J. W. van Wijk

Model for tectonically driven incision of the younger than 6 Ma Grand Canyon

Accurate models for the incision of the Grand Canyon must include characterization of tectonic influences on incision dynamics such as active faulting and mantle to surface fluid 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.

Mantle-driven dynamic uplift of the Rocky Mountains and Colorado Plateau and its surface response: Toward a unified hypothesis

The correspondence between seismic velocity anomalies in the crust and mantle and the differential incision of the continental-scale Colorado River system suggests that significant 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.

Basin migration caused by slow lithospheric extension

Sedimentary basin migration caused by low lithospheric extension rates is investigated using a two-dimensional dynamic numerical model of the lithosphere. We find that continental breakup will eventually occur when larger extension velocities are used. The duration of rifting prior to continental breakup is dependent on the extension velocity. Stretching the lithosphere with lower velocities does not lead to breakup. Instead, the locus of maximum extension migrates. Deformation localizes outside the first formed basin that is in turn uplifted. This basin then becomes a ‘cold spot’ in the area. In this case, syn-rift cooling predominates; the lithosphere regains its strength during stretching instead of becoming weaker, and the lithosphere necking zone becomes stronger than surrounding regions. The transition velocity is, for the cases studied, about 8 mm/yr, while the locus of maximum thinning migrates after about 50^60 Myr. A comparison with observations of the mid-Norwegian, Galicia and ancient South Alpine margins shows a close resemblance of important features. fl 2002 Elsevier Science B.V. All rights reserved.

Melt generation at volcanic continental margins: No need for a mantle plume?

Melt generation in a rifting environment is studied using a dynamic 2-D finite element model. The lithosphere is extended to large, realistic thinning factors assuming a mantle temperature of 1333°C. The focussing of deformation results in a distribution of thinning factors along the margin at breakup time consistent with observations. The timing of melt production (late synrift) and the amounts of melt are consistent with observations at volcanic margins. The dynamical processes related to lithospheric rifting enhance the produced melt volumes sufficiently to explain the sometimes enigmatic melt volumes found at volcanic margins.

Small-scale convection during continental rifting: Evidence from the Rio Grande rift

Recent seismic imaging across the Rio Grande rift, western United States, revealed unexpected structures in the underlying mantle. Low seismic wave velocity anomalies below the Rio Grande rift have been interpreted as being partially of melt origin, and high-velocity structures below the western Great Plains have been proposed to be the result of small-scale convection, i.e., cold downwelling lithospheric material with probably a compositional contribution. We perform a dynamic test of these interpretations using a passive rift model for isochemical convection. The models self-consistently produce a rift localized at approximately the right distance from the border to the nearby thicker Great Plains lithosphere. With realistic upper mantle rheologies, small-scale convection forms, aided by the lithospheric step. The resulting thermal anomalies produce seismic low-velocity anomalies below the rift of amplitudes similar to those imaged seismically, requiring the presence of only small amounts of melt. The lateral extent of the observed low velocities below the Rio Grande rift is as in the models, where it is controlled by the spacing between downwelling limbs of the small-scale convection. The fast velocity structure below the western Great Plains can be produced by cold downwelling lithosphere. The thermal rifting models can predict the amplitudes and size of the main seismic anomalies; compositional heterogeneity may contribute to some of the smaller features observed.

Three dimensional thermal modeling of the California upper mantle; a slab window vs. stalled slab

In order to gain a better understanding of the behavior of microplates after their subduction, we studied two end-member scenarios for the post-subduction history of two offshore California microplates. In the first scenario, Monterey and Arguello microplate remnants are present today below the North America Plate, while in the second scenario subducted microplate remnants are absent. 3-D numerical modeling of the thermal evolution implied by these scenarios results in two different present-day thermal structures of the central and southern California upper mantle. By comparing the model-predicted surface heat flow values and seismic velocities to heat flow data and tomography, we find that we cannot discriminate between the two scenarios as they both are consistent with the data. This result means that the present-day upper mantle temperature field is relatively insensitive to the assumed microplate scenarios. A slabless window is not needed for the generation of partial melt either, which is consistent with earlier 2-D studies for this region.

Development of en echelon magmatic segments along oblique spreading ridges

En echelon magmatic segments commonly develop along obliquely spreading oceanic ridges. To clarify some of the dynamic aspects of this plate boundary, we performed a series of thermo-mechanical numerical tests. When extension of oceanic lithosphere becomes oblique, deformation within the axial region localizes into distinct upwelling centers. Temperatures are elevated in the upwelling cells, which are shallow mantle features that form the new plate boundary. The predicted features are similar to the axial volcanic ridges documented at Mohns and Reykjanes Ridges, and we conclude that they become the new loci of extensional deforma tion, upwelling, and magmatic activity. These ridges, suborthogonal to the plate spreading direction, only develop when the axis rift zone is weak. The subsegment length and spacing depend primarily on obliquity and axial width. Predicted crustal thickness along the subsegmented axis varies discernibly; this might explain the morphology and satellite gravity of the flanks of oblique spreading ridges.

Thermo-mechanical controls on Alpine deformation of NW Europe

Abstract The lithosphere of the Northern Alpine foreland has undergone a polyphase evolution with an intense interplay between upper mantle thermal perturbations and stress-induced intraplate deformation that points to the importance of lithospheric folding of the thermally weakened lithosphere. In this paper we address relationships between deeper lithospheric processes, neotectonics and surface processes in the Northern Alpine foreland with special emphasis on tectonically induced topography. We focus on lithosphere memory and neotectonics with special attention to the thermo-mechanical structure of the lithosphere, mechanisms of large-scale intraplate deformation, Late Neogene anomalies in subsidence and uplift, and links with surface processes and topography evolution.