What: This project will investigate the processes that formed the Salinian arc of central coastal California. Participants will spend two weeks in the Big Sur region making field measurements and identifying and collecting critical samples for laboratory-based microstructural and geochronologic work. The latter half of this project will be spent preparing samples for analysis and analyzing samples at the University of Arizona and the University of Minnesota.
When: tentatively 6/16/2016 – 7/13/2016
Where: Big Sur region, central coastal California
Who: Sarah Brownlee (Wayne State University), Alan Chapman (Macalester College), and 6 students.
Project Overview and Goals
Magmatic arcs are critical players in the formation and evolution of continental crust, and yet many of the processes involved are poorly understood (e.g., Hildreth and Moorbath, 1988; Rudnick and Gao, 2003; Lackey et al., 2005). The Salinia terrane along the central coast of California represents a nearly complete magmatic arc section, including deep crustal magmatic roots (e.g., Kidder et al., 2003; Chapman et al., 2014). Despite its structural completeness, good exposure, and relatively easy access, Salinia remains understudied. Furthermore, a profound structural relationship is observed at the base of the Salinian arc section: a shallowly dipping shear zone underlain by subduction accretion assemblages of the schist of Sierra de Salinias (SdS; Barth et al., 2003; Kidder and Ducea, 2006). The structural-spatial-temporal relationships between the emplacement of melt-fertile materials (SdS schist) beneath Salinia and arc plutonism are not well constrained. We will focus on two questions: 1) Was the SdS schist emplaced by “cold” or “hot” relamination (e.g., Behn et al., 2011; Hacker et al., 2011)? and 2) What were the petrologic and rheologic consequences of schist emplacement?
Our hypothesis is that the SdS schist was emplaced by “cold” relamination, or progressive underthrusting, and that this addition of weak, melt-fertile material profoundly weakened the crust, sparking construction of the late Cretaceous (ca. 93-80 Ma) arc section. This project consists of four specific objectives aimed at testing this hypothesis:
- 1) Distinguish between possible emplacement mechanisms for the SdS schist.
- 2) Constrain the timing of schist emplacement and arc plutonism.
- 3) Characterize the petrologic and rheologic consequences of schist emplacement.
- 4) Determine seismic signature of the SdS schist at depth.
Salinia is a NW-trending composite terrane bounded to the east by the San Andreas fault, to the west by the Sur-Nacimiento fault, and to the south by the Big Pine fault (Vedder et al., 1982). These faults juxtapose the Salinian terrane to the west and east with Mesozoic subduction accretion assemblages of the Franciscan complex. The Salinian terrane traveled northward >300 km during the Neogene along the San Andreas fault system (Dickinson et al., 2005; Hall, 1991; Hall and Saleeby, 2013; Huffman, 1972; Jacobson et al., 2011; Matthews, 1976; Page, 1982; Schott and Johnson, 1998; Sharman et al., 2013). Petrologic, isotopic, geochronologic, structural, and sedimentologic evidence places Salinia near the southern end of the Sierra Nevada batholith in Late Cretaceous time (Barbeau et al., 2005; Chapman et al., 2012, 2014; Hall, 1991; Hall and Saleeby, 2013; James, 1992; Kistler and Peterman, 1978; Sharman et al., 2013; Wood and Saleeby, 1997). Salinian, Mojave, and Peninsular Ranges blocks form a contiguous ~ 1500 km long, NNW-trending Mesozoic batholith – the California arc.
Magmatism began in the Salinian arc at ca. 93 Ma and continued until ca. 80 Ma (Kistler and Champion, 2001; Mattinson, 1978, 1990), during an episode of high flux magmatism that produced >80% of the California arc (Coleman and Glazner, 1998; DeCelles et al., 2009; Ducea, 2001; Ducea and Barton, 2007). The Salinian arc overlies the Late Cretaceous SdS schist along the Salinas shear zone, a Late Cretaceous low angle normal fault responsible for the exhumation of the schist (Barth et al., 2003; Chapman et al., 2010; 2011; 2013; Kidder and Ducea, 2006; Kidder et al., 2013; Fig. 2). The schist of Sierra de Salinas and related schists of southern California (e.g., Jacobson et al., 2011 and references therein) are Late Cretaceous subduction accretion assemblages deposited in the trench and underplated directly beneath the California arc during an episode of shallow subduction (Ducea et al., 2009).
Potential Student Projects
Each student project outlined below will include: 1) mapping, sample collection, and laboratory analysis (geochronology and microstructural data) during the one month summer research period and 2) data interpretation and abstract/poster preparation for the annual Consortium symposium.
