Christopher Gerbi

Geology and structural history of the southwest Precordillera margin, northern Mendoza Province, Argentina

Abstract Rocks and structures in the southwest Precordillera terrane, located in western Argentina, constrain the Paleozoic distribution of continents and the development of the western margin of Gondwana. Detailed mapping of an area in the southwest Precordillera allowed identification of several pre-Carboniferous rock units formed in distinct tectonic environments and were later tectonically juxtaposed. The pre-Carboniferous rock units comprise carbonate metasiltstone, metasandstone, massive diabase, and quartzo-feldspathic gneiss intruded by ultramafic rocks and layered gabbro. Preliminary structural analysis indicates that the present distribution of units is due to two contractional deformation episodes, an east-directed Devonian ductile event and a west-directed Tertiary brittle event. The metasedimentary rocks, which form the structural base of the area and are part of the western Precordilleran passive margin sequence, were juxtaposed along minor ductile shear zones early in the ductile event. Their contact was then folded during continued ductile deformation; at this time the ultramafic/layered gabbro complex and the massive diabase were emplaced over the metasedimentary units along narrow ductile shear zones. Brittle deformation, associated with the Andean orogeny, involved open folding, thrust faulting, and reactivation of some ductile features.

Use of U-Pb geochronology to identify successive, spatially overlapping tectonic episodes during Silurian-Devonian orogenesis in south-central Maine, USA

Silurian-Devonian orogenesis formed the dominant metamorphic and deformational features recorded in the south-central Maine portion of the northern Appalachians. New U-Pb ages for metamorphic monazite and zircon from this high-grade segment of the orogen, coupled with new and previously published igneous and metamorphic ages, reveal the presence of three distinct tectono-metamorphic zones. All three zones experienced an early phase or phases of contractional deformation in the Late Silurian to Early Devonian. In the Eastern and Central Zones, this contractional deformation was synchronous with low-pressure, amphibo-lite-facies metamorphism and accompanied by plutonism in the Central Zone during the waning stages of this phase. Beginning in Middle Devonian time, the kinematics of deformation in the Central and Western Zones became strongly transpressive, accompanied by low-pressure, amphibolite-facies metamorphism and plutonism. The Eastern Zone underwent no dextral transpression or amphibolite-facies metamorphism during this later event. Contemporaneous, spatially partitioned contractional and transcurrent strain is a feature of many transpressional orogens, but in south-central Maine, more than 30 m.y. passed between the peaks of the older contractional and younger dextral deformational episodes. Therefore, the system does not reflect contemporaneous strain partitioning as in a transpressional regime. Rather, our results, along with regional records of Late Silurian-Early Devonian sinistral and Middle Devonian dextral deformation features, are consistent with a significant change in plate kinematics between 430 and 370 Ma.

Computational analysis of nonlinear creep of polyphase aggregates: Influence of phase morphology

The constitutive laws of polyphase aggregates dominantly depend on the operative deformation mechanisms, phase morphology and modes, and environmental conditions. Each of these factors has the potential to dramatically affect bulk mechanical properties as well as the local stress and strain rate distributions. To focus on the effects of phase morphology, we have developed a rigorous multiscale approach based on asymptotic expansion homogenization. The proposed methodology has two fundamental goals: (1) accurately predict bulk behavior in aggregates by explicitly taking into account phase morphology and (2) calculate detailed distributions of strain rates, stresses, and viscosities in heterogeneous materials. The methodology is able to consider general nonlinear phase constitutive laws that relate strain rates to stresses, temperature, and other factors such as water fugacity and grain size. We demonstrate the approach by analyzing power law creep of computer-generated and natural polyphase systems and benchmarking the results against analytical solutions. As an outcome of this analysis, we find that the approximation of an aggregate as a power law material is reasonable for isotropic, homogeneous phase distributions but breaks down significantly with high degrees of phase organization. We also present distributions in strain rate, stress, and viscosity for different applied loading conditions. Results exhibit areas of high internal stresses and substantial localization. We describe and provide a freely available software package supporting these calculations.

