Crustal shortening, exhumation, and strain localization in a collisional orogen: the Bajo Pequeño Shear Zone, Sierra de Pie de Palo, Argentina

The Bajo Pequeño Shear Zone (BPSZ) is a lower-crustal shear zone that records shortening and exhumation associated with the establishment of a new plate boundary, and its placement in a regional structural context suggests that local- to regional-scale strain localization occurred with progressive deformation. A kilometer-scale field and analytical cross section through the ~80 m thick BPSZ and its adjacent rocks indicates an early Devonian (405–400 Ma) phase of deformation on the western margin of Gondwanan continental crust. The earliest stages of the BPSZ, recorded by metamorphic and microstructural data, involved thrusting of a hotter orthogneiss over a relatively cool pelitic unit, which resulted in footwall garnet growth and reset footwall white mica 40Ar/39Ar ages in proximity to the shear zone. Later stages of BPSZ activity, as recorded by additional microstructures and quartz c-axis opening angles, were characterized by strain localization to the center of the shear zone coincident with cooling and exhumation. These and other data suggest that significant regional tectonism persisted in the Famatinian orogenic system for 60–70 million years after one microplate collision (the Precordillera) but ceased 5–10 million years prior to another (Chilenia). A survey of other synchronous structures shows that strain was accommodated on progressively narrower structures with time, indicating a regional pattern of strain localization and broad thermal relaxation as the Precordillera collision evolved.Fil: Garber, Joshua M.. University of California at Davis; Estados UnidosFil: Roeske, Sarah M.. University of California at Davis; Estados UnidosFil: Warren, Jessica. University of Stanford; Estados UnidosFil: Mulcahy, Sean R.. University of California at Berkeley; Estados UnidosFil: McClelland, William C.. University of Iowa; Estados UnidosFil: Austin, Lauren J.. University of Oregon; Estados UnidosFil: Renne, Paul R.. University of California at Berkeley; Estados UnidosFil: Vujovich, Graciela Irene. Universidad de Buenos Aires. Facultad de Ciencias Exactas y Naturales. Departamento de Geología. Laboratorio de Tectónica Andina; Argentina. Consejo Nacional de Investigaciones Científicas y Técnicas; Argentin

Crustal Shortening, Exhumation, and Strain Localization in a Collisonal Orogen: the Bajo Pequeno Shear Zone, Sierra de Pie de Palo, Argentina

The Bajo Pequeño Shear Zone (BPSZ) is a lower-crustal shear zone that records shortening and exhumation associated with the establishment of a new plate boundary, and its placement in a regional structural context suggests that local- to regional-scale strain localization occurred with progressive deformation. A kilometer-scale field and analytical cross section through the ~80 m thick BPSZ and its adjacent rocks indicates an early Devonian (405–400 Ma) phase of deformation on the western margin of Gondwanan continental crust. The earliest stages of the BPSZ, recorded by metamorphic and microstructural data, involved thrusting of a hotter orthogneiss over a relatively cool pelitic unit, which resulted in footwall garnet growth and reset footwall white mica 40Ar/39Ar ages in proximity to the shear zone. Later stages of BPSZ activity, as recorded by additional microstructures and quartz c-axis opening angles, were characterized by strain localization to the center of the shear zone coincident with cooling and exhumation. These and other data suggest that significant regional tectonism persisted in the Famatinian orogenic system for 60–70 million years after one microplate collision (the Precordillera) but ceased 5–10 million years prior to another (Chilenia). A survey of other synchronous structures shows that strain was accommodated on progressively narrower structures with time, indicating a regional pattern of strain localization and broad thermal relaxation as the Precordillera collision evolved

Metamorphism of the Sierra de Maz and implications for the tectonic evolution of the MARA terrane

The Mesoproterozoic MARA terrane of western South America is a composite igneous-metamorphic complex that is important for Paleozoic paleogeographic reconstructions and the relative positions of Laurentia and Gondwana. The magmatic and detrital records of the MARA terrane are consistent with a Laurentian origin; however, the metamorphic and deformation records lack sufficient detail to constrain the correlation of units within the MARA terrane and the timing and mechanisms of accretion to the Gondwana margin. Combined regional mapping, metamorphic petrology, and garnet and monazite geochronology from the Sierra de Maz of northwest Argentina suggest that the region preserves four distinct litho-tectonic units of varying age and metamorphic conditions that are separated by middle- to lower-crustal ductile shear zones. The Zaino and Maz Complexes preserve Barrovian metamorphism and ages that are distinct from other units within the region. The Zaino and Maz Complexes both record metamorphism ca. 430–410 Ma and show no evidence of the regional Famatinian orogeny (ca. 490–455 Ma). In addition, the Maz Complex records an earlier granulite facies event at ca. 1.2 Ga. The Taco and Ramaditas Complexes, in contrast, experienced medium- and low-pressure upper amphibolite to granulite facies metamorphism, respectively, between ca. 470–460 Ma and were later deformed at ca. 440–420 Ma. The Maz shear zone that bounds the Zaino and Maz Complexes records sinistral oblique to sinistral deformation between ca. 430–410 Ma. The data suggest that at least some units in the MARA terrane were accreted by translation, and the Gondwana margin of northwest Argentina transitioned from a dominantly convergent margin to a highly oblique margin in the Silurian

