Integrated geophysical‐petrological modeling of the lithosphere and sublithospheric upper mantle: Methodology and applications

Abstract

[1] A combined geophysical-petrological methodology to study the thermal, compositional, density, and seismological structure of lithospheric/sublithospheric domains is presented. A new finite-element code (LitMod) is used to produce 2-D forward models from the surface to the 410-km discontinuity. The code combines data from petrology, mineral physics, and geophysical observables within a self-consistent framework. The final result is a lithospheric/sublithospheric model that simultaneously fits all geophysical observables and consequently reduces the uncertainties associated with the modeling of these observables alone or in pairs, as is commonly done. The method is illustrated by applying it to both oceanic and continental domains. We show that anelastic attenuation and uncertainties in seismic data make it unfeasible to identify compositional variations in the lithospheric mantle from seismic studies only. In the case of oceanic lithosphere, plates with thermal thicknesses of 105 ± 5 km satisfy geophysical and petrological constraints. We find that Vp are more sensitive to phase transitions than Vs, particularly in the case of the spinel-garnet transition. A low-velocity zone with absolute velocities and gradients comparable to those observed below ocean basins is an invariable output of our oceanic models, even when no melt effects are included. In the case of the Archean subcontinental lithospheric mantle, we show that ''typical'' depleted compositions (and their spatial distribution) previously thought to be representative of these mantle sections are compatible neither with geophysical nor with petrological data. A cratonic keel model consisting of (1) strongly depleted material (i.e., dunitic/harzburgitic) in the first 100-160 km depth and (2) less depleted (approximately isopycnic) lower section extending down to 220-300 km depth is necessary to satisfy elevation, geoid, SHF, seismic velocities, and petrological constraints. This highly depleted (viscous) upper layer, and its chemical isolation, may play a key role in the longevity and stability of cratons.

LitMod3D: An interactive 3‐D software to model the thermal, compositional, density, seismological, and rheological structure of the lithosphere and sublithospheric upper mantle

“…The free-air and geoid anomalies do not change significantly in a transect running from the ocean to the Archon domain, indicating that the average density contrast between these two domains is small. This occurs because of the ''light'' Archean upper compositional layer, which counteracts the density increase associated with the cold thermal structure of this domain [Jordan, 1978;Afonso et al, 2008a]. This is consistent with the weak correlation between the long-wavelength component of the gravity field and cratons [Shapiro et al, 1999].…”

“…These values are within common estimates for Precambrian and Paleozoic crust [Durrheim and Mooney, 1994;Christensen and Mooney, 1995]. The mantle beneath the ocean and Tecton domains is characterized by a typical Phanerozoic composition, whereas the Archon mantle composition displays the following stratification: Archean from the Moho to 150 km depth and Phanerozoic from 150 km to the base of the thermal lithosphere (Table 1) sition, whereas the southern half is representative of an Archon domain [Afonso et al, 2008a;Griffin et al, 2009]. Since we are particularly interested in the effects of sub-Moho features, we assign the same crustal structure for the two continental domains (Figure 6).…”

“…[4] The general methodology used in LitMod3D has been discussed elsewhere [Afonso et al, 2008a]. Here we give only a brief summary of the method and discuss in detail the new implementations relevant to LitMod3D.…”

“…[10] In the mantle, stable mineral assemblages are computed by Gibbs free energy minimization [Connolly, 2005;Afonso et al, 2008a]. Each mantle body is therefore characterized by a specific major element composition (in wt %), which translates into specific bulk rock properties.…”

“…LitMod3D implements an updated version of the recent method presented by Afonso et al [2008a], based on the simultaneous and self-consistent fitting of all the available geophysical and petrological/geochemical observations. Besides allowing for a better control of the lateral and vertical variations of the bulk properties, this method reduces the uncertainties associated with fitting each observable alone or in pairs, as commonly done in the literature.…”