Work Description

Date: 21 September, 2021

Dataset Title: Reverse faulting within a continental plate boundary transform system [Data set]

Dataset Creators: Kirk F. Townsend, Marin K. Clark, & Nathan A. Niemi

Dataset Contact: Kirk Townsend kirkft@umich.edu

Funding: EAR-1528576 (NSF), Department of Earth and Environmental Sciences (University of Michigan), Rackham Graduate School (University of Michigan), Evolving Earth Foundation

Key Points:
- Apatite and zircon helium thermochronometry data from the Western Transverse Ranges yield late-Miocene through Pleistocene cooling ages

- Reverse faulting within the block initiated prior to, and propagates towards, the Big Bend restraining bend in the San Andreas fault

- Rapid post-late-Miocene tectonic exhumation in the region surrounding the Big Bend is localized within the Western Transverse Ranges

Research Overview:
These datasets support the findings of Townsend et al. (in review). Contractional deformation is common along transform plate margins where plate motion is oblique to the plate boundary. While faults that accommodate this deformation are often inferred to be subsidiary to the main plate boundary fault, we typically lack direct geometric or kinematic information. Here we investigate the timing of faulting relative to changes in the orientation of the North American-Pacific plate boundary. Coeval with development of an oblique plate boundary segment (i.e. the “Big Bend” of the San Andreas fault), active shortening is inferred to have initiated at ~5 Ma in the Western Transverse Ranges (WTR). However, new low-temperature thermochronometric transects yield Miocene to Pleistocene apatite (U-Th-Sm)/He cooling ages and partially reset zircon (U-Th)/He ages. Inverse thermal modelling indicate that reverse faulting initiated as early as 10 Ma, several million years prior to our current understanding of the timing of the Big Bend. Reverse faults in the WTR also propagate from west to east, towards the San Andreas Fault, rather than outwards from it. New and existing thermochronometry data delineate the WTR as the locus of rapid post-Miocene exhumation, and demonstrate that similar exhumation is not present in the broader region surrounding the Big Bend. We posit that reverse faulting is localized in the WTR because of a weak underlying lithosphere and predates the more recent geometric anomaly of the restraining bend in the transform margin.

Methodology:
The data are 67 apatite (U-Th)/He low-temperature thermochronometry sample analyses and 16 zircon (U-Th)/He low-temperature thermochronometry sample analyses. Bedrock samples for apatite and zircon (U-Th)/He low-temperature thermochronometry were collected in April 2016 through June 2018 to infer rates of cooling and timing of fault initiation. Samples were crushed, sieved, and separated using standard methods to isolate apatite and zircon grains by exploiting differences in density and magnetic susceptibility. Individual mineral grains were hand-selected under a high-powered binocular microscope to screen for clarity, crystal morphology, and minimal inclusions of other potentially radiogenic minerals. Grains selected for analysis were measured along major and minor axes, photographed, packaged into individual Pt tubes, and analyzed for 4He content using an Australian Scientific Instruments Helium Instrument (Alphachron) at the University of Michigan Thermochronology Laboratory. Grains were heated for 5 minutes at 900°C, released 4He was spiked with 3He, and the 4He /3He ratio was measured on a Pfeiffer quadrupole mass spectrometer to determine the quantity of 4He. Following this initial 4He measurement, these analytical procedures were repeated to check for any additional extraction of 4He that might be indicative of micro-inclusions of high-temperature radiogenic minerals that were not observed optically during grain selection. The Durango apatite age standard was also analyzed with our samples to ensure accuracy of measurements of unknown age. After measurement of 4He, grains were dissolved and analyzed for U, Th and Sm concentrations following standard procedures (Reiners and Nicolescu, 2006) using a Thermo Scientific Elements 2 ICP-MS at the University of Arizona Radiogenic Helium Dating Laboratory.

Individual grain dates were solved for numerically in Matlab using parent and daughter nuclide concentrations and the age equation. Analytical uncertainties were propagated through the age equation using Monte Carlo methods. Grains with low uranium concentrations are particularly susceptible to age biases that result from uranium-implantation from surrounding U-rich phases. Grains with uranium concentrations under 5 ppm are reported but excluded from calculation of mean values. Outliers were identified and excluded following the Dean-Dixon (1951) method based on the 90 percent confidence interval at two significant digits. Using the remaining grain ages, we calculated a mean apatite (U-Th)/He age for each sample. Because the observed variability in our (U-Th)/He ages for individual bedrock samples is larger than the analytical error for single grains, we report mean ages for bedrock samples with uncertainty as the standard error of the mean for the multiple grains analyzed. We consider samples with a standard error greater than 1.0 Ma that is also greater than 20 percent of the mean age to have low reproducibility, and report ranges of individual grain ages instead of mean ages. We also report ranges of individual grain ages instead of mean ages for samples with grain ages that are older than the depositional age of the sedimentary rock from which they were collected, as these ages are likely inherited and do not reflect recent cooling of the sample.

Instrument and/or Software specifications: NA

Files contained here:
One spreadsheet containing thermochronometry sample information, and one spreadsheet containing a codebook for the sample information file:

- Thermochronometry.xlsx: contains thermochronometry sample information for 67 samples.
- codebook.xlsx: contains attributes, descriptions, data type and units for columns in Thermochronometry.xlsx

Related publication(s):
Townsend, K.F., Clark, M.K., & Niemi, N. (in review). Reverse faulting within a continental plate boundary transform system. Tectonics.

Use and Access:
This data set is made available under an Attribution Non-Commercial 4.0 International License (CC BY-NC 4.0).

To Cite Data:
Townsend, K., Clark, M., Niemi, N. Reverse faulting within a continental plate boundary transform system [Data set] [Data set], University of Michigan - Deep Blue Data. https://doi.org/10.7302/ab76-1628