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Item Construction of Late Cretaceous, Mid-Crustal Sheeted Plutons from the Eastern Transverse Ranges, Southern California(2009-01-16T16:57:10Z) Brown, Kenneth Lee; Licht, Kathy J.; Swope, R. Jeffrey; Li, LinDifferential exhumation within the eastern Transverse Ranges of southern California has revealed a tilted crustal section that provides a unique view into the architecture of the Mesozoic arc. At the base of this crustal section is a group of well-exposed sheeted plutons. Well-developed, gentle to moderately dipping magmatic and solid-state fabrics within these plutons are regionally consistent, margin-parallel, discordant to internal sheeting and layering, and are generally parallel to equivalent host rock structures and fabrics. In some plutons, magmatic foliations define regional fold structures, thus recording regional contraction during chamber construction. Collectively, field mapping and fabric analyses within these sheeted plutons show that the observed fabric patterns are better explained by regional deformation rather than internal magma chamber processes. This interpretation is in direct contrast to previous mapping in the region. The host rocks also record complex processes during sheeted pluton emplacement. Deflection of host rock foliations and structures into parallelism with pluton contacts suggest that downward ductile flow played a role in making space for these plutons. However, evidence of regional faulting and shearing is not observed, suggesting that they did not play a significant role. Although there is considerable microstructural variability within each pluton, the observed microstructures are generally consistent with a transition from magmatic to submagmatic/ high-temperature solid-state deformation. Magmatic microstructures are defined by euhedral to subhedral plagioclase, hornblende, and biotite that do not show significant internal crystal-plastic deformation. Evidence for high-temperature solid-state deformation includes high-temperature grain boundary migration in quartz, plagaioclase, potassium feldspar, and hornblende; chessboard extinction in quartz; and ductile bending in plagioclase and hornblende. Microstructural observations also indicate that mafic and intermediate compositions record stronger magmatic fabrics than felsic compositions. Based on the structural and microstructural observations presented in this study, I interpret that these sheeted plutons were emplaced into an active continental arc setting that was undergoing regional contraction. The strong magmatic fabrics and high-temperature solid-state overprinting is likely a consequence of regional deformation during crystallization. The weak fabrics within upper crustal plutons relative to the strong fabrics within the mid-crustal plutons suggest that deformation was largely localized to the more compositionally heterogeneous mid-crustal portions of the arc structure.Item Geochemical evidence for incremental emplacement of Palms pluton, southern California(2010-02-02T18:15:40Z) Roell, Jennifer L.; Barth, Andrew, 1958-; Filippelli, Gabriel M.; Licht, Kathy J.The objectives of this study are, generally, to analyze and understand internal processes that produce melts in an oceanic-continental subduction setting; and, specifically, to understand the assembly of a Cretaceous magmatic arc pluton (Palms pluton), including the timing of melt emplacement(s) and melt evolution from the source. SiO2 concentrations vary from ~ 69-76 % by weight. Whole rock trace element concentrations vary up to 7 times. Zircon analysis shows a minimum age difference in the pluton of 3 my, if considering the uncertainties of the oldest and youngest samples. According to the model made from the HEAT program, this is approximately six times longer than the estimated crystallization time of one batch of melt with the same physical properties as the Palms pluton. Two distinct sources, perceived from chemical analysis of premagmatic zircons, are found throughout the pluton. REE compositional patterns show a hybridization of Proterozoic and Mesozoic sources in some, but not all, Palms pluton granites. This data suggests that the pluton formed from multiple intrusions and the Proterozoic source remained relatively consistent throughout the pluton’s assembly with few additions of younger Mesozoic source material.Item A Holocene record of Pacific Decadal Oscillation (PDO)-related hydrologic variability in Southern California (Lake Elsinore, CA)(Springer, 2010-10-01) Kirby, M. E.; Lund, S. P.; Patterson, W. P.; Anderson, M. A.; Bird, Broxton W.; Ivanovici, L.; Monarrez, P.; Nielsen, S.High-resolution terrestrial records of Holocene climate from Southern California are scarce. Moreover, there are no records of Pacific Decadal Oscillation (PDO) variability, a major driver of decadal to multi-decadal climate variability for the region, older than 1,000 years. Recent research on Lake Elsinore, however, has shown that the lake’s sediments hold excellent potential for paleoenvironmental analysis and reconstruction. New 1-cm contiguous grain size data reveal a more complex Holocene climate history for Southern California than previously recognized at the site. A modern comparison between the twentieth century PDO index, lake level change, San Jacinto River discharge, and percent sand suggests that sand content is a reasonable, qualitative proxy for PDO-related, hydrologic variability at both multi-decadal-to-centennial as well as event (i.e. storm) timescales. A depositional model is proposed to explain the sand-hydrologic proxy. The sand-hydrologic proxy data reveal nine centennial-scale intervals of wet and dry climate throughout the Holocene. Percent total sand values >1.5 standard deviation above the 150–9,700 cal year BP average are frequent between 9,700 and 3,200 cal year BP (n = 41), but they are rare from 3,200 to 150 cal year BP (n = 6). This disparity is interpreted as a change in the frequency of exceptionally wet (high discharge) years and/or changes in large storm activity. A comparison to other regional hydrologic proxies (10 sites) shows more then occasional similarities across the region (i.e. 6 of 9 Elsinore wet intervals are present at >50% of the comparison sites). Only the early Holocene and the Little Ice Age intervals, however, are interpreted consistently across the region as uniformly wet (≥80% of the comparison sites). A comparison to two ENSO reconstructions indicates little, if any, correlation to the Elsinore data, suggesting that ENSO variability is not the predominant forcing of Holocene climate in Southern California.