Influence of microscale weak zones on bulk strength

Shear zones have different rheological properties than the surrounding rocks, indicating that the bulk strength of regions containing shear zone networks cannot be determined by considering the the host rock rheology alone. We demonstrate the value of this concept at the microscale. We first consider the phase arrangements in naturally deformed rocks and document that weak phases exhibit little interconnection within a microstructure. Rather, three-dimensional weak zones, analogous to viscous shear zones, can interconnect or bridge weak phases. These zones typically form at high stress sites, comprise multiple minerals, and deform by mechanisms independent of those in the surrounding minerals. The presence of weak zones strongly affects the bulk strength of the rock, disproportionate to the mode of the weak zones. For example, the development of 1% mode of a weak zone at a high stress site can reduce the bulk strength of the rock nearly an order of magnitude. Calculation of the bulk strength of the rock by some averaging algorithm of the deformation mechanisms operating outside the weak zones will overestimate strength. Instead, accurate calculations and predictions of bulk strength require accounting for the presence and geometry of weak zones. For this reason, we advocate use of the scale-independent conceptual rheological model of interconnected weak zones or layers rather than that of interconnected weak phases. More generally, the way forward in improving quantification of the mechanical properties of the lithosphere requires recognizing and explicitly accounting for the spatial and temporal distribution of deformation mechanisms operating throughout a rock. This article is protected by copyright. All rights reserved.

Timing and conditions of poly-phase metamorphism within the Twelve Mile Bay shear zone: implications for the evolution of mid-crustal decollement zones and western Grenville tectonics

The Twelve Mile Bay assemblage (TMBa) forms the high-strain interior of the Twelve Mile Bay shear zone (TMBsz), a major ductile decollement zone within the western Canadian Grenville orogen. Metasupracrustal gneiss within the TMBa preserves evidence for an early granulite facies (˜10–11 kbar and ˜840°C) metamorphism overprinted by amphibolite facies (˜5–7 kbar and ˜650°C) assemblages that define the high-strain shear zone fabric. U–Pb zircon ages for TMBa samples were determined by LA-ICP-MS. A low-strain amphibolite pod with partially preserved granulite facies assemblage and textures yielded an anchored discordia intercept of 1157 ± 11 Ma and 207Pb/206Pb weighted average of 1146 ± 10 Ma. Three higher strain samples with recrystallized amphibolite facies assemblages all yield younger ages, with 207Pb/206Pb weighted averages of 1125 ± 16, 1110 ± 8, and 1095 ± 17 Ma. Phase equilibrium modelling shows that up to 40 vol.% anatectic melt could have been produced in TMBa pelitic rocks during peak metamorphic conditions, and thus, much of the package likely would have been substantially weakened during the early stages of TMBsz development. Strain apparently continued to accumulate within the TMBa until ca. 1100 Ma, concurrent with pegmatite dike emplacement and hydration along the base of the overlying interior Parry Sound domain (iPSD), perpetuating TMBsz activity during cooling and exhumation to shallower crustal levels. Similarities between the TMBa and the upper parts of the basal PSD (bPSD), in terms of timing and conditions of metamorphism and shearing, as well as structural position relative to the overlying iPSD allochthon, indicate that these units are likely correlative. The composite bPSD–TMBa system appears to have contemporaneously localized strain within the middle orogenic crust during early to middle stages of Grenvillian collision, providing a petrologically constrained mechanism for the long distance transport of mid-crustal nappes predicted in thermal-mechanical models of continental collision for this area.

Chopinite-sarcopside solid solution, [(Mg,Fe)3□](PO4)2, in GRA95209, a transitional acapulcoite: Implications for phosphate genesis in meteorites

