Detachment models for the formation of passive continental margins

Abstract

The inherent asymmetry of extension by detachment leads to contrasting and conjugate classes of passive margins. Upper‐plate margins comprise crust above a deeper detachment. Lower‐plate margins comprise the footwall of the detachment, overlain by faulted upper plate remnants. Such margins have distinctive architectures, structural styles, uplift‐subsidence paths and thermal histories. The wide range in structural styles on passive margins is predicted by five models which incorporate detachment faults linked to flat ductile shear zones, and ductile stretching of the thermal lithosphere below the shear zones. These models provide explanations for enigmatic structural and morphological features of passive margins such as marginal plateaux, outer highs, unstructured synrift sag basins, and perched rift basins. Numerical modelling of isostatic uplift‐subsidence histories shows that different patterns of uplift‐subsidence behaviour can be explained by variations in detachment geometry and change in the amount of lithospheric stretching. Voluminous igneous underplating is predicted if anomalously hot asthenosphere is uplifted. The arrival of such mantle derived melts may cause significant additional uplift. Upper‐plate margins undergo thermally induced uplift, with permanent uplift due to igneous underplating. This uplift may be the origin of passive margin mountains in the adjacent hinterlands. Marginal plateaux are emergent or very shallowly submerged throughout the extension history, with postrift subsidence to intermediate water depths. The lithosphere is extended below a midcrustal detachment, but with little extension of the upper‐plate. The pattern of subsidence on an Atlantic margin requires an extended upper plate superimposed on progressively more stretched subdetachment lithosphere. Conjugate margins are described from the Tasman Sea, the Atlantic Ocean and the Great Southern Ocean, illustrating both the principle of complementary asymmetry and the different patterns of uplift or subsidence on opposing passive margins.

Propagating rift during the opening of a small oceanic basin: The Protector Basin (Scotia Arc, Antarctica)

“…To the north, these small basins become larger and have associated linear magnetic anomalies indicating that an intermediate or even an oceanic crust is developed. The asymmetry of the margins and of the surrounding continental banks suggests that the extension along the basin was asymmetric, and related to a low angle fault dipping towards the Terror Rise, similar to those proposed for continental stretching [38]. This asymmetry may similar to the one proposed for the development of large basins.…”

Localisation of ductile and brittle shear zones along the Szentlőrinc-1 well in the Mecsekalja Zone using quartz microstructural and well-log data

“…Provided that the ductile and brittle structures formed subsequently due to the same tectonic event, the deformed zones may represent a detachment fault. The detachment faults are long-term, low-angle normal faults with significant displacements and subregional-regional extent (Lister et al 1986;Davis 1988;Lister and Davis 1989). These structures develop as the result of continental extension, when the middle and lower continental crust, deformed in a ductile way, is uplifted to the brittle upper crust.…”

Observations from the Basin and Range Province (western United States) pertinent to the interpretation of regional detachment faults

“…The detachment concept is now widely applied in extensional and passive margin settings (e.g. Froitzheim & Eberli 1990;Lister et al 1991;Reston et al 1996;Driscoll & Karner 1998;Hodges et al 1998;Osmundsen et al 1998;Taylor et al 1999;Boncio et al 2000;Manatschal et al 2001;Canales et al 2004). However, the apparent conflict between generally accepted geological interpretations and rock mechanical and seismological considerations has yet to be resolved satisfactorily (Sibson 1985;Jackson 1987;Jackson & White 1989;Collettini & Sibson 2001;Scholz & Hanks 2004;cf.…”