GMEG - Geology and Geophysics
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What is this project about?
The project combines traditional surficial geologic mapping with geophysical investigation. Geologic maps, map databases, and project reports will have widespread application for:
Map showing previous and proposed study areas
|Click on maps and images for a larger view and to see more information.|
| Statement of Problem: The northern Mojave Desert is the least studied and least understood part of the Eastern California Shear Zone, a broad
swath of strike-slip faults that departs from the San Andreas fault near the Transverse Ranges in southern California and
passes east of the Sierra Nevada. This zone accommodates about 20% of interplate motion on poorly defined faults,
and represents a significant seismic hazard for cities through the Mojave Desert and north to Reno, NV, as well as for
military bases and national parks, utility corridors to major coastal population sources, and transportation routes.
Modeling of the zone is crucial for San Andreas fault models, and the northern Mojave Desert represents a critical, but
poorly understood, link in such models.
This northern Mojave Desert part of the zone transitions from several strike-slip faults spaced across a broad area of the southern Mojave Desert to just three major strike-slip faults in the southern Basin and Range province. The nature of this transition, including which faults are active and changing patterns of active faults through time, remain to be identified.
Recently completed 1:100,000-scale USGS mapping of surficial geology across the northern Mojave Desert has identified many new faults, trends in ages of last slip on faults, and active faults that previously were not considered to have slipped during the Holocene. These provocative findings represent a major step forward in understanding and yet are only partly studied due to the scale and time constraints for the previous mapping. Detailed 1:24,000 scale maps of selected faults and their intersections will significantly reduce uncertainties in the new findings, as well as nicely utilize the regionally consistent intermediate-scale mapping (1:100,000). In addition to providing better understanding of the neotectonics of a critical part of the Eastern California Shear Zone, detailed mapping in the northern Mojave Desert will take advantage of the superb desert exposures to examine minor tectonic features that are difficult to study in more vegetated areas. The results may provide lessons for interpreting fault features in urbanized or vegetated areas.
Complete delineation of subtle “off-fault” deformation by minor faults, folds, and warps also has the potential to provide generalized lessons for other areas. Tectonic clues may be determined from geomorphology and extrapolated to other areas.
(To left: Study area map, new fault and geology mapping, an example of a geophysical mapping product, and interpretations of fault activity based on mapping. Click on images for a larger view.)
Earlier USGS surficial geology mapping addressed the northern Mojave Desert part of the Eastern California Shear Zone (ECSZ), identified many previously unknown faults, and began detailed study of several. The ECSZ accommodates about 20% of interplate motion on poorly defined faults, and represents a significant seismic hazard for cities, military bases, and national parks, as well as a potential to disrupt utility corridors and transportation routes to major coastal population centers.
The current project extends detailed mapping to the east margin of the ECSZ. Methods included detailed 1:24,000 scale mapping, geophysical and paleomagnetic studies, study of pre-ECSZ rock units, and geochronology. These studies will aggregate the fault-slip histories providing a more complete evaluation of the tectonics and earthquake hazards. In addition to better understanding the distribution of active faults and their slip histories, we address the broader questions of how dextral and sinistral faults, as well as rotating blocks vs. stably slipping blocks, interact in space and time. Products for this work will be fault maps of the Mesquite Spring, Soda-Avawatz, and Broadwell Mesa faults, and detailed geology of the Cronese Hills area of the two splays of the Cave Mountain fault. Geophysical investigations are expected to be crucial. Our work includes study of Pliocene and Miocene deposits as the pre-faulting template and evidence for changes in paleogeography during long-term faulting. We dovetail with water resource studies at Fort Irwin to help evaluate northward continuations of active tectonics while aiding in ongoing water evaluations in the small basins.
Good landscape exposure in the Mojave region makes it ideal for geological and ecological study!
