This collaborative research effort is integrating seismological, geodetic, and geological information from EarthScope to investigate hypotheses regarding the present-day structure and evolution of the Great Basin region of the western United States. Initial discoveries enabled by the EarthScope program and prototype studies in this region have led to new and potentially related hypotheses that echo prominent research themes in observational and theoretical dynamics of the continents and their margins: (1) the evolution and control of subducting slabs on the mantle flow field, (2) the stability of the lower lithosphere against convective loss, and (3) the nature and extent of subhorizontal decoupling horizons within the lithosphere. The primary motivation for the current is the need to reconcile recent geophysical, geodetic, and geological findings in the Great Basin region directly related to these themes. Beneath the central Great Basin, seismic imaging reveals a cylindrical mass of higher than average wavespeeds east of the actively subducting Juan de Fuca plate near the zone of weakest azimuthal anisotropy in the western United States, along with a swirl-like pattern of fast polarization directions. When considered with other regional geophysical and geologic patterns, hypotheses that may explain these observations include mantle flow around a lithospheric keel, toroidal flow driven by the sinking of the Juan de Fuca slab, mantle downwelling driven by a lithospheric drip, and a number of other possibilities. Recent geodetic data for the Great Basin reveal transient changes in geodetic velocities, which when considered with other local geologic patterns, are consistent with the hypothesis that an active decoupling horizon exists, perhaps localized along the Moho or some other deep decoupling zone beneath the Great Basin. Further, relative to a dynamic model that matches Quaternary rates and orientations of deformation, a time-averaged strain rate solution obtained from campaign and continuous GPS shows a contractional dilatation anomaly in the same vicinity as the geodetic and seismic anomalies. The collocation of such a broad range of geophysical, geodetic, and geologic anomalies beneath the broadly extending Great Basin is unlikely to be coincidental, yet combined they defy conventional models of a classic extensional tectonic regime like the Great Basin. Understanding the relationship between these processes through a comprehensive series of hypothesis testing can transform our general insight of lithospheric dynamics. This project is focused on conducting a comprehensive suite of new investigations to test hypotheses focused on linkages between mantle flow, lithospheric decoupling, and lithospheric destabilization for the Great Basin region. This effort is utilizing new results developed through analyses of EarthScope USArray Transportable Array (TA), EarthScope Plate Boundary Observatory (PBO), and EarthScope Geology data. Specific datasets include seismic imaging (tomography, anisotropy, and receiver functions), continuous GPS, seismotectonics, and patterns of historic and late Quaternary seismic strain release in the upper crust. Results from these analyses will provide the required data for a series of new 3-D and 4-D numerical models developed within this project. This research is inherently integrative, and thus constitutes an important opportunity to combine results from different components of the EarthScope program for a tectonic setting that historically is among the best known and most enigmatic in the world. From a broader impacts perspective, this project represents a new multidisciplinary effort combining four separate Earth science disciplines to draw recent EarthScope-enabled discoveries into a holistic view of Great Basin evolution. Data collected and analyzed for this project will be distributed publicly to the scientific community. The project is enabling the training of several young scientists in multidisciplinary research. The PIs are coordinating with the EarthScope National Office and IRIS to provide findings and discoveries from this project in several forms, including an IRIS Active Earth module that looks into the Basin and Range from the surface through the upper mantle and will serve as an illustration of how continental-scale tectonic forces shape present-day surface deformation and deeper dynamics.
The goal of this project is to look at data generated during one of NSF’s most ambitious earth science projects to data —EarthScope—and see what it tells us about how continents deform through faulting and earthquakes. Most earthquakes and active fault lines occur within localized belts or zones along the boundaries between Earth’s tectonic plates. In the case of EarthScope, the two plates involved are the Pacific and North American tectonic plates. Where one or both of two plates in contact are continental, the belts tend to be diffuse, with considerable geometric and kinematic complexity. The largest active examples of these belts (c. 800-1000 km wide) are the Arabia- and India-Asia collision zones, the Aegean region, the central Andes, and the Basin and Range province of western North America. Of these five examples, the Basin and Range province stands out as one in which the kinematic histories of its active faults, including the timing and amount of slip for the most recent, and even the next two most recent earthquakes, have been determined on a substantial fraction of the total number of faults, especially in the Great Basin region of Nevada and Utah. As has long been noted about this region, the size and spacing of individual, fault-bounded range blocks and intervening valleys is striking in its organization and uniformity—a noteworthy counterexample to the prevailing complexity of continental topography. Because the formation of each range block is controlled by deeply penetrating earthquake faults, the region is particularly well suited for the study and understanding of the distributed style of continental deformation prevalent in places where most of the human race currently lives, like China and India. The new information from EarthScope available for North America provides a singular opportunity to explore the response of the continental lithosphere to plate boundary forces. As predicted by James Jackson of Cambridge University in England prior to the advent of EarthScope, we are beginning to progress "beyond the goal of trying to understand how faulting achieves the present-day large-scale motions, to the more profound question of how the fault configurations have evolved through time." At Caltech, Eugenie Perouse and Brian Wernicke have completed the first regional map of geologic slip rates and pre-historic earthquakes for a segment of the broader Pacific-North America plate boundary zone, which includes that portion of the more extensive Basin and Range province between latitude 35 °N and 42 °N. We have designed and constructed a database for release on the internet, showing an interactive map of active fault segments across the entire Great Basin region, a footprint of order 1000 km x 1000 km. Each fault segment is clickable revealing a detailed description of data for the fault. We have also compiled statistics of fault rupture for more than 120 pre-historic earthquakes, giving a rich picture of how and when earthquakes occur across the region. Lastly, we have constructed an animation showing fault activity as a function of time over the last c. 100,000 yr, four frames of which are shown in the accompanying image. Our map will constitute the first publicly available interactive database of fault slip data for this region or, to our knowledge, for any region on Earth. Perouse and Wernicke’s compilation shows a definite pulse of activity about 30,000 to 60,000 years ago in the middle of the Great Basin, which is now geodetically and geologically only very slowly deforming. Because it must have been deforming while the earthquakes were happening, the data define a timescale of about 50,000 years for cycling between clusters of earthquakes around the exterior of the province versus around the interior. Another major finding of the project is that there appear to be decadal-scale shifts of transient strain variation across the plate boundary zone, with crustal velocity changes concentrated in the west from 1996-2004, and in the east from about 2004 to 2014. A new seismic catalog generated by our collaborators at Aizona State University generally shows that seismic activity is fairly evenly distributed from 2006 to 2009 across the region, as the Earthscope "Transportable Array" crossed the region, the exception being the February, 2008 M6.0 Wells earthquake. Our group is currently in the process of attempting a new synthesis of these data and, in collaboration with William Holt of SUNY Stony Brook, trying to understand the fundamental physics of how earthquakes accommodate relative plate motion between continents, and the implications of this understanding for mitigating seismic hazards.
