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Lithospheric delamination and lithospheric ‘dripping’

This section presents key models demonstrating the occurrence of lithospheric delamination in:
  1. lithospheric excesses composed of the same material as the surrounding lithosphere
  2. hydrated post-collisional sutures

Delamination A - ‘Lithospheric dripping’

This model tests the delamination potential of different lithospheric thickness excesses relative to a surrounding terrane.  Importantly, material in the delamination zones in this model has exactly the same material properties as the surrounding lithosphere.  The model in the upper window presents a 2000km-long cross-section through a section of continental crust featuring three symmetric ‘bulges’ in the lithosphere (representing weak zones following prior crustal thickening).  The bulk of the model features a lithospheric thickness of 90km; composed of 20km of upper crust, 20km of lower crust and 50km of mantle lithosphere.  The left ‘bulge’ has an extra 10km thickness relative to the surrounding lithosphere, the centre an additional 60km, the right an additional 20km.  The lower window presents a 600km-wide zoomed section focusing upon the evolution of the large central bulge.  The model documents behaviour of the lithospheric bulges under 23.4Ma of quiescence (no imposed far-field stress).

The model demonstrates that greater lithospheric excesses delaminate more rapidly than lesser lithospheric excesses.  Delamination of the 60km excess occurs after ~1my, the 20km excess after ~5.5my and the 10km excess much later at ~20my.  The sequence of delamination is consistent with that predicted behaviour (Houseman & McKenzie, 1982; Houseman et al., 1981; Houseman & Molnar, 1997).  Based upon Rayleigh-Taylor, the 10km excess has too small an amplitude:wavelength ratio to delaminate in its own right.  However, its delamination eventually occurs due to overall mantle convection.

The zoomed section of the central bulge highlights patterns of thermal and mechanical relaxation following delamination.  Thermal relaxation operates more rapidly than mechanical relaxation, resulting in increased Moho temperatures (at the base of the mantle lithosphere).  This induces melting at the base of the lower crust, leading to the formation of batholiths, which may be accompanied by extrusive and hypabyssal rocks (yellow).

Delamination B

This model examines the behaviour of a 100km-wide post-collisional suture following an imposed bulk shortening of 0.5cm/a for 6Ma.  The continental crust is composed of 15 km upper felsic and 15 km lower mafic crust. The subjacent asthenosphere and the upper mantle are composed of anhydrous peridotite and are defined by the given temperature profile. The continental lithosphere contains a 100 km wide discontinuity of lowered plastic strength (with respect to the surrounding rocks) that separates the continental crust and underlying mantle into two adjacent blocks.  Such discontinuities (mobile belts) have been widely discussed in the literature and are believed to have formed by the amalgamation of micro-continents and oceanic plateaus (i.e.: Lenardic et al., 2000; Lenardic et al., 2003; Yoshida, 2010; Yoshida, 2012).

The suture zone preferentially partitions the effects of horizontal bulk shortening, resulting in greater proportions of vertical crustal thickening and enhanced geothermal gradients in the upper and lower crust.  Green material emplaced in the mid and upper crust during the first few My represents melt extracted from the molten mantle lithosphere.  Thermal and mechanical relaxation occurs following release of bulk shortening after 6Ma, with melt extracted from the molten mantle lithosphere emplaced into the mid and upper crust.  The thermal relaxation acts more rapidly, leading to melting at the base of the lower crust.  Delamination of the molten mantle lithosphere section of the suture zone occurs from ~19Ma onwards, followed by increased volumes of lower crustal melting.  The isotherm structure across the suture zone re-equilibrates by the end of the model at ~70Ma.

