Strength Of Continental Lithosphere Research Articles

McNutt Receives 2007 Maurice Ewing Medal

It is a great honor for me to have the opportunity to deliver this citation for Marcia K. McNutt, the 2007 recipient of AGU's Maurice Ewing Medal. Marcia has made a significant contribution to our understanding of the structure of oceanic lithosphere, served in leadership roles in the marine scientific community, and instigated new directions and technologies for ocean exploration.Marcia received her undergraduate degree in physics and studied for the Ph.D. with Bob Parker and the late Bill Menard at the Scripps Institution of Oceanography. Her thesis led to two seminal papers on oceanic and continental isostasy. The first showed that flexure due to a young volcanic load could influence the subsidence and uplift history of nearby, preexisting volcanoes. This hypothesis, which explains why some seamounts and oceanic islands show uplift rather than subsidence, has had wide implications for basin development and sea level change. The second used 3‐D spectral methods of analyzing topography and gravity data to argue that the Australian lithosphere responds to loads as a relatively strong structure at short time scales and a weak one at long geological times. This method is now widely used and is at the center of current debates concerning the rheology and strength of continental lithosphere.

The long-term strength of continental lithosphere : "jelly sandwich" or "crème brûlée" ?

There has been much debate recently concern ing the long-term (i.e., >1 m.y.) strength of continen tal lithosphere. In one model, dubbed jelly sandwich, the strength resides in the crust and mantle, while in another, dubbed creme brulee, the mantle is weak and the strength is limited to the crust. The different models have arisen because of conflicting results from elastic thickness and earthquake data. We address the problem here by first reviewing elastic thickness estimates and their relationship to the seismogenic layer thickness. We then explore, by numerical thermomechanical model ing, the implications of a weak and strong mantle for structural styles. We argue that, irrespective of the actual crustal strength, the creme-brulee model is unable to explain either the persistence of mountain ranges or the integrity of the downgoing slab in collisional systems. We conclude that while the creme-brulee model may apply in some tectonic settings, a more widely applica ble model is the jelly sandwich.

The long-term strength of continental lithosphere: “jelly sandwich” or “crème brûlée”?

There has been much debate recently concerning the long-term (i.e., >1 m.y.) strength of continental lithosphere. In one model, dubbed jelly sandwich, the strength resides in the crust and mantle, while in another, dubbed creme brulee, the mantle is weak and the strength is limited to the crust. The different models have arisen because of conflicting results from elastic thickness and earthquake data. We address the problem here by first reviewing elastic thickness estimates and their relationship to the seismogenic layer thickness. We then explore, by numerical thermomechanical modeling, the implications of a weak and strong mantle for structural styles. We argue that, irrespective of the actual crustal strength, the creme brulee model is unable to explain either the persistence of mountain ranges or the integrity of the downgoing slab in collisional. systems. We conclude that while the creme -brulee model may apply in some tectonic settings, a more widely applicable model is the jelly sandwich.

Heterogeneity of the middle crust: Implications for strength of continental lithosphere

Paleoproterozoic rocks in the southwestern United States record tectonism at depths of 10–20 km during 1.75–1.65 Ga amalgamation of arcs to North America. These exposures of middle crust are characterized by great heterogeneity of composition, structure, and metamorphic grade. The crust is dominated by subvertical fabric and is laterally segmented into 10-km-scale blocks by subvertical shear zones. Metamorphic temperature gradients of 200 °C occur within <10 km length scales at various 10–20 km depths. Hottest areas are near granitoid plutons (10 km depths) or dike swarms (20 km depths) and are dominated by subhorizontal foliations. Lowest temperature blocks (depths <10 km) show brittle-ductile thrust belt geometries, and all levels show a range of brittle, semi-brittle, and ductile deformation features. Our synthesis suggests that the synorogenic middle crust exhibits a distinctive heterogeneity of tectonism influenced strongly by advective heating and resulting strength variations. Segmentation of the crust into tectonic blocks is interpreted to be related to the 10 km/10 m.y. space and time scales of plutonism and melt transfer in the middle crust. By analogy to experimental data we hypothesize that the middle crust was a spatially and temporally complex strength beam during deformation, with vertical fabric recording strain hardening during whole lithosphere failure. Heterogeneity and a dominance of vertical fabric are consistent with a general lack of middle crustal reflectivity and may have influenced differential uplift during subsequent tectonism.