Abstract Orthophosphate, (Mg,Fe,Mn)3(PO4)2 with XMg = Mg/(Mg+Fe) = 0-0.89 and Mn/Fe = 0.05-0.3 and chladniite-johnsomervilleite, MnNa8(Ca4Na4)(Mg,Fe,Mn)43(PO4)36 with XMg = 0.44-0.81 and Mn/Fe = 0.3-0.8, are minor constituents of meteorite Graves Nunataks (GRA) 95209, a transitional acapulcoite consisting mostly of forsterite (Fa7) and enstatite (Wo3Fs7-8) with subordinate clinopyroxene (Wo41-45 Fs4-6) and plagioclase (Or1-2An10-19), and cut by Fe,Ni metal veins. Electron backscatter diffraction patterns and maps, together with chemical analyses and Fe-Mg-Mn distribution among phosphates, confirm identification of the orthophosphate as sarcopside, chopinite, and farringtonite; no graftonite was found. Phosphates are found as (1) narrow rims between metal and forsterite or orthopyroxene; (2) aggregates having the same outline as metal; and (3) inclusions and stringers in metal, including a ring around a graphite rosette. Electron microprobe analyses of sarcopside/chopinite-johnsomervilleite/ chladniite pairs give a regular Fe-Mg distribution with KD = (Mg/Fe)Src/Chp/(Mg/Fe)Jhn/Cld = 0.584 consistent with terrestrial sarcopside-johnsomervilleite pairs, whereas analyses of farringtonite-chladniite pairs give KD = 1.51, but the Mg-Fe distribution is less regular. Textural relations suggest that Fe-Mn sarcopside originally formed by oxidation of P in metal and replacement of the metal and, through interaction with silicates, was converted to magnesian sarcopside-chopinite and farringtonite, i.e., the silicate matrix acted as a reservoir of Mg that could be exchanged with Fe and Mn in the sarcopside. Using the farringtonite-chopinite univariant curve determined in hydrothermal experiments by F. Brunet and others, isopleths calculated for the most magnesian chopinite in GRA95209, XMg = 0.65, give 4-7 kbar at 500-1100 °C, pressures far too high for the acapulcoite-lodranite parent body. Two scenarios could explain the discrepancy: (1) chopinite and magnesian sarcopside persisted metastably into the farringtonite stability field as Mg-Fe exchange progressed and the source volume for GRA95209 cooled; (2) a very mild shock event was intense enough to convert Fe-rich farringtonite (XFe = 0.4-0.6) to magnesian sarcopside and chopinite, but not enough to deform olivine in the source volume. Whether metastability could have played a role in chopinite formation would best be answered by experiments on the Mg3(PO4)2-Fe3(PO4)2 system under anhydrous conditions. If the transformation was found to be as kinetically fast as in the hydrothermal experiments, then shock would become the more plausible explanation for the presence of chopinite in this meteorite.

Metamorphosed Ordovician Fe- and Mn-rich rocks in south-central Maine: From peri-Gondwanan deposition through Acadian metamorphism

Abstract The Wilson Cove Member of the Cushing Formation is a thin (up to 120 m thick) metamorphosed Fe- and Mn-rich unit exposed discontinuously over a distance of >75 km in southern Maine. Cathodoluminescence imaging of zircon grains from the unit reveal texturally isolated detrital cores surrounded by distinct metamorphic overgrowths. U-Pb SHRIMP core ages range from 463 to 2058 Ma with a strong peak in the Neoproterozoic-Early Cambrian. This distribution of ages is consistent with a peri-Gondwanan source region and a Middle to Late Ordovician depositional age. Zircon rims have an age of 373 ± 4 Ma, consistent with growth during late Acadian metamorphism. Whole-rock geochemistry reveals considerable major- and trace-element variability with generally elevated abundances of Fe2O3(t) (15-43 wt%), MnO (0.1-12.1 wt%), Ba (4-2503 ppm), and As (7-1161 ppm). Geochemical discrimination diagrams suggest the protoliths were mixtures of hydrothermal exhalatives and terrigenous clastic sediment, with these materials most likely having been deposited in a marine back-arc basin proximal to a peri-Gondwanan continental source region. Late Devonian low-pressure, amphibolite-facies metamorphism of these bulk compositions produced assemblages dominated by grunerite + garnet + biotite + quartz. These and other mineral assemblages found in the Wilson Cove unit in south-central Maine are consistent with peak metamorphic conditions of 550 to 600 °C and 3-4 kbar previously determined from nearby metapelites. Mineral assemblages, mineral modes, and mineral compositions in these Fe- and Mn-rich rocks are strongly influenced by whole-rock bulk compositional variability. In particular, the compositions of co-existing garnet and grunerite vary systematically as a function of whole-rock MnO concentration.

Timing and anatomy of granitic strain gradients in the Grenville Front tectonic zone, Ontario, Canada

