CREST Experiment Probes the Roots and Geologic History of Colorado Rockies

Title: CREST Experiment Probes the Roots and Geologic History of the Colorado Rockies
Authors
: R. Aster, J. McCarthy, Matt Heizler, and Shari Kelley (New Mexico Tech), K. Karlstrom and L. Crossey (University of New Mexico), Ken Dueker (University of Wyoming) and the CREST Team
Publication: The Outcrop, January 2009, p. 6-8, 10-11, 21

Introduction:

General Tectonic Setting of the Colorado Rockies. The Colorado Rockies are the climax of an enigma. They present a major young mountain range located approximately 1000 km from the nearest plate boundary (the San Andreas fault, which separates the North American and Pacific plates). In broad global tectonic context, the Colorado Rockies occupy the easternmost extent of the deformed western United States, an unusually broad tectonically active transition zone lying between the San Andreas Fault system and the stable central and eastern parts of our continent (Figure 2).

The fundamental mechanism for the initial uplift of the Colorado Rockies is widely believed to be low-angle subduction of the Farallon plate during the Laramide Orogeny between approximately 75 and 50 million years ago. Because the oceanic slab was subducting at a low angle, it transmitted sufficient forces to the shallow crust and mantle (the lithosphere) to create great Laramide thrust structures as far east as the Black Hills of South Dakota. Ancillary evidence for low­ angle subduction during the Laramie can be found in a paucity (but not total absence) of volcanic deposits during this period, consistent with a cold slab running along the base of a cool lithosphere .The subducting oceanic slab also lost its volatiles to the overlying continent, adding buoyancy and driving uplift of the western part of the North American plate.

However, the Laramide Orogeny was just the first act in a three-act play that has shaped the rugged topography of the western U.S.The second act began about 35 million years ago when the strike-slip San Andreas Fault system began to form and the Pacific-North America plate boundary transitioned from compressive subduction to the present strike-slip system. This transition had two dramatic effects on western North America. First, the stress across this vast region relaxed and compressively thickened Laramide lithosphere began to gravitationally relax. Dramatic results of this extension include the Great Basin  and the  Rio Grande rift. Second, the sinking of the trailing edge of the Farallon slab permitted mantle upwelling of underlying asthenosphere to heat the previous slab-cooled lower lithosphere that once resided above the subducting slab. The combination of upwelling mantle, increasingly extensional tectonics, and hydrated uppermost mantle was literally explosive, producing vast volcanic across the western United States. This post-Laramide “ignimbrite flare-up” included a San Juan Mountains super-volcano that erupted from multiple calderas 35-25 million years ago. This is one of the largest recognized volcanic complexes in the global geological record; the estimated eruptive volume of the San Juan La Garita Caldera complex is as large as 5000 cubic kilometers. The third act of the play, occurring during the last several million years , involves continued interactions between the North American plate and the underlying flowing mantle. These interactions include mantle plumes like the one under Yellowstone, continued volcanic like the Jemez volcano in New Mexico and ongoing uplifts such as the southern Sierra Nevada Mountains resulting from the upward flow of buoyant asthenospheric mantle replacing downward sinking of parts of the plate. Such upwellings of magma, heat, and  fluid  may be taking advantage of zones of plate weakness, providing glimpses of how old continental features influence young tectonic expressions. In addition to these tectonic and volcanic processes, the Rockies that we see today have, finally, been heavily sculpted by water and glacial erosion, most notably during the last 3 Ma as climate cooled.

