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Enhancing Earthscope by Constraining Vertical Motions of the Continental Crust and Surface J.A. Spotila, Virginia Tech; [email protected] The Earthscope facility will have an enormous impact on geoscience, largely because of its size, systematized approach, and incorporation of new technologies to constrain key geologic problems. With minor modification, the facility can be enhanced using other new technologies and the same systematic strategy to address additional aspects of tectonic and geodynamic processes and a broader spectrum of other scientific questions. One data set that is not heavily emphasized by the current vision of Earthscope is the vertical motion of the continental crust and surface. By expanding the range of observations to systematically constrain decadal surface uplift using space-based observation or absolute gravimetry and the medial-term (103 -105 yr) and long-term (>106 yr) denudational history of the continent using cosmochronology and lowtemperature thermochronology, the scientific value of Earthscope may be enhanced. Three scientific motivations and some basic ideas on implementation of these enhancements are described below. 1) Plate motion partitioning via vertical motion of continental crust That mountain building is representative or even synonymous with active continental tectonics illustrates the importance of vertical crustal motion in some plate boundary systems. In many tectonic settings, including examples along the Pacific-North America plate boundary, surface uplift, exhumation, and mass redistribution via erosion and sedimentation play important roles in accommodating long-term plate motion. The kinematic data collected as the main objective the PBO facility will enhance our understanding of how horizontal plate motions and strain are distributed at the decadal timescale. In addition, surface uplift may locally be rapid enough to be constrained using continuous GPS over the timescale of 5 to 10 years (i.e. >1 mm/yr). In these areas it will be very useful to compare short-term vertical strain with the long-term record of vertical crustal motions, which can be approximated by exhumation histories of rock or depositional histories of adjacent basins. In addition, geologic studies of exhumation may reveal important mechanisms of plate motion accommodation where present-day uplift rates are too small to be measured. Two examples illustrate why long-term vertical motions should be a significant part of PBO's scope. A major problem that all four branches of Earthscope will address is the behavior and mechanics of the San Andreas fault. Key questions include whether the fault is strong or weak, whether it deforms as a wrench or partitioned system, how it accumulates and releases strain, and how it has evolved over the long-term. In some locations, measurable components of the San Andreas' slip budget are manifest as vertical tectonism. This is true where local geometric complexities force convergence, such as at a bend in the Santa Cruz Mountains, at the intersection with the Garlock fault, or at the restraining bend of San Gorgonio Pass. Shortening and related uplift and exhumation are also important along much of the fault's length, because of its transpressive obliquity to fault motion. This includes the 5o obliquity in central California that built the Coast Ranges and the greater obliquity along the big bend in southern California that helped build the Transverse Ranges. In the former, the kinematics of convergent deformation are best constrained by structural reconstruction of fold/thrust belts. In the latter, constraining vertical tectonism as the wholesale uplift of rocks is more important, as convergence in crystalline terranes is well reconstructed using exhumation history. In one part of the Transverse Ranges (Yucaipa Ridge), for example, narrow crustal slivers within the San Andreas fault zone itself have exhumed at rates of up to 5 mm/yr and have thus accommodated a significant component of plate motion via mass redistribution. This partitioning of motion into vertical tectonism within the fault zone has implications for models of the mechanics of transpression as well as for the long-term evolution of the fault system. Constraining exhumation and vertical crustal motions are thus important to understanding the plate boundary processes associated with the San Andreas fault. A second critical topic in tectonics is the importance of surface processes to deformation partitioning in convergent plate settings. Studies have shown that orographically-controlled precipitation gradients can lead to concentration of exhumation along portions of mountain belts, thereby influencing convergent tectonic systems. An example of where this may be true is in the Chugach/St. Elias Mountains of southeast Alaska, where underthrusting of the Yakutat plate has caused intense uplift and exhumation in the North America plate over the past several million years. One reason why this convergence may have been so well accommodated is the rapid erosion of glaciers, which have dominated the mountain range for its entire existence. Whether glacial erosion has played a deterministic role in plate motion partitioning can be tested by comparing exhumation patterns to patterns of precipitation and glaciation. Studying vertical tectonism is thus critical to understanding this portion of the plate boundary, where a transition is made from continental transform to subduction zone and plate motion is manifest mainly as orogeny. This example also illustrates how important the long-term record of vertical tectonism can be. Present surface uplift of southeast Alaska is complicated by relaxation associated with large earthquakes and isostatic rebound associated with deglaciation. To achieve a record of how plate motion is accommodated by mass redistribution at the surface thus requires studies of long-term mountain building. These examples show how it is worth constraining vertical tectonism in some locations to study tectonic processes. The basic tools required for such constraints can include basic geologic data (e.g. measuring vertical motions by correlating offset features), stratigraphy (e.g. basin reconstruction), low-temperature thermochronometry (e.g. apatite [UTh]/He dating, Tc=60-80o C; or apatite fission track dating, Tc=105o C), and drainage basin cosmochronology (e.g. 26 Al and 10 Be, exposure ages and erosion rates over 10 7 -105 yr). Selective application of these techniques in the PBO or SAFOD initiatives may provide useful constraints on vertical tectonism, which may in turn enhance the understanding developed by Earthscope of plate tectonic processes. 