Ridge-push force and the state of stress in the Nubia-Somalia plate system

Abstract

We assessed the relative contribution of ridge-push forces to the stress state of the Nubia-Somalia plate system by comparing ridge-push forces with lithospheric strength in the oceanic part of the plate, based on estimates from plate cooling and rheological models. The ridgepush forces were derived from the thermal state of the oceanic lithosphere, seafloor depth, and crustal age data. The results of the comparison show that the magnitude of the ridge-push forces is less than the integrated strength of the oceanic part of the plate. This implies that the oceanic part of the plate is very little deformed; thus, the ridge-push forces may be compensated by significant strain rates outside the oceanic parts of the plate. We used an elastic finite element analysis of geoid gradients of the upper mantle to evaluate stresses associated with the gravitational potential energy of the surrounding ridges and show that these stresses may be transmitted through the oceanic part of the plate, with little modulation in magnitude, before reaching the continental regions. We therefore conclude that the present-day stress fields in continental Africa can be viewed as the product of the gravitational potential energy of the ridge ensemble surrounding the plate in conjunction with lateral variations in lithospheric structure within the continent regions.

Mechanism for Deep Crustal Seismicity: Insight From Modeling of Deformation Processes at the Main Ethiopian Rift

“…In the EARS the magnitude of deviatoric stress is in a range of 10–20 MPa (Mahatsente & Coblentz, 2015; Stamps et al., 2010), and hence, it appears to be too small to cause brittle failure on seismogenic faults (Craig et al., 2011; Scholz, 2002). Mahatsente and Coblentz (2015) argue that the ridge‐push force from the oceanic part of Nubia (Africa)‐Somalia plate is less than the integrated strength of the African plate and hence additional forces are required to deform the plate. The above studies report magnitude of vertically averaged deviatoric stress over the thickness of the lithosphere.…”

Kinematic Boundary Conditions Favouring Subduction Initiation at Passive Margins Over Subduction at Mid-oceanic Ridges

“…Heidbach et al, 2007) and deformation patterns (Gerbault, 2000;Cloetingh et al, 2005). Furthermore, it is common that forces are calculated using the integral of the horizontal deviatoric stresses (Ghosh et al, 2006;Mahatsente and Coblentz, 2015) that are lower by a factor of two (see Schmalholz et al, 2019) than the differential stress usually used to calculate lithosphere forces.…”

“…Although the above quoted modelling studies successfully simulate the initiation of subduction zones at passive margins upon vertical or horizontal loading, the stress levels required for the nucleation of subduction are usually significantly higher than plate tectonic forces (England and Wortel, 1980;Cloetingh et al, 1989;Mueller and Phillips, 1991;Gerbault, 2000;Gurnis et al, 2004;Zhong and Li, 2019). In fact, the stress needed for subduction initiation at passive margins is in general one order of magnitude larger than the horizontal component of stress generated by ridge push (Mahatsente and Coblentz, 2015). From a mechanical perspective, it is important to note that the critical stress for subduction initiation is also higher by ∼5 TN than the lithospheric yield strength at (slow spreading) mid-ocean ridges (Luttrell and Sandwell, 2012).…”

Rheological and kinematic conditions for forced subduction initiation at continental passive margins

“…Forsyth, 1973;Turcotte & Schubert, 2014). Although the contribution of each mechanism is unclear (Swedan, 2015), ridge push represents integrated deviatoric stress values between 1 and 5 TN/m (Mahatsente & Coblentz, 2015;Mueller & Phillips, 1991;Swedan, 2015), with an average value in the order of 3.5 TN/m for a 75 Myr oceanic lithosphere (Mahatsente, 2017). However, in some places, stress arising from the oceanic plate are of equal magnitude as the GPE from thick continental lithosphere (e.g.…”

“…Although the above quoted modelling studies successfully simulate the initiation of subduction zones at passive margins upon vertical or horizontal loading, the stress levels required for the nucleation of subduction are usually significantly higher than plate tectonic forces (Cloetingh et al, 1989;England & Wortel, 1980;Gerbault, 2000;Gurnis et al, 2004;Mueller & Phillips, 1991;Zhong & Li, 2019). In fact, the stress needed for subduction initiation at passive margins is in general one order of magnitude larger than the horizontal component of stress generated by ridge push (Mahatsente & Coblentz, 2015). From a mechanical perspective, it is important to note that the critical stress for subduction initiation is also higher by ~5 TN than the lithospheric yield strength at (slow spreading) mid-ocean ridges (Luttrell & Sandwell, 2012).…”