Plastic instability arises from processes induced by forces normal to the surface between two media (two sections of the lithosphere).  Plastic delamination therefore occurs due to differences in yield strength between two mediums.  Piriz et al. (2009) analysed the stability of an elastic–plastic plate with respect to the formation of the Rayleigh–Taylor instabilities and showed that the transition from plastic stability to instability is given by the equation:

where: ρ is the density, g is the acceleration due to gravity, ξo is the magnitude of the initial perturbation at the base of the plate, λ is the wavelength of this perturbation and μ is the elastic shear modulus. For this scenario we consider the following values: μ = 6.510 Pa, ρ = 3300 kg m-3,λ = 10 km and Y = 1.09 Pa then ξo = 1.6 m.  Therefore, if the formula of Piriz et al. (2009) is applicable to the model presented here, then a plastic weakness that is 10km thicker than the surrounding lithosphere is always unstable given a small deflection and Rayleigh–Taylor instabilities form once elastic deformation ceases and plastic yielding occurs.  Published in Gorczyk and Vogt (2015)

Delamination C

A model using the same 100km thick lithosphere setup with a 100km-wide suture zone as that in Delamination B, illustrating initially plastic delamination followed by gravitational delamination.  A bulk shortening rate of 1cm/a is applied for 6Ma.  Here, the suture zone is composed of hydrous counterparts of the surrounding lithosphere.  In Delamination B, neither of the lithospheric sections flanking had been thrust under the other prior to release of the bulk shortening at 6Ma.  Here, the lithospheric block to the left has been partially thrust under that to the right by 6Ma, allowing comparison of the model evolution following termination of bulk shortening.

Initial horizontal bulk shortening leads to mechanical thickening of the lithosphere, enabling immediate strain localization and deformation within the suture zone.  Where the hydrated mantle encounters the wet mantle solidus, basaltic melt production is triggered, followed by melt emplacement in mid to upper crustal levels. This, in turn, leads to partial melting of the upper continental crust.  As the crustal material thickens, the base of the Moho reaches the eclogite stability field and part of the lower crust is exposed to eclogitization; here, density increases from 100 kg/m3 to 180 kg/m3.  Because of the density variation between the eclogitizised lower crust and the surrounding mantle, a gravitational instability develops and detached dense material sinks into the deep mantle. The timing and depth at which this occurs vary according to the convergence rate and thickness of the lithosphere. For young lithosphere and high bulk shortening rates, delamination occurs early. Parts of the lower crust melt as the crust encounters greater temperatures (> 700 °C) due to crustal thickening. Thus, the main melting episode takes place after the detachment of the “drip”. Following detachment, thermal equilibration takes place, which leads to a rapid temperature increase at the base of the crust above the delaminated material. Such thermal anomalies lead to partial melting of the lower continental crust at temperatures ranging from 700 °C to 900 °C, forming large felsic batholiths (of up to 200 km in length) at mid to upper crustal levels that may reach the surface in subsequent tectonic events, such as crustal rebounds. The detached part of the lower crust melts and as predicted by Elkins-Tanton (2007) and Elkins-Tanton & Hager (2000), melts are extracted and volatiles are released, potentially contributing to the composition and geochemical evolution of the batholiths formed in the upper crust.  Published in Gorczyk and Vogt (2015)

Delamination D

This model starts with a suture zone that localizes horizontal bulk shortening, leading to deep burial of the lithospheric blocks to either side, followed by subsequent delamination.  It features an 80km-thick lithosphere with a 100km-wide suture zone in the centre, to which a bulk shortening rate of 2 cm/a was applied for 6 Ma.  The suture zone is composed of hydrous counterparts of the surrounding lithosphere.

Initially, during the onset of bulk shortening, melts are produced in the mid and upper crust, resulting from partially molten mantle lithosphere at the base of the hydrous suture zone.  Both blocks of continental lithosphere to flanking the suture are progressively buried during the bulk shortening.  Initially this is asymmetric, with the right block initially buried more than the left.  After ~1.8Ma this is associated with melting at the base of the lower crust.  After ~5.5Ma sections of the hydrated mantle lithosphere, upper and lower crust that were buried begin to delaminate.  This is an effective way of introducing an array of lithologies and melts into the upper crust.

After all the eclogitized material is delaminated, thermal and mechanical relaxation kicks in.  This rebound produces strong asymmetry in the lithosphere, an increase in mafic magmatism and allows the inflow of hot asthenospheric mantle to shallow levels after ~24Ma.  The result is a spatially extensive zone of decompression melting in the asthenospheric mantle, and an extensive province of Ultra High Temperature, Low Pressure melting of the continental crust. Later, delamination of the lithosphere occurs to either side of the high-temperature, low-pressure, province.