Three-dimensional gravity inversion for Moho depth at rifted continental margins incorporating a lithosphere thermal gravity anomaly correction

Abstract

S U M M A R YThis paper describes a method for determining Moho depth, lithosphere thinning factor (γ = 1 − 1/β) and the location of the ocean-continent transition at rifted continental margins using 3-D gravity inversion which includes a correction for the large negative lithosphere thermal gravity anomaly within continental margin lithosphere. The lateral density changes caused by the elevated geotherm in thinned continental margin and adjacent ocean basin lithosphere produce a significant lithosphere thermal gravity anomaly which may be in excess of −100 mGal, and for which a correction must be made in order to determine Moho depth accurately from gravity inversion. We describe a method of iteratively calculating the lithosphere thermal gravity anomaly using a lithosphere thermal model to give the present-day temperature field from which we calculate the lithosphere thermal density and gravity anomalies. For continental margin lithosphere, the lithosphere thermal perturbation is calculated from the lithosphere thinning factor (γ = 1 − 1/β) obtained from crustal thinning determined by gravity inversion and breakup age for thermal re-equilibration time. For oceanic lithosphere, the lithosphere thermal model used to predict the lithosphere thermal gravity anomaly may be conditioned using ocean isochrons from plate reconstruction models to provide the age and location of oceanic lithosphere. A correction is made for crustal melt addition due to decompression melting during continental breakup and seafloor spreading. We investigate the sensitivity of the lithosphere thermal gravity anomaly and the predicted Moho depth from gravity inversion at continental rifted margins to the methods used to calculate and condition the lithosphere thermal model using both synthetic models and examples from the North Atlantic.

The breakup of the South Atlantic Ocean: formation of failed spreading axes and blocks of thinned continental crust in the Santos Basin, Brazil and its consequences for petroleum system development

“…Crustal thickness, continental lithosphere thinning and Moho depth for the Santos Basin and SPP areas of the Brazilian rifted margin were studied by subsidence analysis using flexural backstripping (Kusznir et al 1995;Roberts et al 1998) and gravity inversion performed in the 3D spectral domain (Parker 1972), the latter using a new method incorporating a lithosphere thermal gravity anomaly correction (Greenhalgh & Kusznir 2007;Chappell & Kusznir 2008). …”

Tectonic variation and structural evolution of the West Greenland continental margin

“…The determination of Moho depth, crustal thickness and thinning factor is important at rifted continental margins for understanding the structure and location of the ocean-continent transition (Chappell and Kusznir, 2008).…”

Crustal structure of the conjugate Equatorial Atlantic Margins, derived by gravity anomaly inversion

“…The model results shown in Figure 5c and e correspond to decompression melting assuming 'normal' magmatic addition, in which melting begins at thinning factor 0.7 and produces 7 km of oceanic crust when thinning factor reaches 1. This is the parameterization of melting for normal temperature asthenosphere (McKenzie & Bickle 1988;White & McKenzie 1989) first applied by Chappell & Kusznir (2008). The results shown in Figure 5d and f use decompression melting assuming 'magma-rich' magmatic addition, in which melting begins at thinning factor 0.5 and produces 10 km of oceanic crust when thinning factor reaches 1 ( fig.…”

“…Greenhalgh & Kusznir 2007;Alvey et al 2008;Chappell & Kusznir 2008;Cowie & Kusznir 2012b;Roberts et al 2013;Cowie et al 2015) and so is recapped here only briefly. In Roberts et al (2013) and Cowie et al (2015Cowie et al ( , 2016 gravity inversion has been used alongside other techniques, such as subsidence analysis and analysis of residual depth anomalies, to provide a multifaceted view of rifted margin structure.…”