A sheeted plutonic complex exposed within a tilted mid-crustal section in the eastern Transverse Ranges of California, including Joshua Tree National Park (JTNP), will be examined in detail. The section provides a wonderful opportunity for analysis of (1) the depth dependent physical changes in, and growth of, a magma plumbing system; (2) the relative importance of source region heterogeneity, contamination, and in situ crystal-liquid fractionation in forming compositional diversity in plutonic systems, and (3) the role of mid-crustal detachments and/or vertical deformation partitioning in forming orogen-scale sheeted plutonic complexes. Field mapping and detailed structural analyses combined with geochronology and geochemistry of intrusive units will allow testing of a series of hypotheses. One goal is to determine whether upper and lower crustal plutons and the underlying sheeted complex formed synchronously and thus reflect an abrupt depth-dependent change in petrogenesis and intrusive style during continental margin arc construction. They will also test the hypotheses that the complex represents both a zone of magma transfer, with the creation of vertical pathways, and a site for hybridization that formed magmas parental to upper crustal plutons and explosive volcanic rocks.
The goal is to expand our understanding of orogenesis along plate boundaries in the middle and deeper parts of continental crust. This study will dramatically affect our knowledge of vertical changes in the physical and chemical processes operating in magmatic plumbing systems that feed volcanoes. The research will be concentrated within the boundaries of JTNP and will have important positive impact on public education through geoscience outreach and interpretive activities. Over the last five years, the PIs have developed collaborations with Park Service staff at JTNP and with its interpretive arm, the Desert Institute. They will work with the Park and the U.S. Geological Survey to complete a geologic map and improve understanding of the Park's geology, with a focus on major events in the volcanic and structural evolution that influenced formation of the Park's landscape. Further outreach will extend that commitment through development of field trips and lectures, and will translate the results to assist park staff in enhancing public educational and curriculum materials. At USC and IUPUI graduate and undergraduate education in the geosciences will be strengthened through exceptional field, classroom and laboratory experiences, including excellent opportunities for underrepresented student groups.
Intellectual Merit: During the 3-year tenure of this grant, we completed the following research in Joshua Tree National Park (JTNP): 1. 1:24,000 geologic mapping of rocks in the Keys View and adjacent Indian Cove 7.5-minute quadrangles and in nearby regions, which is helping to establish the geologic characteristics of a faulted and tilted section of crust now exposing near surface to ~20 km depths. Our mapping established that a series of different styles of plutonic "sheeted complexes" formed at both shallow and mid-crustal levels and sometimes amalgamated to form large plutonic bodies and has revealed the existence of a broad zone of pervasive migmatization at mid-crustal levels. This mapping will aid in the construction of an updated geologic map of the Park and provides a geologic base for our research summarized below. 2. Determined the age of the mineral zircon in ~ 30 samples in the mapped areas to help clarify timing relationships among both plutonic, sedimentary, and metamorphic rocks found in JTNP. This dating further established that in JTNP, Proterozoic metamorphic gneisses are intruded by Triassic, Jurassic, and Cretaceous plutons, the latter three forming both large upper crustal plutons and individual sheets in the sheeted complexes. The geochronology has clarified the age of the broad migmatite zone (Cretaceous), enigmatic deformed granite bodies originally interpreted as Cretaceous (now known to be Triassic), and gabbroic bodies originally interpreted as Jurassic or Proterozoic (now known to be Cretaceous). 3. Established the temperatures and pressures (or depth in the crust) at which different rock units formed and were then metamorphosed. Early results indicated that the western part of the Park consists of a tilted crustal section, from around 4 km depth in the northeast to 15 km in the west and 22 km in the south. Our more detailed pressure, temperature, time studies indicates a more complicated history of rock formation and uplift. 4. We have also determined the chemistry of ~100 samples from the mapped areas. Data include oxide percentages for major elements, concentrations of minor elements, and a full suite of rare-earth elements. We have also determined isotopic ratios, including Rb-Sr, Sm-Nd, and Pb-Pb to gain greater understanding of the crustal and mantle processes that each igneous rock has undergone from melt generation to final cooling. Our initial results indicate that the plutonic rocks formed from a mixture of fairly primitive mantle melts plus a component of the remelted Proterozoic continental crust. Different plutonic bodies in both the sheeted complexes plus larger plutons show close geochemical ties. Broader Impacts: The above research provides a better understanding of the how different levels of crust evolve during plate motions, a better understanding of how magmatic arcs (volcanoes and underlying plutons) form along continental margins, and a robust dataset of geochronology and geochemistry to address issues of crustal growth. It also involved a number of educational and outreach components such as the following: 1. Our research provided important training for 6 Ph.D. students and is the focus of one Ph.D. dissertation. 2. It also involved ~75 undergraduates through class field trips and projects, the undergraduate team research (UTR) program at the University of Southern California, 5 undergraduate research projects, and one completed senior thesis. 3. We have interacted with Park personnel to increase their knowledge of Park geology and have organized an international field trip and forum to the area through the Geological Society of America. 4. Our studies aid in evaluating past and present volcanic, hydrothermal, tectonic activity in a region. These estimates are not only valuable for modeling and planning of safe perimeters around active and dormant volcanoes, but also of interest to the electricity generation community in that one can create predictable heat fluxes for geothermal power. 5. Our geologic mapping at Joshua Tree National Park is a starting point for further exploration of biological research. For example since the rocks, their weathered products, and sediment basins define unique microhabitats for biota to thrive: a comprehensive geologic map is very beneficial in discovering and correlating habitats.