Mechanical stability of the Redbank Thrust Zone, Central Australia: Dynamic and rheological implications

Deep seismic reflection profiling and other studies indicate that the Moho has been significantly displaced by intracratonic deformation on the Redbank Thrust Zone in central Australia. A two‐dimensional finite‐element model of the thick‐skinned crustal structure of the Redbank Thrust Zone shows that the gravitational stresses generated by the distribution of crustal masses are in agreement with the tectonic and seismic quiescence of the area. Rock failure is concentrated in the downthrusted, and therefore Theologically weakened, crustal segments. Conversely, the upthrusted slices of lower crust and upper mantle undergo little deformation and no failure and thus constitute a mechanically strong whole‐crustal core. The presence of north‐south oriented intraplate stresses derived from far‐field plate boundary forces does not contribute to the inferred mechanical equilibrium of the crust in central Australia. The results may have implications for the mechanical stability of intracratonic thick‐skinned crustal structures elsewhere, and for the long‐term strength of continental lithosphere in general. The models, however, do predict the possibility of intraplate seismicity in response to small changes in tectonic stress, since parts of the crust are already at or close to the failure limit.

On the stability of isostatically compensated mountain belts

The formation of convergent mountain belts is invariably accompanied by an increase in gravitational potential energy due to part of the work done by the forces driving convergence. The evolution of potential energy stored in an orogen is dependent on (1) the density structure, (2) the thermal evolution, and (3) the way convergent deformation is partitioned between crust and mantle lithosphere. It is now well recognized that this increase in potential energy associated with the mountain building process raises the possibility that significant extension, or collapse, may accompany the relaxation of the forces driving convergence provided the lithosphere is thermally weakened (e.g., England, 1987). In this paper we evaluate the stability of isostatically compensated mountain belts under the assumption that the strength of continental lithosphere is governed by a combination of frictional sliding and creep processes using the “Brace‐Goetze” model for the rheology of the lithosphere. The reference lithosphere, defined to be in potential energy and isostatic balance with the mid‐ocean ridges, changes with different thermal parameters of the lithosphere. The instantaneous extensional strain rate for thermally mature mountain belts is calculated by balancing the horizontal buoyancy force stored in the mountain belts (measured relative to the reference state) with the vertically integrated strength of the lithosphere for initial strengths spanning the probable natural range. It is shown that horizontal buoyancy forces arising in isostatically balanced mountain belts are sufficiently large to induce the collapse at significant rates (greater than a few times 10−16 s−1) and leading to significant finite extension providing the Moho temperatures exceed about 650–700°C, a condition only likely to be attained if the mantle lithosphere has not been thickened to the same extent as the overlying crust. Consequently, processes that thin the mantle lithosphere as a consequence of convergent deformation such as the convective instability of a thickened lower thermal boundary layer greatly increase the possibility of collapse. The calculations presented here suggest that near complete destruction of the mountain system by extensional collapse may be possible if such processes can reduce total lithospheric thickening to less than half the contemporary crustal thickening (i.e., fl ≤ fc/2, where fl and fc are the lithospheric and crustal thickening factors).

Variations of elastic plate thickness at continental thrust belts

After examining 15 reliable estimates of the elastic thickness of the lithosphere where it underthrusts mountain belts in Europe, Asia, and the Americas, we conclude that the strength of continental lithosphere as a function of depth must be very different from that of oceanic lithosphere. Depending on the state of stress, continental lithosphere may be either very much stronger or very much weaker than would be predicted for oceanic lithosphere of the same thermal age. We explain this observation as the result of two competing effects: a thicker thermal plate beneath the continents which allows a deeper elastic/ductile transition for plates more than 100 million years old, and a low activation energy for ductile creep in the continental crust which leads to massive failure of the upper elastic lithosphere at high bending stresses. The 100+ ‐km values for elastic plate thickness of billion‐year‐old lithosphere require a thermal plate thickness of at least 250 km. The magnitude of apparent thinning of the elastic plate at high stress is consistent with that calculated from a simple yield envelope which includes a ductile zone in the lower crust. The low yield strength of continental materials leads to correlations between effective elastic plate thickness and all of the following: dip of the underthrust plate; radius of curvature of the thrust front in map view; and total length of the mountain belt. Using insight derived from laboratory experiments on the buckling of spherical shells and drawing analogies with the geophysical characteristics of oceanic subduction zones, we arrive at the following sequence of causes and effects. The primary variable is the dip of the downgoing plate, presumably prescribed by its own excess mass and pressure exerted by dynamic flow in the mantle. Where this dip is steep, the rigidity of the plate is weakened due to inelastic yielding. The weakened plate can then buckle at small wavelengths into short, arcuate mountain belts. Thus we believe that the mantle ultimately controls the pattern of surface tectonics, but that the nonelastic rheology of the lithosphere is the mechanism by which that control is imposed.