Constraints from Surface Geophysical and Geological Observations on the Role of Lithosphere-Mantle Coupling
A fundamental controversy in plate tectonic theory is the role of the convecting mantle in plate motions, lithosphere stresses, and plate boundary zone deformation. Simple end-member models are often used to describe the importance of coupling. In one case the lithosphere is primarily self-driven, and buoyancy-driven mantle circulation plays little or no role in affecting the lithospheric stress field and plate boundary zone deformation in general. In such a model the lithosphere motion is expected to generally lead the underlying mantle. The other end-member model involves buoyancy-driven mantle convection playing an important role in affecting lithosphere stresses and plate boundary zone deformation. In many cases, such buoyancy-driven convection will result in an underlying mantle flow field that leads the plates. We use the surface measurements of plate motions, plate boundary zone velocity gradient tensor field estimates, World Stress Map observations, earthquake moment tensors, and mantle shear wave anisotropy measurements to investigate the coupling problem. We solve the depth-integrated 3-D force balance equations for depth integrated stresses and plate motions within the lithosphere by addressing both lithosphere and mantle density buoyancies. The role of tractions associated with buoyancy-driven mantle convection are determined by using self-consistent full three-dimensional convection models and applying the traction outputs from these models to the base of the lithosphere model as a lower boundary condition. Lateral viscosity variations within the lithosphere model are used to approximate the plate boundary zones. In order to match (1) lithospheric deformation indicators and WSM observations, (2) plate motions and (3) the toroidal/poloidal velocity ratio, it is necessary to have (A) a low viscosity asthenosphere of order 1e19 Pa-s, (B) a significant viscosity contrast between the continents and oceans, (C) higher viscosity craton regions, (D) higher viscosity old oceans and (E) low viscosity plate boundary zones. When cratons extend more than 200 km deep into the mantle significant problems arise with matching stress observations and deformation indicators. Driving tractions, in which the mantle flow leads the plate motions (associated with sinking mantle density buoyancies), are critically important to completely explain deformation within areas possessing high gravitational potential energy differences, such as the Tibetan Plateau, and the Andes. Driving tractions are also present beneath eastern North America, whereas western North America experiences moderate resistive tractions. Horizontal traction magnitudes are relatively small (1-3 MPa), but they integrate horizontally to provide significant depth-integrated horizontal deviatoric stress magnitudes within the lithosphere. Successful models require that tractions associated with density-buoyancy driven mantle convection contribute about 50% of the total horizontal depth integrated deviatoric stress magnitudes that are acting within the lithosphere on Earth.
Testing Geophysical Hypotheses with Global Models of the Coupled Mantle/Lithosphere System
A detailed understanding of magnitude and distribution of forces causing changes in plate motions over time has become a most important goal in geophysics today. Mantle convection is widely accepted to provide a significant amount of the driving force available to plate tectonics, but the relative magnitudes of other driving and resisting forces still remain unclear. After introducing a recently-developed numerical technique where simulations of global lithosphere dynamics are performed in conjunction with 3D mantle circulation models, I will focus on two convergent-boundary systems: the Nazca/South America margin, and the India/Australia deformation area. For the cases at hand, predictions of recent plate motion changes, as well as of other geophysical observables, compare well with data available through paleomagnetic and geodetic techniques. Numerical results (a) demonstrate that precise budgets of forces acting upon plates can be obtained and (b) support the notion of feedback between mountain belt growth and plate convergence.
