Lithospheric instability beneath the Transverse Ranges of California

Abstract

Abstract. Recent high-resolution seismic experiments reveal that the crust beneath the San Gabriel Mountains portion of the Transverse Ranges thickens by 10-15 km (contrary to earlier studies). Associated with the Transverse Ranges, there is an anomalous ridge of seismically fast upper mantle material extending at least 200 km into the mantle. This high-velocity anomaly has previously been interpreted as a lithospheric downwelling. Both lithospheric downwelling and crustal thickening are associated with the oblique convergence of Pacific and North America plates across the San Andreas Fault, though it seems likely that the lithospheric downwelling is driven at least partly by gravitational instability of the cold lithospheric mantle. We show by means of numerical experiment that the balance between buoyancy forces that drive deforrnation and viscous stresses that resist deformation determines the geometry of crustal thickening and mantle downwelling. We use a simple two-layered lithospheric model in which dense lithospheric mantle overlies relatively inviscid and less dense asthenosphere and is overlain by buoyant crust. External plate motion drives convergence, which is constrained by boundary conditions to occur within a central convergent zone of specified width. A fundamental transition in the geometry of downwelling is revealed by our experiments. For slow convergence, or low crustal viscosity, downwelling occurs as multiple sheets on the margins of the convergent zone. For fast convergence or crust that is stronger than mantle lithosphere a single downwelling occurs beneath the center of the convergent zone. This complexity in the evolution of the system is attributed to the interaction of crustal buoyancy with the evolving gravitational instability. In order for a narrow downwelling slab to have formed beneath the Transverse Ranges within the last 5 Myr, the effective lithospheric viscosity of the convergent region is at most about 1020 Pa s.

Intermediate-Depth Earthquakes in a Region of Continental Convergence: South Island, New Zealand

“…The Pacific-North American plate boundary in Southern California is also thought to consist of a highvelocity uppermost-mantle downwelling (Humphreys and Clayton, 1990;Houseman et al, 2000) but no seismicity deeper than 27 km is observed (Richards-Dinger and Shearer, 2000). The regional Southern California average uppermost mantle P-wave velocity is low (Յ7.8 km/sec) (Hadley and Kanamori, 1977;Hearn and Clayton, 1986;Richards-Dinger and Shearer, 1997) implying an environment too warm to enable deep crustal or mantle seismicity.…”

“…Large horizontal and vertical shear strains have been modeled for downwelling mantle lithosphere below converging continents within a two-dimensional slice parallel to the convergent zone (Houseman et al, 2000). The largest (Table 1) are also plotted.…”

“…At greater depths in the lithosphere, the regions of maximum shear stress underlie the Alpine fault and the eastern edge of the Southern Alps with stresses in the downwelling region comparable to those in a bending layer. Gravitational instability can thicken the lithosphere and result in a deep, high-density body (Houseman et al, 2000). Depending on the rheology, the distributed thickening may exhibit subduction-like behavior in the uppermost mantle (Beaumont et al, 1996).…”

Effects of a low‐viscosity lower crust on topography and gravity at convergent mountain belts during gravitational instability of mantle lithosphere

“…Among the seven dimensionless numbers, the viscosity ratio η 0 affects growth rates, deflections of interfaces, and gravity anomalies most, and past studies with constant viscosity in the layers have considered a range of η 0 from 0.01 to 100, with values near η 0 = 1 seeming to match Earth-like conditions best [Houseman et al, 2000;Molnar and Houseman, 2013;Neil and Houseman, 1999]. We too carried out calculations for five values of η 0 , 0.01, 0.1, 1.0, 10, and 100, but in general, we present results only for η 0 = 0.01, 1.0, and 100.…”

Finite frequency tomography in southeastern Tibet: Evidence for the causal relationship between mantle lithosphere delamination and the north–south trending rifts

“…However, the time scale involved in the convective removal of the mantle lithosphere is uncertain because the viscosity is not well known. Numerical simulations by different authors show that the duration of the removal can vary drastically from 10 Ma to a few hundreds of million years, depending on the rheological parameters in the simulation experiments [Davaille and Jaupart, 1993;Conrad, 2000;Houseman et al, 2000;Morency et al, 2002]. The roughly concurrent onsets of tectonic processes in the surrounding regions have led to the suggestion of a rapid removal of the mantle lithosphere beneath Tibet at about 8 Ma ago [Harrison et al, 1992;Molnar et al, 1993].…”