Mylonitization in the lower plate of the Buckskin-Rawhide detachment fault, west-central Arizona: Implications for the geometric evolution of metamorphic core complexes
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Metamorphic core complex
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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.
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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.
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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.
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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.
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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.
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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.
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<p>Details of U-Pb zircon dating, complete datatables of U-Pb zircon dating, and a compilation of muscovite and biotite Argon dates.</p>
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