Collaborative Research: Elucidating the geologic record of Late Paleozoic subduction initiation along the southwestern margin of Laurentia, southern California and Sonora, Mexico

Project Outcomes NSF-EAR #1322035

INTRODUCTION: The recycling of lithosphere at subduction zones is one of the most fundamental aspects of plate tectonic theory, yet the initiation of subduction at newly convergent margins is poorly understood. This is because the onset of subduction does not get preserved well in the rock record and because modern analogs are few. A unique suite of Permian igneous and metasedimentary rocks, preserved in southern California and Sonora, Mexico, are thought to record the initiation of subduction along the southwestern margin of North America. These rocks were investigated in order to more precisely constrain the development, growth and surficial expression of a nascent subduction zone. The specific questions addressed were: 1) how is the initiation of subduction recorded in the stratigraphic record?; 2) how does the paleogeography of continental margins evolve in response to the development of subduction zones?; and 3) how can incipient subduction be recognized in early arc-magmatic products?  To address those questions, new datasets were generated that constrain the depositional timing and provenance of Late Paleozoic strata, which record a major change in the plate boundary from dominantly transcurrent to convergent motion. Datasets bearing on the timing and mode of formation of earliest arc magmatic products were also generated in order to assess the processes by which new continental arc systems are developed.

NEW FINDINGS / INTELLECTUAL MERIT:  Research done by PI Riggs and her students at Northern Arizona University primarily focused on the analysis of Permian sedimentary strata in Sonora, Mexico, with lesser focus on early arc magmatism in the Sierra Nevada of California.  

In Sonora, graduate student Stephen Dobbs documented a marine record in the Monos Formation of upsection deepening-and-shallowing, culminating in development of a carbonate reef complex.  Overall, this corresponds to the expected succession related to subduction initiation.  We were also able to constrain the earliest sedimentary record of magmatism to c. 266 Ma (Fig. 1), which, although younger than the age of earliest plutonism, may reflect evolution of volcanism as plutons remained unexposed.  We infer that asymmetry in the incipient arc meant that detritus remained on the forearc side until Early Triassic time. 

Hf analysis of previously dated zircon grains in a middle Permian pluton indicates that magma composition was strongly influenced by continental crust.  Initial ε_Hf values that average -7 indicate a highly evolved source; arc-derived zircon within the sedimentary strata similarly have initial ε_Hf values that average -10 (Fig. 2).  We use these data to infer that the western margin of Laurentia experienced induced nucleation of subduction as the California-Cohuila fault yielded to compressional stresses, causing the proto-Pacific plate to begin subducting under continental Laurentian crust.

In the Sierra Nevada, we continue to unravel the complex Mesozoic history of roof pendants, as well as constraining the timing of earliest magmatism to c. 230 Ma in this part of the arc.  Earliest arc magmatism produced subaqueous dome fields represented by mafic breccias and caldera complexes whose products may extend across two or more pendants.

BROADER IMPACTS:  Broader impacts center on research experiences for graduate students and work experience for undergraduates. The grant supported MS theses by Steve Dobbs (MS 2016; matriculated F16 to Stanford University) and Derek Field (MS 2017, now instructor at Cal Lutheran College).  Both students presented their research at Geological Society of America meetings; Dobbs is currently finishing a manuscript from his thesis.  Several undergraduate and graduate students were employed in the mineral-separation lab and as field assistants.   Students received training in field methods (sample collection, measuring stratigraphic sections, mapping) and in lab methods (mineral separation, U-Pb analysis).  Dobbs spent one week at California State University-Northridge working with co-PI Kathie Marsaglia, who also served as a member of this thesis committee.  CSUN graduate student Eric McDonald was funded by the grant to be Dobbs?s field assistant in Mexico.

Partially as a result of this work, PI Riggs has been invited to be a keynote speaker at the GEOPANGEA conference to be held at the UNAM-Mexico City in January, 2019.  Riggs has also begun a collaboration with a UNAM-Hermosillo researcher to investigate late Permian sedimentary rocks in Baja California.

Last Modified: 12/01/2018
Modified by: Nancy R Riggs