Multiple migmatite events and cooling from granulite facies metamorphism within the Famatina arc margin of northwest Argentina

The Famatina margin records an orogenic cycle of convergence, metamorphism, magmatism, and extension related to the accretion of the allochthonous Precordillera terrane. New structural, petrologic, and geochronologic data from the Loma de Las Chacras region demonstrate two distinct episodes of lower crustal migmatization. The first event preserves a counterclockwise pressure-temperature path in kyanite-K-feldspar pelitic migmatites that resulted in lower crustal migmatization via muscovite dehydration melting at ~12 kbar and 868°C at 461 ±1.7 Ma. The shape of the pressure temperature path and timing of metamorphism are similar to those of regional midcrustal granulites and suggest pervasive Ordovician migmatization throughout the Famatina margin. One-dimensional thermal modeling coupled with regional isotopic data suggests Ordovician melts remained at temperatures above their solidus for 20–30 Ma following peak granulite facies metamorphism, throughout a time period marked by regional oblique convergence. The onset of synconvergent extension occurred only after regional migmatites cooled beneath their solidus and was synchronous with the cessation of Precordillera terrane accretion at ~436 Ma. The second migmatite event was regionally localized and occurred at ~700°C and 12 kbar between 411 and 407 Ma via vapor saturated melting of muscovite. Migmatization was synchronous with extension, exhumation, and strike-slip deformation that likely resulted from a change in the plate boundary configuration related to the convergence and collision of the Chilenia terrane

Computed Tomography (CT) of five samples of the Sutter's Mill CM2 chondrite.

These files supplement Jenniskens et al. (2012), a comprehensive description of the April 22, 2012 fall and the petrology of the Sutter‟s Mill CM2 chondrite breccia. Here, we present 3-dimensional scans of individual stones of this meteorite. A “Methods” document in this repository records particulars of CT (see Ebel and Rivers 2007). In the Science paper, we note that “samples SM3 and SM9 appear to contain a dominant lithology characterized by abundant 200 to 400 μm diameter clasts (chondrules or CAIs), and 0.05 - 0.15 μm metal oxide or sulfide grains. A second lithology, with higher average atomic mass (Z) matrix and more abundant clasts, appears as irregular, angular lithic fragments many mm in size. At least one metal grain ~250 μm across, was observed, surrounded by a halo ~750 μm wide, of oxidized or sulfidized metal. It is unlikely that such a grain would be sampled by random cutting. Several clasts larger than 1 mm include a low-Z spherical object that appears to be concentrically zoned, and a similar object with zoned high-Z (metal) and low-Z (silicate) layers. While the samples are fractured, and metal grains appear to be altered, no high-Z veins (e.g., FeO-rich) are observed.” And, “the meteorites studied so far exhibit a dominant, primary lithology that is the host for multiple types of exotic lithic clasts.” This lithology is evident in most of SM3. In SM3_13A, at ~30/45 sec running time, more lithic clasts appear, and a large metal-cored grain rimmed by metal sulfide or oxide, appears briefly. The oriented sample SM51 illustrates the asymmetry of fusion crust, thick on the trailing side, very thin on the leading side (top of movie), and thickest at the „lip‟ between these surfaces (image right). A slightly brighter clast (higher average Z) that intersects the leading side fusion crust at ~60/100 sec illustrates a small effect of its composition on crust thickness and composition. A clast-poor lithology is prevalent through the first half of the stack. A large metal grain is present at ~57/100. Two large chondrules appear in SM51 at ~72/100 sec, and the lithology between there and the end is rich in low Z (forsterite-rich?) spherical clasts. In SM54S, fusion crust is very prominent, sweeping left to right in the first few seconds. Several lithologies are present, perhaps four at ~11/83 sec. This sample has some low-Z terrestrial contamination, a reddish clay, that thinly fills small depressions in the sample at the bottom of the images. References Ebel, D. S. and M. L. Rivers. 2007. Meteorite 3-dimensional synchrotron micro-tomography: Methods and applications. Meteoritics and Planetary Science 42: 1627-1646. Jenniskens, P. and 69 coauthors. 2012. Radar enabled recovery of the Sutter‟s Mill meteorite, a carbonaceous chondrite regolith breccia. Science 21 December 2012: Vol. 338 no. 6114 pp. 1583-1587. DOI: 10.1126/science.122716