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Drivers of lithospheric deformation

Deformation, metamorphism and orogenesis have long been considered responses to plate boundary processes (subduction, collision and/or accretion).  Here, ephemeral switches in tectonic mode between approximately vertical and horizontal bulk shortening in the overriding plate lead to the creation and subsequent destruction of orogenic belts, orthogonal fold and foliation development and basin development and inversion (e.g. Collins, 2002). Tectonic mode switches may be triggered by alternations between slab hinge advance and slab hinge retreat due to the transient subduction of bathymetric anomalies such as oceanic plateaus or ridges (Gibson, 1991; Collins, 2002; Schellart et al., 2002; Lister & Forster, 2008; Schellart, 2008; Rosenbaum & Mo, 2011).  They may also be instigated by progressive recumbent folding of the subducted oceanic slab at the ~670km upper-lower mantle boundary (Schellart, 2005).  Alternatively, intervening episodes of ~vertical bulk shortening may result from gravitational (orogenic) collapse, acting to alleviate lateral contrasts in gravitational potential energy produced during prior crustal thickening (e.g. Bell & Johnson, 1989; Aerden & Malavieille, 1999; Liu et al., 2000; Bell & Newman, 2006; Bell & Sapkota, 2012).

In addition to that associated with plate boundary margins, the occurrence and importance of intense deformation, metamorphism, basin development and magma generation in intracratonic settings has become increasingly recognized (e.g. Gorczyk, 2012; Gorczyk et al., 2013).  Unlike classical plate margin tectonics, our understanding of the fundamental underlying processes responsible for intracratonic geodynamics has largely remained a mystery - until more recently.  Lithospheric deformation in plate interiors must represent a response to either:

  1. far-field stress transference from plate boundaries, and/or
  2. processes stemming from pre-existing intra-plate architectures
  3. Processes related to flow within the upper mantle (thermal anomalies, fluid propagation from metasomatized parts of the mantle or stagnated parts of the slab, or delaminated lower crust)

Processes associated with older lithospheric architecture (2) may either operate entirely independently of plate boundary activity (1) or may partition and enhance its effects.  Intracontinential geodynamics independent of plate boundary forces are attributed to interaction and density contrasts between the lithosphere (encompassing the upper crust, lower crust and lithospheric mantle) and the asthenospheric mantle, and include:

  1. lithospheric delamination – removal of sections of mantle lithosphere in an asymmetric manner, exposing the lower crust to the underlying mantle asthenosphere
  2. Rayleigh Taylor instability of the lithosphere, or ‘lithospheric dripping’

Sites of anomalous intra-cratonic deformation and magmatism have been correlated with post-collisional suture zones, interpreted as ‘fossil’ subduction/shear zones.  Post-collisional intra-cratonic suture zones represent zones of intense shearing and strong mantle lithosphere metasomatism (Gorczyk et al., 2013).  Consequently, these suture zones possess significantly different rheological and chemical properties (Ranalli, 1995) to the more competent terranes they are sandwiched between.  These pre-existing lithospheric weaknesses therefore respond differently when exposed to subsequent far-field stresses, serving as a locus for strain.