Relevance and Impact: The Eastern California Shear Zone (Dokka and Travis, 1990) part of the San Andreas fault system accommodates more than 20% of North American – Pacific plate motion (Sauber and others, 1994), and thus represents a significant seismic hazard and a necessary component of predictive seismic hazard models for the San Andreas system. Since 1990, several major earthquakes have occurred in the southern part of this system in the southern Mojave Desert (Landers, Big Bear, Hector Mine). Studies associated with these major earthquakes have led to improved understanding of the many faults and their interactions in this area (e.g., Haukson and others, 1993). Studies in the southern Basin and Range province have led to improved understandings in this area. However, the northern Mojave Desert represents a gap in current understanding. For instance, microseismicity does not lie along the previously mapped faults in many cases, and high rates of geodetically-determined strain occur in areas (McClusky and others, 2001; Miller and others, 2002; Peltzer and others, 2001) where previously mapped faults have not ruptured during the late Quaternary (Oskin and Iriondo, 2004). The lack of agreement raises questions about very fundamental understandings such as which faults are active. Recent ruptures in the southern Mojave Desert have crossed from fault to fault in unexpected patterns that some have speculated means new fault systems are forming (e.g., Nur and others, 1993). Are similar new fault systems forming in the northern Mojave Desert? In addition, the manner in which active faults transition from one province to the next is not understood, both to the north (Garlock fault; Garfunkle, 1974; Clark, 1973; Louie and Qin, 1991) and the south (Barstow-Bristol trough; Howard and Miller, 1992). Another outstanding problem is the poor understanding of how the faults in different structural domains interact. In the northern Mojave Desert, a set of east-striking faults lies in the general area of Fort Irwin southeast to the Cady Mountains. Structural blocks in this area have undergone significant vertical-axis rotation, unlike those in surrounding tectonic domains with northwest-striking faults (e.g., Carter and others, 1986; Luyendyk, 1991; Schermer and others, 1996). These observations have led to models that suggest that the east-striking faults and intervening blocks originated as conjugates to the northwest-striking faults, and have rotated to a position where they are no longer active. However, faults of the two orientations in some places are currently active and interlaced in space and time (Miller and others, 2005), requiring vastly more complex models than currently used (McCluskey and others, 2001; Schelle and Grunthal, 1996).
Our understanding of neotectonics of the northern Mojave Desert is more primitive than adjoining areas, resulting in a limited ability to predict seismic hazards here and in the San Andreas system as a whole. Improved understandings from mapping and related studies will improve our ability to describe seismic hazards, which is necessary to reduce social and economic costs from these hazards.
We use geochronology of offset markers, Lidar and detailed air photo study of geomorphology, and geophysical investigations of sedimentary basins associated with faults to improve interpretations. All data will be added to the fault database for the northern Mojave Desert and made available to earthquake hazard scientists and land-use planners.
We have defined the principal scientific questions for understanding the neotectonic framework and evolution of the
northern Mojave Desert. Using this list, and moderating it with locations of superior
data sets on timing, distribution, and other aspects of faults, we have created a prioritized list of fault targets. We have also defined a set of tectonic geomorphology questions that will drive field studies and synthesis
studies. These include differentiating geomorphic signatures for different kinds of faults and their decay with time.
Location of detailed mapping investigations in the Mojave Desert region.
The ecology connection: Detailed surficial mapping also has application for ecological evaluation of the Mojave Desert. Properties of surficial material (soils, alluvium, stream channel deposits, etc.) affect how precipitation and runoff infiltrate and retained in soil. This in turn affects how plants utilize water which reflects on the architecture of the ecosystem. Combined studies of geologic mapping and ecology provide insights into ecosystem recovery in disturbed lands and assist in evaluating the impact of climate change and other ecosystem functions in the future.
Click here to take a "Science Tour" of maps and images highlighting examples of geologic mapping and science investigation capabilities in the Mojave Desert region.
Selected references cited above:
Bull, W. B., 2007, Tectonic geomorphology of mountains: A new approach to paleoseismology: Blackwell Publishing, 324 p.
Bull, W. B., and Wallace, R. E., 1985, Tectonic geomorphology: Geology v. 13, p. 216-219.
Carter, J. N., Luyendyk, B. P., and Terres, R.R., 1987, Neogene clockwise rotation of the eastern Transverse Ranges, California, suggested by paleomagnetic vectors: Geol. Soc. Am. Bulletin, v. 98, p. 199-206.
