that the footwall was exhumed through about ~300°C between 22 and 18 Ma by progressive incisement of the footwall of the detachment shear zone. Combined geochronological and oxygen and hydrogen stable isotope data of metamorphic silicate minerals reveal that mylonite recrystallization occurred in the presence of a deep‐seated metamorphic/magmatic fluid, and experienced a late stage meteoric overprint during the development and exhumation of the detachment shear zone. Quartz‐biotite and quartz‐hornblende geothermometry from the base to the top of the detachment shear zone yield equilibrium temperatures ranging from 630 to 415°C, respectively. This temperature trend is attributed to an insulating effect caused by rapid slip and juxtaposition of cool hanging wall on top of a hot footwall. It is suggested that rapid cooling of the top of the detachment shear zone caused strain to migrate toward lower structural level by incisement of the footwall of the shear zone. Progressive strain front migration into the ductile footwall produced hydromechanical anisotropies parallel to the detachment shear zone, effectively saturating the footwall with magmatic/metamorphic fluids and preventing downward flow of meteoric fluids. The combined microstructural, geochronological, and stable isotope results presented in this study provide insight on the dynamic feedback between deformation and fluid flow during the evolution of a detachment shear zone.
Mylonite
Metamorphic core complex
Detachment fault
Hornblende
Cite
Citations (12)
A sinuous zone of gently southeast-dipping low-angle Tertiary normal faults is exposed for 100 km along the eastern margins of the Anaconda and Flint Creek ranges in southwest Montana. Faults in the zone variously place Mesoproterozoic through Paleozoic sedimentary rocks on younger Tertiary granitic rocks or on sedimentary rocks older than the overlying detached rocks. Lower plate rocks are lineated and mylonitic at the main fault and, below the mylonitic front, are cut by mylonitic mesoscopic to microscopic shear zones. The upper plate consists of an imbricate stack of younger-on-older sedimentary rocks that are locally mylonitic at the main, lowermost detachment fault but are characteristically strongly brecciated or broken. Kinematic indicators in the lineated mylonite indicate tectonic transport to the east-southeast. Syntectonic sedimentary breccia and coarse conglomerate derived solely from upper plate rocks were deposited locally on top of hanging-wall rocks in low-lying areas between fault blocks and breccia zones. Muscovite occurs locally as mica fish in mylonitic quartzites at or near the main detachment. The 40 Ar/ 39 Ar age spectrum obtained from muscovite in one mylonitic quartzite yielded an age of 47.2 + 0.14 Ma, interpreted to be the age of mylonitization. The fault zone is interpreted as a detachment fault that bounds a metamorphic core complex, here termed the Anaconda metamorphic core complex, similar in age and character to the Bitterroot mylonite that bounds the Bitterroot metamorphic core complex along the Idaho-Montana state line 100 km to the west. The Bitterroot and Anaconda core complexes are likely components of a continuous, tectonically integrated system. Recognition of this core complex expands the region of known early Tertiary brittle-ductile crustal extension eastward into areas of profound Late Cretaceous contractile deformation characterized by complex structural interactions between the overthrust belt and Laramide basement uplifts, overprinted by late Tertiary Basin and Range faulting.
Mylonite
Detachment fault
Metamorphic core complex
Breccia
Conglomerate
Country rock
Cite
Citations (30)
Abstract The article describes the characteristics of the Yagan metamorphic core complex, especially the associated detachment fault and various extensional structures in its footwall. The age of the complex is discussed in some detail as well. The basic features of the Yagan metamorphic complex (Jurassic in age) are similar to those of the metamorphic core complex (Tertiary in age) in the Cordilleran area; they are as follows: (a) mylonitic gneisses in the footwall, (b) chloritized sheared mylonitic rocks, (c) pseudotachylites and flinty cataclasites or microbreccias, (d) unmetamorphosed or epimetamorphic rocks in the hanging wall with a layer of fault gouges or incohesive fault breccia next to the detachment fault. In contrast to its Cordilleran counterpart, however, there are many extensional faults with different styles (from dactile low‐angle normal faults through brittle — ductile to brittle high — angle normal faults) in the footwall.
