Western Basin & Range - Eastern California Shear Zone

Science Center Objects

 

The Eastern California Shear Zone (ECSZ) Mapping project, funded by the National Cooperative Geologic Mapping Program, combines surficial and bedrock geologic mapping, geophysical surveys, and high-resolution topographic data analysis with neotectonic, geomorphic, structural, volcanic, and geochronologic studies to better understand the tectonic framework and landscape evolution of the ECSZ in the central and eastern Mojave Desert, California.

 

We are using these approaches to address map-based research questions like:

What are the timing and spatial distribution of fault slip across the northern portion of the ECSZ, and how do faults interact with one another, particularly at fault intersections?

 

What is the imprint of early Mesozoic compression and Cenozoic extension on the Quaternary and active tectonics of the region?

 

What are the distribution and geometry of groundwater basins in the northern Mojave Desert, what are the tectonic controls, and how do they fit into the context of the ECSZ?

 

What are the characteristics of contemporaneous Quaternary depositional units between the northern Mojave Desert and the Lower Colorado River Corridor? How are those units correlated?

 

The core of the Manix Fault (dark band in foreground) separates two Pliocene sedimentary sequences

The core of the Manix Fault (dark band dipping to the left in the middle of the image), which is an east-trending left-lateral fault in the central Eastern California Shear Zone, separates two Pliocene sedimentary sequences near Afton Canyon, central Mojave Desert

(Credit: Dave Miller, USGS. Public domain.)

Scientific & Societal Relevance:

Landscape Evolution

Erosion and tectonic uplift have profoundly shaped the Mojave Desert region. The weathering, transport, and deposition that shaped the topography also govern the enrichment and availability of many critical mineral deposits, the extent of groundwater resources, and the distribution of nutrients critical to ecosystems. Understanding how surface processes have been influenced by tectonics and past climate variability will improve managers’ ability to make effective land-use decisions related to the utilization and conservation of natural resources. High-resolution topographic data enable coupling of quantitative surface process models to the geologic record, facilitating the use of geologic maps and databases to characterize natural resources and to mitigate natural hazards. 

 

Magmatism

The Mojave Desert Region has been significantly affected by myriad ages and styles of igneous activity that are associated with different suites of mineral deposits, including those recently identified as “critical” for the United States’ national and economic security. Igneous rocks also provide important dateable stratigraphic markers for delineating the temporal evolution of basin formation, tectonic deformation, and sedimentary provenance. Better understanding the history of magmatism through geologic mapping can also be combined with 3D data to characterize potential geothermal energy resources.

 

USGS Research Geologist uses a field meter to measure magnetic susceptibility of basalt flows in the Mojave Desert.

Research Geologist Geoff Phelps measuring basalt flow thickness and magnetic susceptibility of individual basalt flows of the Broadwell Mesa Basalt in the central Mojave Desert near Ludlow, CA. 

(Credit: Kevin Schmidt , USGS. Public domain.)

Surface and Ground Water

Surface water and groundwater within the Mojave Desert serve as water supplies for population centers, National defense infrastructure, and native habitats of endangered species. The combination of rapid population growth, high water use, and arid climate has led to an increased dependence on groundwater, resulting in locally severe groundwater depletion and declining groundwater levels. Management of surface water and groundwater resources requires knowledge of the groundwater system, which in turn requires an understanding of the configuration and properties of aquifers.

 

Tectonic Evolution and Plate Boundary Kinematics

The geology and physiography of the northern and central Mojave Desert record all of the major episodes of orogeny, magmatism, continental extension, and basin formation that have shaped the Pacific-North American plate boundary since the Early Paleozoic. Active crustal deformation continues to shape the region, with direct consequences for geologic hazards, earth surface processes, and water resources. Increasingly sophisticated models of the tectonic evolution of the intermountain west require integrated regional geologic synthesis, subsurface geologic characterization, and quantitative description of geologic processes.