The CREST Project. A major new science effort, CREST (Colorado Rockies Experiment and Seismic Transects), recently begun, is designed to help answer questions about how mantle processes beneath the Colorado Rocky Mountains have influenced their tectonic history. We are currently collecting and analyzing seismic, geochronologic, geochemical, and topographic data to study Earth’s crust and mantle in the region. A key aspect of this work is to understand when and why changes in buoyancy forces and dynamic mantle convection forces have occurred since the end of Laramide subduction, as well as to understand current mantle dynamics. CREST was partially motivated by the prior discovery of a poorly resolved low velocity mantle seismic anomaly underlying the central Colorado Rockies, which we call the Aspen Anomaly (Figure 1; Figure 2). The Aspen Anomaly lies somewhere between the uppermost mantle and, perhaps, as deep as many hundreds of kilometers. It is part of a collection of major upper mantle velocity anomalies beneath the western United States that include Yellowstone (recently confirmed to be a mantle plume extending at least 600 km into the mantle)  and  the Rio Grande Rift-Jemez region (which is largely limited to the uppermost 300 km of the mantle). Testable hypotheses regarding the nature of the Aspen Anomaly tend to involve entangled processes that are difficult to isolate. It may be a region of broad scale mantle upflow associated with global convective flow: the flow pressures dynamically raising and continuing to support the Rockies today. Alternatively, the Aspen Anomaly may manifest hydrated lower lithosphere associated with the Colorado Mineral  Belt, which is a Proterozoic lithosphere-scale zone hot springs. Because 3He represents escaping primordial gases that are surface of the planet. In parallel with this revolution in instrumentation, a the Colorado Mineral Belt, which is a Proterozoic lithosphere-scale zone of weakness that likely originated at the time of continental accretion. Ancient architectural sutures within the continents are known to widely influence recent and present-day tectonics and volcanism (the Jemez Lineament in northwestern New Mexico  is a prominent example). Emerging evidence that the high Rockies above the Aspen Anomaly may be tectonically active and  currently uplifting includes active degassing of deeply derived gases rich in CO2 and with high 3He/4 He ratios that are found  in most of the  region’s hot springs. Because 3He represents escaping primordial gases that are a  relic of  planetary  accretion, high 3He/4He ratios are a clear signature of volatile connections between the mantle and the surface.

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Figure 1. Western United States P-wave seismic velocity estimated at a depth of 100 km (c/o Ken Dueker; after Humphreys et al., 2003). Major low velocity anomalies in beneath the Rocky Mountain Region are labeled. CREST is focusing on the nature and influence of the presently poorly resolved Aspen Anomaly beneath the Colorado Rockies. CREST, EarhtScope USArray, and other ongoing experiments are presently making dramatic strides in improving the resolution and accuracy of such mantle maps.

CREST is taking place during a revolutionary period in seismic imaging that seeks to clarify how the North American continent is and has been shaped by mantle processes. Advances in portable seismographic instrumentation during the  past 20 years have now made it possible for researchers to deploy arrays of up to thousands of state-of-the-art seismic recorders  anywhere on the solid surface of the planet. In parallel with this revolution in instrumentation, a variety of innovative techniques have been developed to form detailed seismic images. Generally, these imaging methods are of two types: 1) we can look for seismic layering that shows up as velocity discontinuities (for example the Moho discontinuity at the base of the continental crust, a zone of interesting complexity in the Rockies) and 2) we can create tomograms (analogous to CAT scans) that reveal bulk mantle seismic velocity structure (for example the low velocity domain of the Aspen Anomaly). Much of this imaging is done seismic “listening” by recording how long it takes seismic waves to reach a given instrument from large earthquakes in Indonesia, South America, and elsewhere around the globe. The distribution  of  global earthquake sources, as well as use of ambient make it possible to form state-of-the art images of mantle structure after about a year and a half of continuous seismic “listening.”

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Figure 2. Close-up view of the topography and underlying mantle velocity structure at 100 km (from Figure 1), superimposed on gray-scale topography. Note the geographic association of the Aspen Anomaly with the high Rockies. Symbols show extent of existing or planed (dashed) seismograph stations.

