Slab

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

Post-Laramide removal of the Farallon slab, western United States

Abstract: I propose that post-Laramide removal of the subhorizontally subducting Farallon slab occurred by buckling downward along an approximately east-northeast–trending axis. This process was accomplished by a tearing or necking separation of the subducted slab near the northern and southern boundaries of the United States and propagation of the separated edges toward the central axis of downwelling, accompanied by aesthenosphic upwelling behind the trailing edges. Initial buckling probably began near 50 Ma, and slab removal was complete by 20 Ma. This model is based primarily on the space-time evolution of the “ignimbrite flare-up” (a major mid-Tertiary igneous event of mantle origin), which involved two propagating fronts of initiation of volcanism that followed the proposed motions of the separated slab edges as they converged on central Nevada from the north and southeast. Post-Laramide uplift, extension, establishment of the Cascadia subduction zone, and active magmatism may be consequences of lithosphere-scale modifications caused by the Laramide removal of the slab and the resulting asthenospheric upwelling.

Origin of the slab component in arc lavas from across-arc variation of B and Pb isotopes

Abstract: AT convergent margins, the subducting oceanic slab is thought to dehydrate, producing fluids which metasomatize the overlying mantle wedge where island-arc magma forms. However, the nature and origin of the metasomatizing fluid, its source composition and its relation to the genesis of the chemical characteristics of arc magmas are largely controversial. Across-arc variation in the chemistry of arc lavas provides a useful key to this problem, because it may reflect the changes in the physical conditions of the subducting slab that control mass transfer from slab to mantle wedge as a function of depth. Here we report clear across-arc variations in the concentrations and isotopic compositions of boronand lead observed in lavas from the Izu arc (Japan). Our data suggest that a homogeneous slab fluid contributes to all Izu volcan-oes, but that the amount of this fluid decreases continuously with increasing depth of the subducting slab. Whereas the Izu slab fluid conies primarily from altered oceanic crust, our data for high-Mg andesites from the Setouchi volcanic belt (a nearby fore-arc) indicate a significant involvement of sediment in the fluid source.

Subduction history of the Tethyan region derived from seismic tomography and tectonic reconstructions

Abstract: [1] In the mantle underneath the Tethyan suture zone, large volumes of positive velocity anomalies have been imaged by seismic tomography and interpreted as the present-day signature of subducted Tethyan lithosphere. We investigate the Mesozoic-Cenozoic subduction history of the region by integrating independent information from mantle tomography and tectonic reconstructions. Three different subduction scenarios for the Tethyan oceanic lithosphere, representative for the available tectonic reconstructions, are used to predict the present thermally anomalous volumes associated with the lithospheric surface subducted since the late Mesozoic. Next, these predicted thermal volumes and their expected positions are compared to the relevant anomalous volumes derived from seismic tomographic images. In this analysis we include, among others, the possible effects of ridge subduction and slab detachment after the Cenozoic continental collisions, absolute plate motion, and slab thickening in the mantle. Our preferred subduction model comprises the opening of large back-arc oceanic basins within the Eurasian margin. The model points to slab thickening by a factor of 3 in the mantle, in which case the estimated volumes allow for active oceanic spreading (∼1–2.5 cm/yr) in the Tethyan lithosphere during convergence. Our results further indicate the occurrence of early Oligocene slab detachment underneath the northern Zagros suture zone, followed by both westward and eastward propagation of the slab tear and diachronous Eocene to Miocene slab detachment below the eastern to western Himalayas. Free sinking rates of the detached material of ∼2 cm/yr in the lower mantle provide the best fit between the tomographic mantle structure and our Tethyan subduction model.

Source component mixing in the regions of arc magma generation

Abstract: Most recent workers attribute the main features of island arc basalt geochemistry to variable contributions of at least two source components. The major source appears to be the peridotitic wedge of upper mantle overlying the subducted slab, but the nature of the second component and the processes by which the sources become mixed during genesis of arc magmas are in dispute. A metasomatic addition to the wedge resulting from devolatilization in the slab is the simplest explanation of the marked enrichment of the alkali and alkaline earth elements with respect to the rare earths in island arc basalts, together with the variably developed trends in Pb, Sr, and Nd isotopic data toward sedimentary contaminants. However, lack of the correlations between relative degrees of trace element fractionation and radiogenic isotopic ratios expected of such processes requires a more complex explanation. Alternative models that suggest that all of the characteristics of island arc basalts can be accounted for by melting of an intraoceanic, hot spot type of mantle source also face specific difficulties, particularly with regard to the strong depletions of trace high-field-strength elements in arc compared with hot spot magmas. A possible resolution of these specific geochemical difficulties may lie in dynamic transport processes within the wedge linked with the slab through coupled drag, and the marked depression of mantle isotherms in subduction zones. Inefficient escape of melts and subsequent repeated freezing within the overturning wedge can lead to local mineralogic and geochemical heterogeneity of the peridotite overlying the slab. Fluids released from the slab may infiltrate the heterogeneous wedge and preferentially scavenge the alkalis and alkaline earths with respect to the rare earths and high field strength elements from locally enriched portions of the wedge. Incorporation of such metasomatic fluids in renewed melting at shallower but hotter levels within the wedge can give rise to the trace element and isotopic systematics generally observed in arc basalts. Furthermore, subsequent melting of wedge-type peridotite in nonsubduction zone environments can result in complementary enrichment of the high field strength elements compared with arcs, and in the general isotopic similarity of hot spot and arc magmas. Although it is likely that the wedge-type peridotite in any arc is heterogeneously veined by previous inefficient melt extraction episodes, it is possible that the subduction zone environment is most conducive to the generation of veining.