2) Geodynamics of the crust and lithosphere Unprecedented data on the thermal, compositional, and rheological structure of the crust and upper mantle to be developed by USArray will greatly enhance our understanding of the lithosphere's dynamic behavior. It will be interesting to see, for example, how topography relates to crustal thickness or mantle density structure. Although surface elevations may be useful in understanding the structure and processes in the lithosphere below, a greater understanding may be achieved if the history and modern rates of change in surface elevation can be measured. There are numerous examples of where structure and processes within the upper mantle and crust may be reflected in topography and surface uplift. The 2-km-high Colorado Plateau of the western U.S. is one, as its origin has been linked to crustal thickening via magmatic injection or intra-crustal flow, buoyancy associated with mantle density gradients and thermal structure, delamination of the mantle lithosphere, or simply thickening via convergent tectonics. USArray may shed light on what is actually beneath the Plateau today, and it would be useful to know whether the surface elevations are increasing or decreasing accordingly. A similar (non-U.S.) example of the importance of lithospheric structure on orogenic plateaus is Tibet, where topography may directly represent the viscosity of the flowing lower crust. Other examples of where topography and uplift may be important in understanding the behavior of the crust and upper mantle include California's Transverse Ranges, where topography appears associated with a high density anomaly in the upper mantle that may represent a sinking slab, or the southern Sierra Nevada, where a high density anomaly does not correlate with topography. For much of the western U.S. and Alaska, surface elevation change at the decadal timescale may be measured by continuous GPS of the PBO and InSAR, depending on the magnitude of motion and the timeframe studied. To fully understand the dynamic behavior of the lithosphere that will be imaged by Earthscope, however, it may be important to constrain the decadal change in surface elevation at locations spread across the entire continent. In almost all cases, we would expect such change to be unmeasurable, given the lack of geologic evidence for sustained vertical motion. There are locations, however, where rapid elevation change has been suggested by leveling studies over the previous century, such as the Adirondack Mountains in New York, the Blue Ridge Mountains in North Carolina, and the coast of Maine. Some of these may represent change associated with glacial rebound, an important process worth constraining. Some of these examples have also been interpreted by geodesists as representing epeirogeny, in which broad scale warping of the continental surface occurs rapidly over short, but unsustained periods. An example of this view can be seen in a widely-used undergraduate geology textbook (Understanding Earth; Press and Siever, 3rd Ed., 2001), as a map of present-day uplift and subsidence rates across the entire U.S. is shown, including rates of over 1 cm/yr uplift across the midwest and southern Appalachians (Figure 21.19, p. 500). Although this map is not well constrained, it illustrates that epeirogeny as a dynamic lithospheric process is still so poorly known that it remains a poorly defined element in introductory geologic instruction. Expansion of decadal surface uplift measurements beyond the region covered by the PBO could lead to an ultimate goal of creating a map of surface uplift and subsidence rates for the entire U.S. For example, continuous GPS stations (vertical accuracy gives resolution of about 1 mm/yr in 5-10 yr) or sites of repeated measurements with absolute, fallingmass gravimetry (accuracy of 0.01 µGal can give vertical resolution of up to 1 mm/yr) could be spread across a moderate-resolution grid (~500 km) throughout areas not covered by the PBO, either as an expansion of PBO or a broadening of the tools of USArray. Observations of this type are already being made along the East Coast, where absolute surface motions are being measured to constrain the stability of tide gauges. The resulting pattern could subsequently be compared to numerous structural aspects of the crust and upper mantle. It would also be interesting to compare the observations of recent surface uplift across a continent-wide grid with constraints on long-lived vertical crustal motion. Geomorphic studies could thus be useful, as sustained topographic change is generally associated with perturbation of erosional and depositional systems. For example, at each location where decadal surface uplift is measured, it would be worthwhile to constrain long-term (106 -107 yr) exhumation rates using low-temperature thermochronometry or medial-term (103 -105 yr) erosion rates using basinscale cosmochronology and sedimentology. A comparison of present-day surface uplift and long-term could explore the long-term evolution of geodynamic processes and would also relate importantly to other scientific problems. 3) Landscape evolution of the continent Inclusion of measurements of decadal surface uplift or long-term exhumation will improve our knowledge base and understanding of tectonic and geodynamic processes. It will also open doors to study other Earth processes in a systematic way. The long-term evolution of landscapes is of considerable interest but is still only partly understood. Major steps in our understanding of the processes involved are coming from many sources, including complex numerical simulations of landscape evolution, theoretical and empirical studies of individual erosional processes (e.g. fluvial bedrock incision), and quantification of how particular landscapes have developed through time (e.g. at the basin scale or mountain scale). Earthscope now presents the opportunity to systematically control how the surface of the continent is actually changing and has changed over the past thousands and millions of years. For example, if basin-scale erosion rates (103-105 yr timescale) are measured across even a minimum-resolution grid spanning the continent, the role of climate and other parameters in erosional processes can be systematically tested. If longer-term exhumation is constrained across the continent, maps of exhumation rate may be compared to known tectonic histories to investigate the timescale of topographic decay in mountain belts. The keys to these examples are that the data will be collected across regions where it may not yet exist and will be collected in a systematic way that can be easily accessed and synthesized. Other types of data important for surface processes, such as hydrologic or meteorologic data, could also be collected systematically at each USArray and PBO site. These data would be particularly useful given that high-resolution topographic and remote sensing data may become available for many of the PBO and USArray localities. This approach would thus take advantage of the research strategy of Earthscope, to increase our understanding of geomorphic and atmospheric processes.