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Cenozoic tectonics of the northern Mojave Desert

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Previous studies - Neotectonics of the northern Mojave Desert project

Geologic maps and chronology of selected active faults

Statement of problem: 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. However, detailed understandings of the faults in terms of their specific geometries, related features such as splay faults and folds, their history of slip since inception, and how they interact in space and time remain poorly understood. Detailed geologic and geophysical mapping in conjunction with chronological studies should significantly advance our understanding of the neotectonics.

Objectives: We wish to prepare and publish detailed geologic maps of key fault systems and their interactions. We also plan to better determine the history of faulting through improved chronology and mapping, and describe these histories in journal articles. The two avenues of study will culminate in research on the neotectonic evolution of the region. Planned products (actual number and titles depend on outcomes of workshop)

• 24K geologic maps
• journal article: histories of key faults
• journal article: interactions among faults and rotating tectonic blocks
• journal article: Pliocene paleogeography and tectonics of the central Mojave Desert


HOW …is the task being conducted (examples of methods, software, laboratory, etc.)

Geologic maps depict the spatial patterns of deposits and rocks of varying age that provides a synoptic representation of evolution of faults. We plan to target map areas that have the greatest potential to show offset in deposits ranging in age from pre-faulting (middle Miocene) to modern (Holocene). Faults will be prioritized by:

a) potential to yield information that can be extrapolated to broader areas or more faults,
b) potential to provide offset history by using deposits of many ages,
c) potential to create greater understanding of tectonics of the region, and
d) have superior current knowledge such as dated deposits, identified scarps, high-resolution geophysical data.

We expect that many map targets will be fault intersections, where evolution of fault interactions may be addressed. Geophysical methods will help with locating faults in areas with little geomorphic expression, and likely will be especially important for broad basin floors. Chronological emphasis will be on Pliocene and Quaternary deposits, each requiring innovation. We will compile and evaluate new geodetic data (GPS, InSAR) and historical seismology data.

We will define the principal scientific questions for understanding the neotectonic framework and evolution of the northern Mojave Desert in a workshop. Using this list, and moderating it with locations of superior data sets on timing, location, and other aspects of faults, we will create a prioritized list of fault targets. This will form the basis for selecting mapping targets for the first field season.

We will conduct field mapping, including acquisition of geophysical, chronological, topographic, and paleontologic data.

Highlights and Key Findings:

Initial mapping has shown that uplift rates are between 0.1 and 1 mm/yr at corners of tectonic blocks where faults interact. In addition to providing information on tectonic rates, these data allow uplifted deposits to be used as a guide for fault locations that are otherwise obscure.

Accomplishments:

Mapping underway. We have prioritized mapping targets and built connections with the Northern California Earthquake Hazards Project, geodetics and paleoseismology faculty in several schools, and identified some projects for students.

Progress:

This task focuses forward on mapping to illuminate neotectonics of the northern Mojave Desert, while addressing the past in seeing legacy maps from the Surficial Geology project (FY2001-2007) through publication. Accomplishments are divided into those two categories, and the illustration of map areas only shows the neotectonics part of the deliverables.

Legacy geologic maps:

Ivanpah 30x60: http://pubs.usgs.gov/sim/3206/

Amboy 30x60: http://pubs.usgs.gov/sim/3109/

Mesquite Lake 30x60: http://pubs.usgs.gov/of/2006/1035/.

Cuddleback Lake 30x60: http://pubs.usgs.gov/sim/3107/

Newberry Springs 30x60: http://pubs.usgs.gov/of/2011/1044/

Owlshead 30x60: (in USGS review and publication cycle).


Extrapolating neotectonic setting with regional geologic and topographic databases

Statement of Problem: We aim to extrapolate our new field data from a few specific sites to the remainder of the northern Mojave Desert that lacks site-specific studies. Extrapolating can be done after developing tectonic models, and will be guided by previous mapping at intermediate scales and by tectonic geomorphology interpretations of the region. Topography carries powerful information for understanding deformation history. At the regional scale, recently constructed Quaternary map data bases at 1:100,000 scale and DEMs at 30 m resolution offer the potential for describing neotectonics attributes such as: a) style of faulting, folding, and warping; and b) time-integrated history of deformation. The degree to which this approach succeeds depends on the quality of the regional datasets and our understanding of the geomorphic signal. We will improve the 1:100,000-scale tectonics databases by populating the fault data model with field data collected during past mapping and the limited published studies. We will study carefully chosen tectonic geomorphic settings to improve our ability to discriminate tectonic signals such as thrust vs. strike-slip vs. normal faults, folds, and warps, as well as better understand the decay of these landforms with time.

In addition, this task will house the Venture Capital studies of D. R. Bedford, who is developing high-resolution remote sensing methods with applications to mapping neotectonic features.

Objectives: Our first goal is to populate an intermediate-scale (1:100,000) fault database. The data collected during field mapping, previously published data, and attributes visible by stereoscopic study of aerial photographs will be gathered and used to populate the draft data model. Each fault segment will thus be queriable for data ranging from age information, topographic characteristics, structural data, and certainty assignments.

We wish to establish the tectonic geomorphic signals associated with active faults in several locations (mountain front, within mountains, mid piedmont, distal piedmont) and of several fault types (thrust, strike-slip, normal, and mixed modes). Best candidates for each situation will be established in the workshop and targeted for study. These will all be compared with well-characterized geomorphology in non-tectonic settings, such as the eastern Mojave Desert. We hope these studies will yield generalized characteristics that will allow assignment of fault properties partly from geomorphology alone. We also expect that very subtle features of tectonic geomorphology, including off-fault strain, will be quantified and described for use in urban and vegetated areas where subtle topography is difficult to describe. We hope to evaluate the effect of time on decay of geomorphic features for old faults that are well dated. This decay study will include fault features themselves such as scarps and shutter ridges, but also mountain-front characteristics that are longer-lived.

Finally, the data derived from the above studies will be used with topographic data (30-m DEMs) to evaluate the intermediate-scale fault database and improve it.


Methodology: Workshop to brainstorm methods and mapping targets
Field work using standard mapping methods and new topographic surveying if needed (e.g. targetless laser) Compare results with non-tectonic settings (eastern Mojave Desert) previously studied (e.g., Globe Fan) by quantifying shapes of alluvial landforms and patterns of sediments of various ages, as well as evaluating hillslopes and catchment characteristics Develop preliminary models for discriminating tectonic signals and evaluate existing models Implement regional fault data model and analyze.

Eventual products:
Journal article: Fault history of the northern Mojave Desert inferred from intermediate-scale mapping and geomorphic analysis

Journal article: Tectonic geomorphology metrics of fault style and age and implications for recognizing tectonic features in urban areas

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