Clark, M. M., 1973, Map showing recently active breaks along the Garlock and associated faults, California: U.S. Geological Survey Miscellaneous Geologic Investigations Map I-741, scale 1:24000.
Dibblee, T. W., Jr., 1961, Evidence of strike-slip movement on northwest-trending faults in the western Mojave Desert, California: U.S. Geological Survey Professional Paper 424-B, p. B197-199.
Dokka, R. K., and Travis, C. J., 1990, Role of the eastern California shear zone in accommodating Pacific-North American plate motion: Geoph. Res. Letters, v. 17, p. 1323-1326.
Garfunkel, Z., 1974, Model for the late Cenozoic tectonic history of the Mojave Desert, California, and its relation to adjacent areas: Geol. Soc. Am. Bulletin, v. 85, p. 1931-1944.
Howard, K. A., and Miller, D. M., 1992, Late Cenozoic faulting at the boundary between the Mojave and Sonoran blocks: Bristol Lake area, California, in Richard, S. M., ed., Deformation associated with the Neogene eastern California shear zone, southwestern Arizona and southeastern California: Redlands, CA, San Bernardino County Museum Special Publication 92-1, p. 37-47.
Hauksson, E., Jones, L. M., Hutton, K., and Eberhart-Phillips, D., 1993, The 1992 Landers earthquake sequence: Seismological observations: Journal of Geophysical Research, v. 98, p. 19835-19858.
Jennings, C. W., 1994, Fault activity map of California and adjacent areas with location and ages of volcanic eruptions: California Geologic Data Map Series, Map No. 6, California Div. Mines Geology, scale 1:750,000.
Louie, J. N. and Qin, J., 1991, Subsurface imaging of the Garlock Fault, Cantil Valley, California: Journal of Geophysical Research, v. 96(B9), p. 14,461-14,479.
Luyendyk, B. P., 1991, A model for Neogene rotations, transtension and transpression in southern California: Geol. Soc. Am. Bulletin, v. 103, p. 1528-1536.
McClusky, S. C., Bjornstad, S. C., Hager, B. H., King, R. W., Meade, B. J., Miller, M. M., Monastero, F. C., and Souter, B. J., 2001, Present day kinematics of the Eastern California shear zone from a geodetically constrained block model: GOP.Geophysical Research Letters, v. 28, p. 3369-3372.
Miller, D. M., and Yount, J. L., 2002, Late Cenozoic tectonic evolution of the north-central Mojave Desert inferred from fault history and physiographic evolution of the Fort Irwin area, California: Geol. Soc. Am. Memoir, 195, p. 173-197.
Miller, D. M., Menges, C. M., Amoroso, L., Schmidt, K. M., Phelps, G. A., Lidke, D. J., Dudash, S. L., 2005, New Quaternary geology map of faults, northern Mojave Desert, California: Geol. Soc. Am. Abstr, Prog., v. 37, no. 4, p. 98.
Nur, A., Hagai, R., and Beroza, G. C., 1993, The nature of the Landers-Mojave earthquake line: Science, v. 261, p. 201-203.
Oskin, M. and Iriondo, A., 2004, Large magnitude transient strain accumulation on the Blackwater fault, Eastern California shear zone: Geology, v. 32, p. 313-316.
Peltzer, G., Crampe, F., Hensley, S., and Rosen, P., 2001, Transient strain accumulation and fault interaction in the Eastern California shear zone: Geology, v. 29, p. 975-978.
Sauber, J., Thatcher, W., Solomon, S., and Lisowski, M., 1994, Geodetic slip rate for the eastern California shear zone and the recurrence time of Mojave Desert earthquakes: Nature, v. 367, p. 264-266.
Schelle, H., and Grunthal, G., 1996, Modeling of Neogene crustal block rotation: Case study of southeastern California: Tectonics, v. 15, p. 700-710.
Schermer, E. R., Luyendyk, B. P., and Cisowski, S., 1996, Late Cenozoic structure and tectonics of the northern Mojave Desert: Tectonics, v. 15, p. 905-932.