Reports: UNI851941-UNI8: The Paleogeographic Origin of the Nacimiento Block, Central California Coast

Scott Johnston, PhD, California Polytechnic State University

Research activities

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 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 first year of this project, my students and I focused on the Late Jurassic/Early Cretaceous Toro formation and equivalents, working on sandstones and quartzite conglomerate clasts from three sections spanning the north–south limit of forearc basin sediment exposures in the Nacimiento Block. Through this work, my undergraduate students from Cal Poly San Luis Obispo have been trained on the techniques of sample collection, mineral separation, and the basics of laser ablation inductively-coupled-plasma mass spectrometry (LA-ICPMS) operation at the University of California, Santa Barbara, as well as received guidance on the research process including the presentation of results in report format. To date, this work has supported paid summer research projects for two motivated students seeking extensive research experience, as well as funds to support research activities during the school year for five additional unpaid undergraduate students working toward the completion of their senior projects.

Preliminary results

U–Pb geochronology using LA-ICPMS was completed on detrital zircons from six sandstones from the Toro formation. Zircons from five of the six samples display Cordilleran probability maxima of 140–160 Ma that yield maximum depositional ages consistent with Tithonian–Valanginian fossils in adjacent outcrops. In these samples, ~60% of the analyzed grains are pre-Cordilleran with broad peaks at 0.3–0.6, 1.0–1.2, ~1.45, and 1.6–1.8 Ga, and indicate significant input from recycled Triassic/Jurassic and Neoproterozoic–Early Paleozoic miogeoclinal strata. Subtle variations observed in these samples suggest an increasing component of Cordilleran arc detritus in younger, coarser, and more northerly samples; coarser sandstones are characterized by conspicuously lower percentages of 0.3–0.6 Ga grains. In addition to conventional LA-ICPMS, split-stream analyses were completed on one sample to supplement zircon U–Pb ages with REE geochemistry. Split stream analyses indicate Mesozoic grains with significant trace element variability, although bulk averages are characterized by higher Yb/Gd during Permian–Triassic arc magmatism, and higher Th/U with lower U/Yb during Late Jurassic magmatism. The sixth Toro sandstone sample, collected from the San Rafael Mountains, is dominated by 110 Ma zircons with < 5% pre-Cordilleran grains, and suggests that the lithic-rich Toro formation may locally extend temporally into the Albian, or that the area is more structurally complex than recognized in published geologic maps.

In addition to Toro formation sandstones, detrital zircon ages were also determined from 10 sandstone and quartzite clasts sampled from Toro formation conglomerates. LA-ICPMS analyses indicate a suite of Jurassic and Triassic sandstones complemented by quartzites that are dominated by pre-Cordilleran ages with 1.0–1.2, 1.4 and 1.6–1.8 Ga ages. One granitic clast from the San Rafael Mountains yielded a crystallization age of 164 ± 2 Ma.

These initial results indicate that Late Jurassic and Early 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. The lack of 0.3–0.6 Ma grains in coarser Nacimiento block sandstones and the observed geochemical trends are broadly consistent with previously published detrital zircon results from the McCoy Mountains of Southern California, and suggest that a Southern Sierra or Mojave provenance for Nacimiento Block sediments is more likely than a northern Sierran source.

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 will be presented as a talk by the PI at the GSA Fall Meeting in Denver, CO on October 29, 2013.

Future plans

Data collection in the second and final year of the project will focus on three research goals: 1) placing new constraints on Late Cretaceous Nacimiento Block provenance through detrital zircon geochronology and REE geochemistry of the Campanian Atascadero formation, 2) exploring the temporal extent of Early Cretaceous forearc greywackes through detrital zircon geochronology of a detailed transect from the San Rafael Mountains, and 3) reconnaissance sampling and detrital zircon geochronology of the Early Cretaceous Espada formation from the Santa Barbara area for comparison with correlative sediments in the Nacimiento Block. This work will be completed over the course of the school year in conjunction with three additional Cal Poly students recruited during the fall quarter. Following the completion of data collection, the PI will compile the data and prepare a manuscript for publication in a peer-reviewed journal during the summer of 2014.