Observations of physical and chemical changes across strain gradients can provide information about the processes that lead to localization and therefore provide better tools for prediction of spatial and temporal strain patterns. In contrast to the many chemical and microstructural studies of natural shear zones in metapelites and mafic lithologies, few have described kilometer-scale granitic strain gradients despite the fact that granitoid bodies make up much of the orogenic crust. This study reports microstructural and compositional data across two amphibolite-facies strain gradients from middle crust of the Grenville orogen. The kilometer-scale gradients, defined by stretched enclaves and foliation intensity, form parts of the Boundary and Bad River shear zones, located along the east and west borders of the Bad River granite in the southwestern Grenville Front tectonic zone in Ontario, Canada. The granite is bounded on both sides by older orthogneiss (ca. 1720 Ma igneous crystallization age). Zircon U-Pb ages indicate that the granite crystallized from a magma at ca. 1465 Ma, with metamorphic growth at ca. 1020 Ma. Zircons in the orthogneiss record metamorphic growth during these two younger episodes. These age determinations are consistent with other regional dates and indicate that the strain gradients in the Bad River granite formed during the Grenville orogeny. Whole-rock analyses reveal minor heterogeneity in major-element distribution in the granite that can be attributed to igneous processes, and homogeneity in the trace elements, indicating that strain did not affect the bulk-rock composition. In contrast, microstructural and chemical analyses across the two strain gradients indicate some correlation with strain: slight changes in mineral compositions, development of crystallographic preferred orientations in quartz, an increase in recrystallized fraction, a reduction in recrystallized grain size, and the development of a mixed-phase matrix. However, most of these variations are subtle and occur at different positions across the gradients. The spatial distribution of the microscale changes suggests a change in deformation mechanisms toward the margin, accompanying increased localization. The rheological weakening at the Bad River granite margins was the product of microstructural, rather than mineralogical, change, in contrast to nearby shear zones in more mafic units. INTRODUCTION Strain localization operates at a wide range of scales and locations throughout the lithosphere, including in middle to lower orogenic crust (e.g., Coward, 1980; Austrheim and Griffin, 1985; Holdsworth and Strachan, 1991; Wittlinger, 1998; Calvert, 2004). Most kilometer-scale localization, such as the Legs Lake shear zone in Saskatchewan, Canada (Mahan et al., 2006), the Coast shear zone in southeast Alaska (Klepeis et al., 1998), and in the Cabo Ortegal region in northwest Spain (Puelles et al., 2009) initiates at preexisting strength heterogeneities (cf. Molnar and Dayem, 2010). However, for a zone of incipient localization to develop into mechanically and geologically significant structures such as those just mentioned, rheological weakening of an order of magnitude or more (e.g., Gerbi et al., 2010; Platt, 2015) must occur. Candidate weakening processes are well established, including metamorphism to weaker phases and a deformation mechanism switch, perhaps due to grain-size reduction and/or the development of a crystallographic preferred orientation (e.g., Poirier, 1980; White et al., 1980; Montési and Zuber, 2002; Regenauer-Lieb and Yuen, 2004). Weakening mechanisms need not operate in isolation; rather, multiple factors, such as reaction softening and changes in the spatial patterns of deformation mechanisms (e.g., Tsurumi et al., 2003; Menegon et al., 2006; Culshaw et al., 2010; Kilian et al., 2011; Gerbi et al., 2016) can combine to generate the rheological change necessary for kilometer-scale localization. Investigations of granitic shear zones, mainly at the millimeter to meter scale in greenschist and lower amphibolite facies, have identified granular flow of feldspars (Stünitz and Fitz Gerald, 1993), competency contrasts between different plutonic phases (Belkabir et al., 1998), coeval brittle and viscous processes (Berthé et al., 1979), and fluid-assisted mass transfer (Kwon et al., 2009) as weakening factors. Other causes include preexisting fabrics as pathways for fluids (e.g., Segall and Simpson, 1986), inherited strength heterogeneities (e.g., Christiansen and Pollard, 1997), rock fabrics and microstructures (e.g., Schulmann et al., 1996; Martelat et al., 1999), melt (e.g., Garlick and Gromet, 2004; Rosenberg and Handy, 2005; Závada et al., 2007; Schulmann et al., 2008), and continuous recrystallization of feldspars (Vauchez, 1987; Oliot et al., 2010). Despite the large body of work at the outcrop and smaller scale, few studies have investigated whether and how these local weakening mechanisms operGEOSPHERE GEOSPHERE; v. 13, no. 6 doi:10.1130/GES01413.1 14 figures; 6 tables; 1 supplemental file CORRESPONDENCE: deborah .shulman@maine .edu CITATION: Shulman, D., Gerbi, C., Marsh, J., Yates, M., and Culshaw, N., 2017, Timing and anatomy of granitic strain gradients in the Grenville Front tectonic zone, Ontario, Canada: Geosphere, v. 13, no. 6, p. 1949–1972, doi:10.1130/GES01413.1. Received 11 August 2016 Revision received 21 June 2017 Accepted 7 August 2017 Published online 25 September 2017 For permission to copy, contact Copyright Permissions, GSA, or editing@geosociety.org.