Metamorphic core complex
Mylonite
Detachment fault
Breccia
Cite
Citations (39)
Mylonite
Metamorphic core complex
Detachment fault
Cite
Citations (49)
The previously undiscovered Waziyu metamorphic core complex in the Yiwulu Shan, a mountain range in western Liaoning Province, consists of a master, west-dipping, low-angle normal fault, the Waziyu detachment, that separates a hanging wall of dominantly Early Cretaceous sedimentary and volcanic units from a footwall of mylonitic and non-mylonitic units. Exposures of the Waziyu detachment fault (previously called the Sunjiawan-Shaohuyingzi fault) along the western flank of the range have dips of 10～40° and excellent kinematic indicators that give consistent top-to the west-northwest sense of shear (ca. 290°). Mylonitic units in the footwall associated with Early Cretaceous crustal extension yield the same top-to the west-northwest sense of shear and are kinematically related to the Waziyu detachment fault. The timing of extension and metamorphic core complex development in the Yiwulu Shan is broadly constrained as Early Cretaceous (ca.127～116 Ma) by published and unpublished U-Pb geochronology,~(40)Ar/~(39)Ar thermochronology, and stratigraphic age determinations based upon biostratigraphy in the hanging wall supradetachment Fuxin basin. We have found no evidence for a symmetrical Yiwulu Shan mcc as reported in earlier literature. Recognition of the Waziyu mcc and its WNW-rooting detachment fault adds to our understanding of the extensional behavior of the North China crust. Future work in the Yiwulu Shan should include field-based structural studies to define the spatial extent of both the detachment fault and the kinematically related footwall mylonites, collection of additional samples for geo/thermochronology, and petrologic studies of the plutons within the range. We attribute formation of the Waziyu metamorphic core complex to collapse of an orogenically thickened crust that was facilitated by thermal weakening due to Early Cretaceous magmatism and paleo-Pacific plate boundary reorganization.
Mylonite
Metamorphic core complex
Detachment fault
Thermochronology
Geochronology
Cite
Citations (60)
Although crystalline rocks dominate the footwall of the Buckskin-Rawhide detachment fault in west-central Arizona (USA), we estimate that thin (<1 to 100 m thick) calcite-rich metasedimentary mylonites were present along 25%–35% of the detachment fault, and in parts of the footwall they were continuous for ∼30 km in the slip direction. New field observations, geochronology, and detailed microstructural data provide insight into the origin of these metasedimentary rocks and their role in the structural evolution of the detachment fault system. We propose that calc-mylonites along the Ives Peak footwall corrugation were derived from locally overturned Pennsylvanian–Permian strata that were buried to mid-crustal depths beneath a southeast-vergent Cretaceous thrust fault, which was reactivated in the Miocene by the parallel Buckskin detachment fault shear zone. In some areas these laterally persistent calc-mylonites were smeared out along the detachment fault during incisement into the crystalline footwall, forming a thin carapace of rheologically weak rocks structurally below the original weak zone. Metasedimentary mylonites consistently record top-to-the-northeast simple shear parallel to the detachment fault slip direction. Strain, synmylonitic veins, and paleostress recorded in these mylonites increase toward the detachment fault. Marble mylonites <1 m below the detachment fault preserve strong calcite crystallographic preferred orientations and lack cataclastic deformation that characterizes quartz-rich rocks along the detachment fault. In addition, unlike quartzofeldspathic mylonites, calc-mylonites typically lack extension via postmylonitic normal faults and associated horizontal axis rotation. Paleopiezometry and rheological modeling of the metasedimentary mylonites suggest that when quartzite layers were being sheared at ∼100 MPa and 10−13 to 10−14 s−1 near the brittle-plastic transition, marble layers could have been strained ∼100× faster at ∼20 MPa. Detachment fault strain localized within the metasedimentary rocks, and the calcite marbles exerted significant control on the rheology of the footwall shear zone. This study highlights the important role that inherited weak zones may play in influencing the location, geometry, rheology, and style of deformation associated with detachment fault systems.
Mylonite
Detachment fault
Metamorphic core complex
Cataclastic rock
Lineation
Cite
Citations (20)
The kinematics of ductile shearing is not compatible with that of detachment faulting in the detachment systems of the Louzidian metamorphic core complex, which is different from the Cordillera metamorphic core complex in North America. Structural analysis of the detachment systems on the two sides of the core complex shows consistent top-to-the-northeast ductile shearing. Three biotites separated from mylonitic rocks in the detachment systems on the two sides of the core complex yielded 40Ar/39Ar plateau ages between 126-128 Ma and one hornblende separated from mylonitic rocks on the west side yielded a 40Ar/39Ar plateau age of 134 Ma. Four 40Ar/39Ar plateau ages, which are concordant with corresponding isochronal ages, represent a range of ductile shearing and suggest ductile extension. Studies show that the ductile shear zones on the two sides of the Louzidian core complex have the same formation and kinematics and ductile shearing is an important stage of the formation and evolution of the core complex, which provides chronic evidence for the constraints of ductile extension.
Mylonite
Metamorphic core complex
Detachment fault
Shearing (physics)
Hornblende
Cite
Citations (9)
<p>Details of U-Pb zircon dating, complete datatables of U-Pb zircon dating, and a compilation of muscovite and biotite Argon dates.</p>
Mylonite
Metamorphic core complex
Detachment fault
Muscovite
Shearing (physics)
Cite
Citations (0)