Field Studies of Friction and Geochemistry Impacts on Serpentinite Weathering

John Christensen, Harrison Lisabeth, Diane Moore, Benjamin Gilbert and Jill Banfield

Overview: This project will leverage the natural laboratory of the Bartlett Springs Fault to mount an observational study of serpentine-containing rocks deformed and altered under conditions of chemical and mechanical disequilibrium. We will identify locations of where mineralogical transformations can be associated with metamorphic grade and influence of fault motion, and use high-resolution characterization to infer the roles of chemical and mechanical deformation and alteration processes in order to test insights from laboratory experiments.

Background

The Bartlett Springs fault is a ~170-km-long right-lateral strike-slip fault, i.e., mainly rightward horizontal motion, situated on the northeast side of the San Andreas fault system in northern California. In common with the San Andreas Fault, actively creeping sections are associated with regions of serpentinized ultramafic rocks. The fault has been monitored by the USGS for surface expressions and tectonic activity (Fig. 1) (Lienkamper 2010) and recent field studies have sought to understand the mechanism for rock deformation coupled to aseismic slip (Moore et al. 2018; Melosh 2018). Field observational studies and petrographic mounts indicate that fault motion involves the shearing and alteration of serpentinite matrix around silicic clasts in the fault core (Melosh 2019). However, the principal motivation for these studies is to infer the frictional properties of deep faults and the links between stress, geochemical disequilibrium and serpentinite alteration remain far from understood.

Figure 1. (a) Location of the Bartlett Springs Fault in the northern California Coast Ranges (Lienkamper 2010). Red lines show tectonically active sections. Red polygon shows location studied by Melosh (2019). Red circle shows Lake Pillsbury location studied by Moore et al. (2018) (b) Simplified bedrock geology including serpentinite matrix mélange, chaotic block mélange, Great Valley Group, and the Bartlett Springs fault (Melosh 2019). (c) Geomorphic expression of the Bartlett Springs Fault along the 170 km transect shown in (a) (Lienkamper 2010).

Goals

The goal of this effort is to use field observation, sampling plus high-resolution mineral characterization to identify the influence of stress and geochemical disequilibrium on serpentinite alteration. We will identify locations at the Bartlett Springs fault exhibiting gradients in climate, metamorphic intensity and fault activity and perform high-resolution mineralogical and geochemical characterization to infer alteration pathways. We will seek natural analogs to laboratory studies of friction assisted serpentinite transformation.

Dr. Diane Moore Research Geologist (emerita) will be a collaborator on this project (See Letter)

Proposed Work

Field Site Selection

The first task will be to identify and collect samples from the field area. The Cache Creek area of the Bartlett Springs Fault offers accessible exposures of the fault that juxtapose Franciscan ultramafic rocks against silicic Great Valley rocks. Our goal will be to collect samples representing gradients in shear and weathering.

Figure 2. Schematic of fabric evolution from undeformed mesh texture to sheared ribbon texture within a shear zone. Images on the top right are photomicrographs with a gypsum plate inserted to emphasize crystallographic orientations. The schematic on the bottom shows mesh cores in pink and two orientations of serpentine rims and the response of this structure to progressive shearing. After Viti et al. 2018.

Serpentinite samples will be identified with a range of fabrics, from relatively undeformed to highly sheared. The mélange zone at this field site contains a range of deformation textures. Rigid, undeformed clasts float in a matrix of deformed serpentinite (Melosh 2018). Within anastomosing shear zones, a range of shear fabrics should be available for sampling, from small strain within wide shear bands and close to rigid bodies to large strain where deformation has been focused into thin mylonitic bands (Viti et al. 2018). The relative amount of strain in each sample can be surmised by assessing the progress of the development of ribbon textures from undeformed mesh textures (Fig. 2).

Unsheared samples can be collected from the cores of intact serpentinite kernels. The so-called kernels are concentrically altered clasts of relatively undeformed serpentinite floating within the sheared matrix. The alteration rims will provide an example of chemical reactions occurring in a relatively isotropic strain environment.

Sample Collection and Analysis

Small-diameter (51 mm) rock cores will be collected using a portable drill (Shaw Tool of Yamhill, Oregon, USA), labeled and photographed. The fracture density in the borehole will be logged with a home-built optical viewer. The samples will be analyzed for major and trace elements, mineralogy and prepared as petrographic thin sections, polished surfaces for scanning electron microscopy (SEM) and polished thin sections for low-dose HAADF-STEM.

Studies of chemical variation among the samples will seek to characterize correlations between geochemical gradients, serpentine mineralogy and defect structure as well as deformation fabric. As disequilibrium tends to promote chemical exchange, we hypothesize that boundaries between mineralogically different zones should be sites of enhanced deformation. Gradients in chemistry should be resolvable with EDS, while gradients in deformation should be resolvable with EBSD.

The HAADF-STEM studies will map the specific polytypes of serpentine present as well as the distribution and type of defects in the crystal structure. We hypothesize that highly deformed regions within chemical gradients will exhibit defect structures reflecting that chemical environment.

References


Lienkaemper, J. J., Williams, P. L., & Guilderson, T. P. (2010). Evidence for a Twelfth Large Earthquake on the Southern Hayward Fault in the Past 1900 Years. Bulletin of the Seismological Society of America, 100(5A), 2024-2034.
Melosh, B. L. (2019). Fault Initiation in Serpentinite. Geochemistry, Geophysics, Geosystems, 20(6), 2626-2646.
Moore, D. E., McLaughlin, R. J., & Lienkaemper, J. J. (2018). Serpentinite-Rich Gouge in a Creeping Segment of the Bartlett Springs Fault, Northern California: Comparison With SAFOD and Implications for Seismic Hazard. Tectonics, 37(12), 4515-4534.
Viti, C., Collettini, C., Tesei, T., Tarling, M., & Smith, S. (2018). Deformation Processes, Textural Evolution and Weakening in Retrograde Serpentinites. Minerals, 8, 241.