A large number of studies in  the western United States and elsewhere during recent years have abundantly demonstrated that strong mechanical, volcanic, and geochemical coupling exists between deep crustal and mantle processes such as extension, lower crustal eclogitic delamination, and small-scale convection and upper­ crustal tectonics. Partially driven by these remarkable observations of dynamism at the lithosphere­ asthenosphere scale, EarthScope , a  vast deployment of geophysical instrumentation, including GPS and seismometery across the United States was initiated in 2003. EarthScope will probe the deep structure of the entire conterminous U.S. with a 2000 station moving network of seismographic stations dubbed USArray, through 2013, with a subsequent deployment planned for Alaska. At present, USArray is deployed between the northern Rockies and the southwestern U.S., and CREST has recently complemented this seismograph deployment with an additional 59 stations (Figure 3) specifically focused on the region of the Aspen Anomaly. The combined USArray/ CREST instrumentation constitutes one of the largest and densest seismic arrays currently deployed on the planet.

CREST will produce new 3-d images of the present Rockies. However, the fourth dimension is time. In imaging the state of the lithosphere, we also need to understand the rich prior history of North America . An important CREST component is a new suite of geochronology measurements designed to clarify patterns of Cenozoic magmatism and refine the timing of basaltic volcanism. This is being carried out with 40Ar/39Ar dating, focusing on a wide distribution  of Laramide-age pluton within the Colorado Mineral Belt. The timing of uplift and erosion is being explored using samples collected from key locales throughout the study area via apatite fission track and the (U+Th)/ He thermochronology methods which measure rock cooling histories related to exhumation. Additional key constraints on uplift history are being gathered from incision rate estimates derived from drainage profiles in the major river systems and other topographic constraints. CREST will ultimately produce, we expect, a newly clarified understanding and level of geodynamic modeling of the current tectonic forces and processes of the Colorado Rocky Mountains, and their ties to deep Earth processes. We also hope that this will significantly enlighten our understanding of the formation and evolution of this region. During the past decade much has been learned , and many further questions have arisen,about the complex interactions between continental  history and structure and today’s active tectonics. The Rockies are a key locality to study these  interactions, because they represent a region where  coupling between old structures (even those dating back to the Proterozoic) and recent tectonics are strong. As such, the region is a natural laboratory to help us not only better understand one of America’s  iconic scenic landscapes, but also to inform global understanding of plate tectonics and the histories of the continents. The enigma of the Colorado Rockies is that they are a major mountain range over 1000 km from the plate boundary. The resolution of this enigma is coming from a better realization that the mountains are a manifestation of a wide deforming plate margin domain that is strongly coupled to deep mantle processes, and demonstrates that critical plate boundary processes exist not only at the edges, but also at the bases of moving plates.

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Figure 3. Michael Johnson (New Mexico Tech IRIS PASSCAL Instrument Center) installing a CREST seismograph near Silverton, Colorado, in August 2008

 


Acknowledgments

CREST is supported by the National Science Foundation Continental Dynamics Program under award 0607693. We thank the IRIS PASSCAL Instrument Center at New Mexico Tech for facility support and field assistance. Data collected will be available through the IRIS Data Management Center. The facilities of the IRIS Consortium are supported by the National Science Foundation under Cooperative Agreement EAR- 0552316, the NSF Office of Polar Programs and the DOE National Nuclear Security Administration. The CREST team includes Andres Aslan, Rick Aster, Clem Chase, Dave Coblentz, Laura Crossey, Ken Dueker, Lang Farmer, Matt Heizler, Eric Kirby, Eric Leonard, Colin Shaw, and Jolante van  Wijk,  and graduate students Jonathan MacCarthy, Andy Darling, Joshua Feldman, Rebecca Garcia, Steve Hansen, and Zhu Zhang. For more information, see http://www.ees.nmt.edu/Geop/CREST. More information on Earthscope can be found at http://www.earthscope.org.

References

Humphreys et. al., How Laramideage hydration of the North American lithosphere by the Farallon slab controlled subsequent activity in the western United States, Int. Geology. Rev., 45, 575-595, 2003.

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