Surface constraints on the temporal and spatial evolution of the Farallon-Pacific-North America plate boundary
Extension and volcanism are two surface derived data sets used to infer mantle processes back in time. We integrate two new large GIS-based datasets to create palinspastic restorations of extension and volcanism allowing us to readdress the relationship between plate-boundary deformation, intra-plate extension and magmatism in western North America. Using ArcGIS and custom software, we retrodeformed the NAVDat (North American Volcanic Database, navdat.geongrid.org) using the western North America reconstruction of McQuarrie and Wernicke (2005). We compare this data to strain rates calculated over a 50 km-grid forward- deformed from 36 Ma to present. With the deformed grid and palinspastically restored volcanic dataset we quantitatively compare rates of magmatism and deformation and evaluate the age, location, and migration of Cenozoic volcanic arcs. A first order conclusion from this study is that magmatism, throughout the Basin and Range, is primarily driven by plate boundary effects. The plate boundary effects include subduction and rollback of the Farallon plate, creation and expansion of slab windows as the Pacific plate intercepts the North American plate and re-establishment of the ancestral Cascade arc along the eastern margin of the Sierra Nevada at ∼ 15 Ma. Notable exceptions include the Yellowstone hotspot system along the northern boarder of our study area and late-stage (<8 Ma) passive, extension related asthenospheric upwelling that accompanied a thinning lithosphere along the eastern and western margins of the Basin and Range. The palinspastic reconstructions presented here highlight that the classic, high-angle, Basin and Range faulting that comprises most of the physiographic Basin and Range province commenced during a remarkably amagmatic period. These observations largely contradicts the active rifting model where magmatism triggers Basin and Range extension
Thermal isostasy and the elevation of continents
Simple models of crustal thickness and density assuming an Airy-type isostasy in continental regions do not fully account for the observed surface topography. This is not surprising, because continental elevation is also determined by lithospheric temperature, and by stresses associated with convection deeper in the mantle. Observed surface topography is the sum of two components: the isostatic 'lithospheric' contribution and the dynamic 'mantle' contribution. Local upper mantle temperature variations result in local variations in upper mantle density, which control the observed spatial variations in surface elevation. The underlying assumption is that the predominant source of upper mantle density variations is temperature rather than composition. Regional surface heat flux, which may be downward continued to estimate temperatures at the crust-mantle interface, thus provides an important constraint on the thermal contribution to surface elevation. In this study, we develop thermal lithospheric models for North America based on our compilation of heat flux data and recent seismic crustal models in an attempt to reconcile the 'static' or 'isostatic' contribution to the observed continental elevation. For stable continental lithosphere, we use a steady-state thermal model while for active regions experiencing extensional tectonics, we use transient thermal models. The difference between observed and predicted continental elevation gives the dynamic contribution of topography for North America. We present differences in gravitational potential energy between crustal columns and the resulting distribution of lithospheric stresses in the continent.
Constraints on Early Tertiary Incision and Uplift of the Grand Canyon Region of the Colorado Plateau From Apatite (U-Th)/He Thermochronometry
The mechanism(s) responsible for the elevation gain of cratonic plateaus is a controversial problem. The 1.9 km of surface uplift of the Colorado Plateau since Late Cretaceous time serves as a classic example of this debate. We used apatite (U-Th)/He thermochronometry (36 samples, 230 single-grain analyses) to resolve the regional unroofing history of the southwestern Colorado Plateau, and its relationship to plateau uplift and Grand Canyon incision (up to 1.5 km). Our data document overall southwest to northeast unroofing from plateau margin to plateau interior, during denudation phases in the Late Cretaceous/Early Tertiary (80 to 55 Ma), mid Tertiary (28 to 16 Ma), and Late Tertiary (<6 Ma). Distributions of apatite dates simulated using the radiation accumulation and annealing model for apatite He diffusivity suggest that eastern Grand Canyon samples from the basement and the Kaibab surface nearby had similar Early to mid-Tertiary thermal histories, despite their ~1500 m of stratigraphic separation. If these models are correct, they indicate that a significant (at least 1000 m deep) "proto"-Grand Canyon was carved in post-Paleozoic sediments in this region during the Early Tertiary. Kilometer-scale Early Tertiary topographic relief would require substantial uplift during Sevier/Laramide time. These results are compatible with the mid-Tertiary episode of plateau interior unroofing documented by our data, and evidence for significant mid-Tertiary relief in northeast flowing drainages along the plateau margin, together suggesting the southwestern plateau interior had acquired substantial elevation by at least mid-Tertiary time. Our data are inconsistent with models that invoke mechanisms requiring exclusively Late Tertiary surface uplift of the plateau. Sevier-Laramide (80-50 Ma) denudation, incision, and elevation gain of the plateau record the northeastward expansion of the Cordillera's orogenic highlands into its cratonic foreland. Subsequent stability of the landscape from ca. 50-30 Ma, a time interval that includes the onset of Laramide flat slab removal at ca. 40 Ma, suggests that inferred slab demise did not significantly alter the landscape of the southwestern plateau. Renewal of regional unroofing at ca. 28 Ma is synchronous with significant crustal extension southwest of the plateau and the reversal of drainage across the modern southwestern plateau margin. The return to a relatively stable landscape from 16-6 Ma appears to indicate that the phase of significant extension in the central Basin and Range had limited influence on the plateau landscape. The opening of the Gulf of California and integration of a west-flowing Colorado River induced the final phase of significant unroofing of the plateau interior starting at ca. 6 Ma. Thus, the most visible phase of unroofing and generation of topographic relief recorded by our data occurred in Sevier-Laramide time, pointing toward Laramide-age mechanisms as the initial cause of plateau elevation gain. Although our data do not preclude additional post-Laramide uplift, the subsequent regional unroofing phases can be explained by drainage reorganization associated with rift- related lowering of adjacent regions without additional surface uplift of the plateau.
