The Colorado plateau is a large, tectonically intact, physiographic province in the southwestern North American Cordillera that stands at ∼1,800-2,000 m elevation and has long been thought to be in isostatic equilibrium. The origin of these high elevations is unclear because unlike the surrounding provinces, which have undergone significant Cretaceous-Palaeogene compressional deformation followed by Neogene extensional deformation, the Colorado plateau is largely internally undeformed. Here we combine new seismic tomography and receiver function images to resolve a vertical high-seismic-velocity anomaly beneath the west-central plateau that extends more than 200 km in depth. The upper surface of this anomaly is seismically defined by a dipping interface extending from the lower crust to depths of 70-90 km. The base of the continental crust above the anomaly has a similar shape, with an elevated Moho. We interpret these seismic structures as a continuing regional, delamination-style foundering of lower crust and continental lithosphere. This implies that Pliocene (2.6-5.3 Myr ago) uplift of the plateau and the magmatism on its margins are intimately tied to continuing deep lithospheric processes. Petrologic and geochemical observations indicate that late Cretaceous-Palaeogene (∼90-40 Myr ago) low-angle subduction hydrated and probably weakened much of the Proterozoic tectospheric mantle beneath the Colorado plateau. We suggest that mid-Cenozoic (∼35-25 Myr ago) to Recent magmatic infiltration subsequently imparted negative compositional buoyancy to the base and sides of the Colorado plateau upper mantle, triggering downwelling. The patterns of magmatic activity suggest that previous such events have progressively removed the Colorado plateau lithosphere inward from its margins, and have driven uplift. Using Grand Canyon incision rates and Pliocene basaltic volcanism patterns, we suggest that this particular event has been active over the past ∼6 Myr.

The correspondence between seismic velocity anomalies in the crust and mantle and the differential incision of the continental-scale Colorado River system suggests that signifi cant mantle-to-surface interactions can take place deep within continental interiors. The Colorado Rocky Mountain region exhibits low-seismic-velocity crust and mantle associated with atypically high (and rough) topography, steep normalized river segments, and areas of greatest differential river incision. Thermochronologic and geologic data show that regional exhumation accelerated starting ca. 6-10 Ma, especially in regions underlain by low-velocity mantle. Integration and synthesis of diverse geologic and geophysical data sets support the provocative hypothesis that Neogene mantle convection has driven long-wavelength surface deformation and tilting over the past 10 Ma. Attendant surface uplift on the order of 500-1000 m may account for ~25%-50% of the current elevation of the region, with the rest achieved during Laramide and mid-Tertiary uplift episodes. This hypothesis highlights the importance of continued multidisciplinary tests of the nature and magnitude of surface responses to mantle dynamics in intraplate settings.

Accurate models for the incision of the Grand Canyon must include characterization of tectonic infl uences on incision dynamics such as active faulting and mantle to surface fl uid interconnections. These young tectonic features support other geologic data that indicate that the Grand Canyon has been carved in the past 6 Ma. New U-Pb dates on speleothems are reinterpreted here in terms of improved geologic constraints and understanding of the modern aquifer. The combined data suggest that Grand Canyon incision rates have been relatively steady since 3-4 Ma. Differences in rates in the eastern (175-250 m/Ma) and western (50-80 m/Ma) Grand Canyon are explained by Neogene fault block uplift across the Toroweap-Hurricane system. Mantle tomography shows an abrupt step in mantle velocities near the Colorado Plateau edge, and geodynamic modeling suggests that upwelling asthenosphere is driving uplift of the Colorado Plateau margin relative to the Basin and Range. Our model for dynamic surface uplift in the past 6 Ma contrasts with the notion of passive incision of the Grand Canyon due solely to river integration and geomorphic response to base-level fall.

Ar/ 39 Ar dates on basalts of Grand Canyon provide one of the best records in the world of the interplay among volcanism, differential canyon incision, and neotectonic faulting. Earlier 40 K/ 40 Ar dates indicated that Grand Canyon had been carved to essentially its present depth before 1.2 Ma. But new 40 Ar/ 39 Ar data cut this time frame approximately in half; new ages are all <723 ka, with age probability peaks at 606, 534, 348, 192, and 102 ka. Strategic sampling of basalts provides a semicontinuous record for deciphering late Quaternary incision and faultslip rates and indicates that basalts fl owed into and preserved a record of a progressively deepening bedrock canyon. The Eastern Grand Canyon block (east of Toroweap fault) has bedrock incision rates of 150-175 m/Ma over approximately the last 500 ka; western Grand Canyon block (west of Hurricane fault) has bedrock incision rates of 50-75 m/Ma over approximately the last 720 ka. Fault displacement rates are 97-106 m/Ma on the Toroweap fault (last 500-600 ka) and 70-100 m/Ma on the Hurricane fault (last 200-300 ka). As the river crosses each fault, the apparent incision rate is lowest in the immediate hanging wall, and this rate, plus the displacement rate, is subequal to the incision rate in the footwall. At the reach scale, variation in apparent incision rates delineates ~100 m/Ma of cumulative relative vertical lowering of the western Grand Canyon block relative to the eastern block and 70-100 m of slip accommodated by formation of a hanging-wall anticline. Data from the Lake Mead region indicate that our refi ned fault-dampened incision model has operated over the last 6 Ma. Bedrock incision rate has been 20-30 m/Ma in the lower Colorado River block in the last 5.5 Ma, and displacement on the Wheeler fault has resulted in both lowering of the Lower Colorado River block and formation of a hanging-wall anticline of the 6-Ma Hualapai Limestone. In modeling long-term incision history, extrapolation of Quaternary fault displacement and incision rates linearly back 6 Ma only accounts for approximately two-thirds of eastern and approximately onethird of western Grand Canyon incision. This "incision discrepancy" for carving Grand Canyon is best explained by higher rates during early (5-to 6-Ma) incision in eastern Grand Canyon and the existence of Miocene paleocanyons in western Grand Canyon. Differential incision data provide evidence for relative vertical displacement across Neogene faults of the Colorado Plateau-Basin and Range transition, a key data set for evaluating uplift and incision models. Our data indicate that the Lower Colorado River block has lowered 25-50 m/Ma (150-300 m) relative to the western Grand Canyon block and 125-150 m/Ma (750-900 m) relative to the eastern Grand Canyon block in 6 Ma. The best model explaining the constrained reconstruction of the 5-to 6-Ma Colorado River paleoprofi le, and other geologic data, is that most of the 750-900 m of relative vertical block motion that accompanied canyon 40

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