Miocene exhumation of northeast Pamir: Deformation and geo/thermochronological evidence from western Muztaghata shear zone and Kuke ductile shear zone
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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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The tectonic setting of a transcontinental belt of 1500–1300 Ma granitic intrusions that extends from southern California to Labrador is controversial; however, the granites are conventionally considered to be “anorogenic.” Detailed field, microstructural, and geochronologic data from the 1425 Ma Beer Bottle Pass pluton, southern Nevada, indicate that major mylonite zones recording dextral‐contractile strains were probably active during and/or shortly after pluton emplacement and suggest that the anorogenic interpretation for this pluton requires reevaluation. Mylonite zones up to 100 m thick strike northeast, dip moderately northwest, and contain a consistent west plunging elongation lineation. Mylonites occur along 15% of the exposed granite‐wall rock contact and extend into both the pluton and the wall rock. Mesoscopic and microscopic kinematic indicators record an oblique, dextral/reverse (pluton side down) movement sense. Synkinematic mineral assemblages of hornblende and biotite and dynamic recrystallization of feldspars suggest that deformation occurred minimally under amphibolite facies conditions. A K/Ar biotite date of 1399±32 Ma, obtained from a sample of mylonitic granite, suggests that deformation took place during or soon following pluton crystallization. We reject forcible emplacement of the Beer Bottle Pass pluton as a mechanism for formation of the mylonite zones because (1) rocks near the granite‐wall rock contact are largely unstrained, (2) the mylonite zones conform only locally to the pluton‐wall rock contact, (3) mylonite zones strike at high angles to, and truncate, the intrusive pluton‐wall rock contact, (4) the pluton‐side‐down shear sense is more compatible with a uniform sense simple shear zone than a forcibly intruding pluton, and (5) fabrics indicative of noncoaxial deformation dominate over flattening fabrics. We suggest that the Beer Bottle Pass pluton is fundamentally synkinematic with respect to either (1) a local, contractile deformational event or (2) regional strains produced by distant plate tectonic processes operative between 1500 and 1300 Ma as suggested by Nyman and Karlstrom (1994). The timing and kinematics of shear zones associated with other circa 1400 Ma plutons in the western United States (e.g., Graubard and Mattinson, 1990; Nyman et al., 1994; Kirby et al., this issue) support the latter interpretation. Our kinematic data are more consistent with a contractile or transpressive tectonic setting at circa 1400 Ma (Nyman and Karlstrom, 1994; Nyman et al, 1994) than with models involving regional extension (e.g., Windley, 1993).
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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 near NS ductile shear zone was first discovered in Proterozoic Bendong granodiorite pluton. It dips toward 240°～280° at the dip angle of 40°～63°. The lineations, which represent the shear direction, plunge to 216°～226° at the plunge angle of 39°～46°. The kinematics of Bendong ductile shear zone is characterized by sinistral-normal shearing, and shows a sliding from NE to SW. Typical granitic mylonites were developed in the ductile shear zone and clearly show a zoning from phyllonite through mylonite to initial mylonite from center to both walls. The discovery of Bendong ductile shear zone and mylonite indicates an important structural event superimposed on Proterozoic Bendong pluton.
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The eastern margin of the Chitradurga schist belt is marked by a NNW-SSE trending sub vertical crustal scale ductile shear zone. The kinematic indicators indicate a predominant sinistral sense of strike-lip movement along the shear zone. Syntectonically emplaced granitic rocks are converted to mylonites and ultramylonites as a result of crystalplastic deformation in the shear zone. In contrast, there are localized zones of brittle failure with attendant functional heat generation exemplified by the development of thin but conspicuous bands and veins of pseudotachylytes, which are emplaced either subparallel with or transgressing the C-planes of the mylonites. From our field and petrographic studies it is interpreted that these two coexisting rock types, namely the mylonite and pseudotachylyte, which are the results of contrasting deformational mechanisms, have generated near synchronously in a progressively developed ductile shear zone. The pseudotachylytes represent the brief interlude of sudden increase in strain rate in an overall ductile regime.
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