Constraints on Late Tertiary Elevation of the Colorado Plateau From Carbonate Clumped- Isotope Thermometry
Topography is a first-order expression of the buoyancy of the lithosphere, and the timing and pattern of elevation change can place fundamental constraints on mantle flow and continental dynamics. We investigate the timing of Colorado Plateau uplift using clumped-isotope thermometry to independently constrain both the temperature and isotopic composition of ancient surface waters based on the 13C-18O bond enrichment in carbonates. Analyses of ancient lake sediments from the plateau interior and adjacent lowlands are compared to signals recorded by modern sediments collected over 3 km of elevation in the region. Comparison of modern and ancient samples deposited near sea level provides an opportunity to quantify the influence of climate on changes in temperature, and therefore more accurately assess the contribution from changes in elevation. Both modern and ancient (Miocene-Pliocene) carbonates record near-surface spring/summer lake water temperatures that vary strongly with elevation. Modern and ancient lake carbonate temperature lapse rates of -4.2±0.7°C/km and -4.1±0.6°C/km, respectively, suggest that little if any post-16 Ma change in elevation of the southern plateau is required to explain the data. Agreement of δ18O data for modern and ancient surface waters supports this interpretation. The zero-elevation intercept of the ancient trend is 7.7±2.0°C warmer than the modern trend, indicating significant cooling due to climate change since Late Miocene time. The temperature data are permissive of up to 450 m of uplift or 250 m of subsidence of the plateau interior since 6 Ma, but do not support km-scale changes. Combined with previous constraints, the data suggest that most uplift of the south-central plateau occurred during Late Cretaceous/earliest Tertiary time, favoring uplift mechanisms such as crustal thickening by channel flow, hydration of the mantle lithosphere due to volatile flux from the Laramide flat slab, or dynamic topography associated with slab foundering. The data do not support explanations that ascribe most uplift to ca. 40-0 Ma disposal of the Farallon or North American mantle lithosphere.
The Ethiopia Afar Geoscientific Lithospheric Experiment (EAGLE): Probing the Transition From Continental Rifting to Incipient Sea Floor Spreading
The Miocene-Recent East African Rift in Ethiopia subaerially exposes the transition between continental rifting and sea floor spreading within a young continental flood basalt province. As such, it is an ideal study locale for continental breakup processes and hotspot tectonism. Here I review the results of a recent multidisciplinary, multi-institutional effort to understand geological processes in the region: The Ethiopia Afar Geoscientific Lithospheric Experiment (EAGLE). In 2001-3 dense broadband seismological networks probed the structure of the upper-mantle, while controlled source wide-angle profiles illuminated crustal structures both along-axis and across-rift crustal structure of the Main Ethiopian Rift (MER). These seismic experiments, complemented by gravity and magnetotelluric surveys, have provided important constraints on variations in rift structure and melt distribution prior to continental break-up and have advanced our understanding of hotspot tectonism and rifting processes in East Africa. Quaternary magmatic zones observed at the surface within the MER are underlain by high-velocity, dense gabbroic intrusions that accommodate extension without marked crustal thinning. Magnetotelluric studies reveal the presence of partial melt in the Ethiopian crust, consistent with an overarching hypothesis of magma assisted rifting. Looking deeper, into the mantle, tomographic images reveal a ∼500 km wide low P- and S-wave velocity zone at 75 to >400 km depth in the upper-mantle that extends from close to the eastern edge of the Main Ethiopian Rift (MER) westwards beneath the uplifted and flood basalt-capped NW Plateau. The low-velocity zone does not interact simply with the Miocene-Recent (rifting) related base of lithosphere tomography, but provides an abundant source of partially molten material that drives the extension of the MER to the present day.