Slab

About: Slab is a research topic. Over the lifetime, 31617 publications have been published within this topic receiving 318693 citations.

Continental magmatism, volatile recycling, and a heterogeneous mantle caused by lithospheric gravitational instabilities

Abstract: [1] Removal of the lower lithosphere (mantle lithosphere with or without portions of the crust) through ductile gravitational instabilities can produce magma under continents. Using numerical experiments approximating the rheology of continental crust and lithosphere and underlying asthenosphere and using phase equilibria from the literature, I investigate the topographic and magmatic results of lithospheric gravitational instabilities, along with the fate of the sinking material. Lithospheric removal, commonly referred to as delamination regardless of the mechanism, may allow asthenospheric material to rise and to melt adiabatically, and this asthenosphere can conductively heat portions of the lithosphere previously at lower temperatures. The size and rheology of the sinking material greatly influence the resulting surface topography as well as whether or not melting occurs. The sinking material may devolatilize as it reaches higher temperatures and pressures, just as a subducting slab does, triggering further melting. Gravitational instabilities are possible causes of nonmagmatic basins, continental magmatism of varying volume and composition in the absence of subduction, areas of high heat flow and uplift, and creation of an upper mantle heterogeneous in major and trace elements and volatiles. Magmatism can be simultaneous with topographic subsidence and possibly with subsequent uplift. In those cases that produce magma, melting can occur over a depth range from ∼30 to 200 km, though anomalously hot mantle is required to reach the volume of flood basalts.

Mesozoic plate-motion history below the northeast Pacific Ocean from seismic images of the subducted Farallon slab

Abstract: The high-resolution seismic imaging of subducted oceanic slabs1,2 has become a powerful tool for reconstructing palaeogeography3. The images can now be interpreted quantitatively by comparison with models of the general circulation of the Earth's mantle4. Here we use a three-dimensional spherical computer model of mantle convection5,6 to show that seismic images of the subducted Farallon plate provide strong evidence for a Mesozoic period of low-angle subduction under North America. Such a period of low-angle subduction has been invoked independently to explain Rocky Mountain uplift far inland from the plate boundary during the Laramide orogeny7. The computer simulations also allow us to locate the largely unknown Kula–Farallon spreading plate boundary, the location of which is important for inferring the trajectories of ‘suspect’ terrain across the Pacific basin8.

Plastic instabilities: Implications for the origin of intermediate and deep focus earthquakes

Abstract: Adiabatic or catastrophic plastic shear has been reported in metals, polymers, and metallic glasses. The phenomenon is associated with rapid stress drops and audible pings or clicks as the material deforms in a plastic manner. The driving force for the plastic instability is the stored elastic strain energy of the loading system, and in many respects the behavior is reminiscent of the shear stress response arising from stick slip events during unstable frictional sliding, although the precise mechanism is different. It is shown here that adiabatic plastic shear is capable of explaining the detailed distribution of intermediate and deep focus earthquakes within subduction zones, the earthquake events being the result of instabilities in material undergoing plastic flow. It is argued that for a particular strain rate there exists a critical temperature, TC, which is depth dependent; for temperatures below TC the material is strain rate softening and, for a soft enough loading system, may undergo catastrophic plastic shear. For temperatures above TC the material is strain rate hardening and is always stable during plastic shear. The cutoff depth for deep focus earthquakes then corresponds to the transition from strain rate softening to strain rate hardening material, and for commonly accepted geothermal gradients within the slab corresponds to approximately 800 km. The precise distribution of earthquakes within the slab is a function of the subtle interplay between the geothermal gradient and the TC gradient. In particular, a decrease in seismic activity is to be expected below about 300 km in the slab with total stress drops decreasing from a maximum of 700 MPa above 300 km to a maximum of ≈ 50 MPa below 300 km. The differences in foci distribution between subduction zones such as Tonga, New Hebrides, and Peru result from minor differences in the geothermal gradients within the slabs. The model predicts the development of triple seismic zones high in the slab, double seismic zones down to approximately 300 km, and single seismic zones down to ≈ 800 km. Such a distribution is to be expected of relatively young, cool slabs; as the slab heats up, the seismic activity retreats up the slab. The paper only proposes a deformation mechanism for earthquake generation, it does not address the stress field within the slab but only the distribution of strength. Thus the distribution of focal plane mechanisms is not considered, only the locations where earthquakes due to plastic instabilities are possible. The absence of earthquakes does not necessarily mean that the slab does not exist, it only means that the slab is too hot to undergo plastic instability. This means that aseismic subduction is a distinct possibility in many regions of high geothermal gradient within the slab (i.e. > circa 3°C km−1).