The Homestake shear zone, one of the principal Precambrian structures within the Colorado mineral belt, has a history of tectonism that extends from the Proterozoic to the Tertiary. New field mapping, microstructural analysis, and electron-microprobe U-Th-Pb monazite dates define the style, kinematics, and timing of multistage Proterozoic ductile deformation. Early subhorizontal shearing (D1) progressed to northwest-southeast crustal shortening (D2) during a protracted ca. 1710-1630 Ma orogeny involving high-temperature metamorphism, partial melting, and emplacement of the ca. 1.7 Ga Cross Creek batholith. The subvertical, northeast trend of the shear zone was established by D2 shortening. A major episode of simple-shear displacement occurred ca. 1.4 Ga during emplacement of the St. Kevin batholith. Complex kinematics are recorded by 1380 Ma, southeastside-down, dextral mylonite zones (D3) and post-1380 Ma southeast-side-up, dextral ultramylonite-pseudotachylyte zones (D4) that were both localized in D2 foliation domains. The succession of ductile tectonites in the Homestake records Ͼ300 m.y. of episodic deformation at progressively shallower crustal levels as Colorado evolved from an active accretionary margin (1.8-1.7 Ga), through crustal assembly (1.7-1.6 Ga), to a tectonically active continental interior (post 1.45 Ga). The Homestake shear zone constitutes a classic example of ductile shear-zone reactivation and the development of persistent tectonic zones in the continental lithosphere.
A major geologic boundary has been proposed in the Southern Rocky Mountains separating Proterozoic crustal provinces with different ages and tectonic histories. These provinces probably correlate with the Yavapai (1.8-1.7 Ga) and Mazatzal (1.7-1.6 Ga) provinces of Arizona. Geologic, geochemical, geochronologic, and xenolith data suggest that the boundary lies within a ~3 0 0 km-wide zone that trends northeastward through southern Colorado and northern New Mexico. This zone also seems to have focused later tectonic and thermal effects. However, no major shear zone that might represent a discrete tectonic suture has been identified in the area, and there is no agreement on precisely where the boundary is or what tectonic significance it may have.We present a review of evidence supporting extrapolation of the Yavapai-Mazatzal boundary through the Southern Rocky Mountains. Limitations in the precision, quantity, and interpretation of available data probably contribute to disagreement over the location of the boundary. However, the disparity in boundaries defined by different data sets may partly reflect a complex or gradational transition between crustal domains. We propose a speculative model for the boundary based on a preliminary structural analysis. lkctonic fabrics appear to be consistent with the initial juxtaposition of arc terranes of the Yavapai and Mazatzal provinces on a lowangle thrust system with later modification and steepening of the boundary during continued crustal shortening. This model explains the diffuse isotopic boundary as a manifestation of a vertically heterogeneous crustal column that might promote isotopic mixing. The cryptic structural expression of the suture may result from a layer-parallel style of suturing and complex post-accretionary tectonic overprinting.
Mantle-derived helium in hot springs of the cordillera Blanca, Peru: Implications for mantle-to-crust fluid transfer in a flat-slab subduction setting, ABSTRACT Fault-controlled hot springs in the Cordillera Blanca, Peru provide geochemical evidence of mantle-derived fluids in a modern flat-slab subduction setting. The Cordillera Blanca is a ~200 km-long mountain range contains the highest peaks in the Peruvian Andes, located in an amagmatic reach of the Andean arc. The Cordillera Blanca detachment defines the southwestern edge of the range and records a progression of top-down-to-the-west ductile shear to brittle normal faulting since ~5 Ma. Hot springs, recording temperatures up to 78 °C, issue along this fault zone and are CO 2 -rich, near neutral, alkaline-chloride to alkaline-carbonate waters, with elevated trace metal contents including arsenic (≤11 ppm). Water 18 O SMOW (-14.2 to -4.9 ‰) and D SMOW (-106.2 to -74.3 ‰), trends in elemental chemistry, and cation geothermometry collectively demonstrate mixing of hot (200-260°C) saline fluid with cold meteoric water along the fault. Helium isotope ratios ( 3 He/ 4 He) for dissolved gases in the waters range from 0.62 to 1.98 R A (where R A = air 3 He/ 4 He), indicating the presence of up to 25% mantle-derived helium.Given the long duration since and large distance to active magmatism in the region, and the possible presence of a tear in the flat slab south of the Cordillera Blanca, we suggest that mantle helium may originate from asthenosphere entering the slab tear, or from the continental mantlelithosphere, mobilized by metasomatic fluids derived from slab dehydration.
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A northeast-striking system of subvertical mylonites and ultramylonites, which formed in the Mesoproterozoic, provided a zone of weakness and conduit for the Paleocene to Oligocene magmatism and mineralization that are the Phanerozoic expressions of the Colorado Mineral Belt. The mylonites overprinted higher temperature Paleoproterozoic high-strain domains of similar orientation. Here, we distinguish the Phanerozoic Colorado Mineral Belt from a Proterozoic 'Colorado Mineral Belt shear zone system.' In each segment of the shear zone system, Mesoproterozoic mylonite strands, which are meters to tens of meters wide, overprint highertemperature Paleoproterozoic high-strain domains, which are several kilometers wide. In situ electron microprobe monazite dating of the mylonites and higher temperature high-strain domains, and field studies of relative timing of shearing and pluton emplacement, show two main periods of shearing that each involve ~100 Ma of deformation. Higher temperature high-strain domains record pulses of deformation that occurred at 1.71-1.69 Ga, 1.67 Ga, 1.65 Ga, and 1.62 Ga. Mylonites record movement at 1.45 Ga synchronous with emplacement of the Mt. Evans pluton, at 1.42 Ga synchronous with emplacement of the Silver Plume pluton, and at 1.38 Ga synchronous with emplacement of the St. Kevin pluton. Post-1.38 Ga movements created ultramylonites. This shear zone system may be analogous to modern-day intracontinental zones of weakness like the Tien Shan of central Asia, which record both original assembly of tectonic blocks and recurrent reactivation during later plate convergence at a distant margin.
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