- 1) Emplacement mechanism of the SdS schist (2 students)
These students will distinguish between two possible emplacement mechanisms for the SdS schist. This module requires characterizing the foliation orientations through the SdS schist and surrounding rocks. Our hypothesis is that the schist was emplaced by “cold relamination.” This emplacement mechanism predicts a consistent foliation orientation throughout the SdS schist. It also predicts that the schist foliation will be slightly oblique to the foliation in the overlying material, and have dominantly non-coaxial strain. The alternative emplacement mechanism is “hot relamination,” or diapiric rise. This emplacement mechanism predicts a change in foliation orientation with depth in the schist. The foliation in the upper part of the schist would be nearly parallel to the contact between schist and overlying material, and would have dominantly coaxial deformation. In contrast, the lower part of the schist would have nearly vertical foliation, and potentially demonstrate constrictional strain. To distinguish between these two emplacement mechanisms, the students will do two things: 1) map the foliation trajectories within the SdS schist and surrounding material, and 2) characterize the strain using traditional strain markers and measurements of mineral crystallographic preferred orientations.
- 2) Temporal relationship between SdS schist emplacement and arc plutonism (2 students)
Our hypothesis is that emplacement of the schist played a key role in initiating arc magmatism. If this is the case, schist emplacement should precede arc magmatism. Only a handful of modern U-Pb zircon ages are available from the CRB (Kidder et al., 2003), the remainder are analyses of multigrain age splits (Mattinson, 1978, 1990; Kistler and Champion, 2001). As a result, the timing of arc plutonism is poorly constrained to a time window of 93-80 Ma. The timing of schist emplacement, metamorphism, and migmatization are also poorly constrained by a small number of detrital zircon ages (144 detrital zircon ages total; Barth et al., 2003; Grove et al., 2003). One student will be responsible for dating ~6 samples from the CRB and the other student will be responsible for dating ~6 samples from the SdS schist.
- 3) Characterize the petrologic and rheologic consequences of schist emplacement (1 student)
Our hypothesis is that emplacement of the SdS schist beneath the Salinian arc dramatically decreased the strength of the crust. This can be addressed by determining the amount of water released during metamorphic reactions, as well as the composition, and amount of partial melt coming out of the schist. This component will involve sample petrography (point counting and bulk composition estimation using EDS measurements of mineral compositions) and petrologic modeling using PerpleX.
- 4) Determine the seismic signatures of the SdS schist and lower arc crust at depth (1 student)
Our hypothesis is that the SdS schist has a specific seismic signature that is different than what is expected for the lower crust. Because the schist will have high mica content, we expect it to have high magnitude of seismic anisotropy in both Vp and Vs, and we expect the elastic symmetry to be approximately transversely isotropic, but highly non-elliptical. To address this we will measure mineral crystallographic preferred orientations and calculate average rock elastic tensors for SdS schist and CRB assemblages. We will use these tensors to predict the seismic signature of the Salinian crust and compare our predictions to passive seismic studies currently underway in the area (e.g., Hoots et al., 2014).
The project will begin and end in San Francisco, CA with the group members meeting at the airport on June 16 and departing immediately for the University of California, Santa Cruz Big Creek Research Facility. The co-directors and students will stay in the research cabin on the Big Creek research station property during the field component of the project. On day two, the research group will discuss the significance of the project and the geology of the Salinian block from the research cabin, which overlooks many important geologic elements of the Santa Lucia Range. In the following days, students and faculty will work in groups of three, tentatively pursuing the research avenues described above, to evaluate where the students’ interests lie and to refine project ideas.
Upon finalizing research projects and finishing fieldwork, we will return to Macalester College to regroup and pool maps and notes, to begin separating minerals for geochronology, and to prepare polished thin sections for electron backscatter diffraction (EBSD) work. Samples selected for geochronology will be processed and imaged by SEM prior to laser ablation-inductively coupled plasma-mass spectrometry (LA-ICP-MS) analysis at the University of Arizona.
Co-director Chapman and 2-3 students will work for two days in shifts during the University of Arizona session to complete analysis of selected samples. All students involved in this work will assist in reducing the data while at the University of Arizona. All data resulting from this session will be shared with all students involved in the project.
Co-director Brownlee and three students will analyze crystallographic preferred orientations of minerals in 12-15 samples of the SdS schist using EBSD facilities available at the University of Minnesota. Samples will be selected to cover a range of compositional and structural heterogeneity within the SdS schist. Each sample will be mapped using EBSD at ~100 μm step size to cover as much of the thin section as possible in a reasonable amount of time. The EBSD mapping is automated, but requires a short setup time, so while maps are running, students can begin working on data processing. EBSD results will be used to determine shear sense using quartz preferred orientations, and to calculate rock elastic tensors using all mineral data combined with single crystal elastic tensors. All EBSD data and results of calculations will be shared with the entire project group.
Days 1-14: Fieldwork
Days 15-25: Making and polishing thin sections, separating out minerals for geochronology, petrologic analyses and characterization, mineral petrography and petrologic modeling.
Days 26-30: LA-ICP-MS geochronologic analysis at the University of Arizona, EBSD analyses at the University of Minnesota.
The potential projects described above require familiarity with concepts introduced in courses focused on igneous and metamorphic petrology and structural geology. Previous coursework in these areas is encouraged for participation in this project. Experience at a field camp or in a field geology course is also encouraged.
Sarah Brownlee (sarah[email protected])
Alan Chapman ([email protected])
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