 

Lineations on a vertical strand of the Soda-Avawatz Fault Zone exposed in Northern Soda Mountains, California.

Lineations on a vertical fault strand of the Soda-Avawatz Fault Zone, exposed in the northern Soda Mountains, Mojave Desert, southern California. The fault can be seen cutting both late Tertiary sand and gravel units, but is buried by an overlying mid-Pleistocene alluvial gravel deposit.

(Credit: Andrew Cyr, USGS. Public domain.)

Integrated, Regional-Scale Geologic Map Database

Existing geologic map coverage of the ECSZ is inconsistent, mismatched across administrative borders, and lacks adequate surficial geologic detail, preventing comprehensive regional characterization of hazards associated with recent and active tectonic deformation. Seamless, multi-scale surficial and bedrock geologic mapping is essential for land management decisions. Complementary subsurface interpretations provide the template for regional geologic framework models of the earth’s composition, structure, and evolution. Geochronology, geochemistry, geophysics, and numerical modeling of Earth’s physical systems provide the analytical framework for understanding the timescales and physical properties of processes critical to mineral, water, and energy resource management, environmental health, hazard mitigation, and ecosystem impact.

 

Methodology:

We use a variety of techniques for building on recent 1:100k scale geologic mapping in the region. These include everything from traditional “boots on the ground” type field work to the application of geophysical and other remote sensing techniques, and targeted geochronology. These efforts are coordinated and compiled in a way that will contribute to a seamless, multi-scale geologic map database of the ECSZ. Specific approaches include:

Interpretation of remote sensing, including aerial and satellite imagery, DEM’s (including LiDAR and Structure for Motion), and spectral imagery. Many contacts and faults are visible on photo-imagery in the desert environment and can be accurately mapped as an aid to fieldwork.

Field mapping of contacts, faults, and folds, measurement of bedding and foliation attitudes, making detailed lithologic observations, and collecting samples for petrographic, geochemical, and geochronologic analysis. Most of this mapping is done directly on air photos or on field tablets with user-configured base maps, aided by the collection of many spot observations that are recorded using GPS.

 

Research geologist using satellite imagery on tablet computers to conduct geologic mapping on a mule pack train.

Research Geologist Kevin Schmidt and Geoff Phelps examine satellite imagery on tablet computers during geologic mapping field work near Broadwell Mesa while supported by mule pack train.  

(Credit: Andrew Cyr, USGS. Public domain.)

Collection, analysis, and modeling of gravity and aeromagnetic data. This includes standard techniques such as depth-to-basement modeling and gradient and frequency analysis, as well as novel forward modeling techniques.  These analyses yield models of subsurface geologic structure, which can be integrated with both surface and 3D mapping.

 

Simplified geologic, gravity, and aeromagnetic maps of the eastern-most fault of the Eastern California Shear Zone

Simplified geologic, gravity, and aeromagnetic maps of the eastern most fault of the Eastern California Shear Zone showing the locations and geometries of pull-apart basins. The locations, shapes and sizes of these basins provide constraint on fault step-overs and total offset along the Soda-Avawatz - Bristol-Granite Mountains Fault Zone. Red lines - faults; dashed blue lines - playas; dark blue lines - 0.5 km contours of basin thickness; plus signs - locations of maximum horizontal gradients, with smaller symbols denoting gradient values below the mean value and larger symbols denoting gradient values above the mean value. 

(Credit: Victoria Langenheim, USGS. Public domain.)

 

Digital compilation of geologic mapping as field mapping progresses. As needed, field mapping is modified in a GIS, based on digital imagery and GPS-based locality information. This is conducted in cooperation with other USGS geologic mapping projects in order to build towards a seamless geologic map database of the region.

Analytical work, including appropriate numeric chronology and geochemistry from both sedimentary and igneous rocks is used to correlate map units and to constrain the timing and rate of offset of faults of the ECSZ.