59th Annual Report on Research 2014

The California Mesozoic arc is commonly cited as an archetypal example of a convergent margin tectonic setting, although the juxtaposition of Nacimiento Block subduction mélanges and Salinian Block arc rocks across the enigmatic Sur–Nacimiento Fault in the central California coast serves as a reminder of the complexity of the Cordilleran Mesozoic forearc. Our study employs detrital zircon geochronology and geochemistry, and sandstone petrography to investigate the provenance of forearc basin sediments scattered throughout the Nacimiento Block, and is ultimately designed to place new paleogeographic constraints on the evolution of the Sur–Nacimiento fault.

During the second year of the project, our work focused on detrital zircon analysis from a dense sample array in the San Rafael Mountains that details the most complete known section of the Toro formation, sample collection and preparation of Upper Cretaceous sandstones, and sandstone petrography of all collected samples. Through this work, my undergraduate students from Cal Poly San Luis Obispo have been trained on the techniques of sample collection and field work, petrography, mineral separation, and the basics of laser ablation inductively-coupled-plasma mass spectrometry (LA-ICPMS) operation at the University of California, Santa Barbara. In addition to the direct work on this project, students have received guidance on the research process including the presentation of results in report format. To date, this ongoing work has supported paid summer research projects for four motivated students seeking extensive research experience, as well as funds to support research activities during the school year for six additional unpaid undergraduate students working toward the completion of their senior projects.

Preliminary results

U–Pb detrital zircon geochronology from a dense sample array in the San Rafael Mountains documents a previously unidentified section of Albian greywackes included in the Toro formation. These younger rocks appear in a fault sliver along the Cachuma fault and again at the top of the section, and are dominated by Cordilleran detritus with 80% of the grains in four samples yielding ages of 112-105 Ma. In addition to this detailed analysis of the Lower Cretaceous forearc section, we have also sampled a detailed section through the Upper Cretaceous section exposed in the San Rafael range in conjunction with similar rocks exposed in the Atascadero formation farther north in the central coast ranges. Our preliminary U-Pb detrital zircon geochronology results from one sample of the Atascadero formation yields grains that are dominated by Cordilleran detritus with ~90% of the grains yielding ages of 110-80 Ma.

To further constrain the provenance of individual detrital grains, we have also collected trace element data from over 700 detrital zircon grains from 10 samples using split-stream ICPMS. Our initial results indicate generally decreasing Yb/Gd during Triassic-Cretaceous history of the Sierran arc, and higher Th/U and lower U/Yb ratios in late Jurassic grains.

New sandstone petrography has also been performed on most of the rocks from the study. Our preliminary results indicate that the Tithonian-Albian Toro formation sandstones are quartz-rich greywackes with > 45% lithic fragments, while the Upper Cretaceous Atascadero formation and equivalents are more arkosic with < 40% lithic fragments.

When combined with our results the first year of the project, our initial results indicate that Late Jurassic and Lower Cretaceous Nacimiento Block sediments were dominated by input from miogeoclinal source terranes in the continental interior and from the growing Middle–Late Jurassic Sierran/Salinian arc. Following a brief hiatus, forearc sedimentation in the Nacimiento block resumed in the Albian, although by this time, continental sources had largely been blocked, and Sierran/Salinian arc sources dominated. Our petrographic results indicate that unroofing of the plutonic core of the arc did not occur until the Late Cretaceous as documented by the arkosic nature of the Upper Cretaceous sandstones. These detrital zircon age and geochemical trends are consistent with similar trends in sediments from the McCoy Mountains of southern California. Furthermore, the flood of Late Cretaceous arc detritus that dominates the Albian-Campanian sediments in our study is similar to results from the Peninsular Ranges farther south, but markedly different from Late Cretaceous sandstones of the Sacramento and San Joaquin valleys which are dominated by Jurassic detritus. Together, these lines of evidence suggest that a Southern Sierra or Mojave provenance for Nacimiento Block sediments, and support thrust-sense displacement along the Sur-Nacimiento fault.

Presentation of results

Our preliminary results were initially presented in a 20 minute talk by the PI at the GSA Cordilleran Section Meeting in Fresno, CA on May 22, 2013. A second progress report was presented as a talk by the PI at the GSA Fall Meeting in Denver, CO on October 29, 2013.

Future plans

To better suit the needs of the students at Cal Poly, we will continue this project for one more year during a one-year grant extension. Data collection during this grant extension will include 1) completing sandstone petrography on several remaining samples, and 2) completing U-Pb detrital zircon geochronology and geochemistry on the last suite of Upper Cretaceous sandstones from the San Rafael Range. This work will be completed over the course of fall and winter quarter with two Cal Poly undergraduates continuing to work toward their senior thesis reports. Following completion this data collection, the PI will compile the data and prepare a manuscript for a peer-reviewed journal in the spring and summer of 2015.