Controls on the Thermomechanical Evolution of Hyperextended Lithosphere at Magma‐Poor Rifted Margins: The Example of Espirito Santo and the Kwanza Basins

Abstract

High‐quality, long offset seismic data from many distal rifted margins show evidence for hyper‐extended, <10‐km‐thick crust. Direct observation of such domains is challenging as they lie, at great water depth, buried beneath thick sedimentary sequences and formed by rock‐assemblages that are hydrated and geophysically indistinguishable. Only a few drill holes have penetrated basement at ultradistal rifted margins. These observations, together with outcrops of preserved analogs exposed in collisional orogens, suggest that the complex interaction of detachment faults rooted in a subhorizontal shear zone in the hyperextended crust or, in the serpentinized mantle controls the formation of the ocean continent transition. While depth‐dependent thinning controls the early phases of rifting conforming to classical rift models, we still have a superficial understanding of how normal faults and subhorizontal shear zones form and evolve during rifting and lithospheric breakup. Here we develop a rheological parameterization to simulate the formation of, and slip‐on, large offset normal faults rooted in growing brittle to ductile shear zones. The evolution of these structures leads to the creation of a hyperextended crust and eventually exhumed serpentinized mantle. We also propose a simplified formulation to simulate magmatic underplating and seafloor spreading. The resulting numerical models provide a self‐consistent picture for the evolution of magma‐poor rifted margins from initiation of rifting to seafloor spreading. The model results are compared with first‐order observations of the Kwanza and Espirito Santo conjugate margins in the South Atlantic as well as of magma‐poor margins globally.

Geodynamic evolution of a wide plate boundary in the Western Mediterranean, near-fieldversusfar-field interactions

“…Such weak levels might have nevertheless been used as decoupling levels during convergence (Tugend et al, 2014;Manatschal et al, 2021). Models of formation of magma-poor passive margins suggest that weakening of the crust and weakening of the upper mantle by serpentinisation during extension can lead to lower crust and mantle exhumation with low-angle extensional shear zones (Lavier et al, 1999(Lavier et al, , 2019. Although large uncertainties remain on the mechanical stratification of the pre-convergence crust, it was thus probably in average rather weak with several decoupling levels and a weak upper crust.…”

The role of inheritance in forming rifts and rifted margins and building collisional orogens: a Biscay-Pyrenean perspective

“…Their results show that the fluid history as well as the related temperatures were very similar between both sites, suggesting that both sites had similar high heat flow during hyperextension and mantle exhumation. Thus, the high T max values from the Pyrenean-Cantabrian domain are certainly linked to a high heat flow during hyperextension, which is to be expected in (sediment-rich) hyperextended domains and predicted by numerical modelling (e.g., Lavier et al, 2019;Lescoutre et al, 2019;. Thus, while the heat flow may have been similar to that of other hyperextended and exhumed mantle domains, the high T max values may have been anomalous in the Biscay-Pyrenean domain.…”

Mantle Deformation Processes during the Rift-to-Drift Transition at Magma-Poor Margins

“…However, two mechanisms that complement each other are preferred by the community. These are the magma assisted diking or weakening of the mantle (Buck, 2006;Piccardo et al, 2007) and depth-dependent thinning (DDT) (Huismans & Beaumont, 2011;Huismans & Beaumont, 2008Kusznir et al, 2005;Lavier et al, 2019;Royden & Keen, 1980). DDT is the process by which the mantle lithosphere initially thins passively and weakens through the localization of deformation along lithosphere-scale shear zones and the upwelling of a buoyant mantle (Huismans & Beaumont, 2011).…”

“…This latter mechanism is preferred for magma poor margins where little volcanism is expressed at the rift surface before the transition to oceanic crust. Observations of exhumed or denuded mantle lithosphere in the southern European Alps (Lanzo peridotite) show that melt migration through porous mantle lithosphere at depths between 50 and 15 km can also weaken the mantle by reducing viscosity (Kaczmarek & Müntener, 2008;Piccardo et al, 2007) and likely lead to DDT (Huismans & Beaumont, 2011;Lavier et al, 2019;Ros et al, 2017;Svartman Dias et al, 2015). The mantle lithosphere in this model thins and delaminates, reducing the lithosphere's yield stress and allowing the continental rift to split the lithosphere into two plates (Davis & Kusznir, 2016;Svartman Dias et al, 2015.…”