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Delamination, Slab Break-Off, and Slab Roll-Back Jeff Bowman 1 Outline • Delamination • Slab Break-Off • Slab Roll-Back –Discussion of related papers –Discuss papers assigned 2 Delamination • Lithospheric delamination is… – the detachment of thickened lithospheric mantle from overlying crust during continental collision (Bird, 1978). – In other words, a chunk of lithosphere peeling off and sinking into the asthenosphere (Bowman, Today) 3 Ways to Approach Delamination • Geochemical analysis of metamorphic and igneous provinces. • Orogenic processes –Rapid uplift • Seismicity –Earthquakes/Tomography –Seismic refraction –Seismic reflection (Cook et. al., 1998) 4 Analogy • Glue a piece of steel to the bottom of a block of wood that is floating in water. http://www.geology.um.maine.edu/geodynamics/analogwebsite/Projects2002/Gerbi20 5 02/lith_delam.html Taking A Step Back • Oxburgh 1972 introduced – Flake Tectonics and Continental Collision • Large sheets of material (flakes) shearing off the top of one of the colliding plates. 6 The Accredited Start • ‘Delamination’ was introduced as a theory by Peter Bird of UCLA in 1978 and 1979. – Detachment of thickened lithospheric mantle from overlying crust during continental collision (Bird, 1978). 7 P. Bird - Delamination • His concept has come to include a broader range of processes, including… –Detachment of oceanic slabs (Sacks and Secor, 1990; Davies and von Blackenburg, 1995) –Foundering of mafic lower crust and upper mantle driven by phase changes (Kay and Kay, 1993; Nelson, 1991). 8 Continental Delamination and the Colorado Plateau, Bird 1979 • Delamination cannot begin until some process breaks through the mantle lithosphere, allowing asthenosphere to contact the crust. 9 Bird, 1979 Four possible causes of delamination: •Convective instability •Rifting •Plume erosion •Continental collision http://peterbird.name/publications/1979_d elamination/0010s_4_ways_to_initiate.jpg 10 Continental Delamination and the Colorado Plateau, Bird 1979 • Instability is expected to propagate at plate tectonic rates, 5 cm/yr 11 Bird, 1979 •The delamination model implies a fast rise of asthenosphere to replace the sinking lithosphere; 24 cm/yr •Figure shows the similarities between subduction and delamination 12 Meissner, Mooney 1998 13 Consequences of Delamination Bird, 1979 • Regional Uplift – Excess pressure brought by newly risen asthenosphere • Increased Heat Flow • Reduced Seismic Velocities • Mafic volcanism 14 Kay and Mahlburg Kay, 1993 • Same idea of delamination as P. Bird – “Lithospheric delamination is the foundering of dense lithosphere into less dense asthenosphere.” • One step further: – Cause for the density inversion • thermal • compositional • phase changes 15 Kay and Mahlburg Kay, 1993 • Other existing models rely on thermal expansion to create density changes. • However, compositional variations and phase changes more easily generate density differences. – Mafic granulite facies -> eclogite facies 16 Kay and Mahlburg Kay, 1993 • A prediction of delamination events are tough • Field observations are better for identifying where delamination has occurred – Observable structures – Thermal and magmatic events 17 Delamination Magmatism: Andes • 2-3 m.y. ago, the Argentine Puna changed – Thickened crust -> Crustal shortening (Isacks, 1988) •Mafic volcanism across region •Topographic high compared to the northern Puna (Isacks, 1988) •This area outlines delamination Kay and Mahlburg Kay, 1993 18 Cross-section of Andean Crust • Seismic studies show a gap – Slab too hot due to asthenosphere Kay and Mahlburg Kay, 1993 19 Geochemical Classification • Three general chemical groups based on trace element characteristics – Oceanic Island Basalt (OIB), Calc-Alkaline, and Shoshonite Kay and Mahlburg Kay, 1993 20 Geochemical Classification • Melting percentages • OIB – highest – Relates to seismic gap on x-section • CAC-ALK – intermediate – Flanking OIB on margins • Shoshonites – smallest – Northeastern area where lithosphere is thicker Kay and Mahlburg Kay, 1993 21 Kay and Mahlburg Kay, 1993 • Delamination events exist in few places – Basin and Range – Tibet – Andes • Delamination events may have been more common (Archean) 22 Lustrino 2005 • Paper explaining the geochemical compositions of basaltic magmas found • Uses a model involving delamination to explain peculiarities • Density increase – Basalt -> amphibolite -> eclogite • Sound familiar? 23 Lustrino 2005 •A) Initial situation •Cont. -Oceanic -Cont. •B) Oceanic slab subducting •C) Cont. – Cont. collision •D) Lithos. thickening •Phase changes •E) Density increase leads to instability 24 Top: temperature (degree centegrate) field and some isotherms. Middle: composition by tracers, blue = upper crust, yellow = lower crust, red = mantle; in the mantle lithosphere the initially vertical tracer columns are almost not deformed due to the high viscosity. Bottom: viscous dissipation function (non-dimensionallog10), zoomed-in into the upper half of the model. http://www.geophysik.unifrankfurt.de/~schmelin/orogeny/orogenyneu.h tml 25 Schott and Schmeling, 1998 Collisional Delamination in New Guinea: The Geotectonics of Subducting Slab Breakoff • Mark Cloos et. al., 2005 • Lithotectonic belt relationships • Mechanical properties of the crust and lithospheric mantle • Basis for series of lithospheric-scale cross sections showing collisional delamination 26 Cook et. al., 1998 • Describes results from an ~725 km long ~100 km deep seismic reflection profile recorded by LITHOPROBE 27 Cook et. al., 1998 • Enlargement of data in Slave–Wopmay crustal wedge 28 Slab Break-Off • A.K.A. – Slab Detachment • Subducted slab becomes detached from the surface slab • Observed volcanism • Regional tectonics • First hypothesized based on (1970’s) – Hypocentral distribution – Tomographic images 29 Davies & Blanckenburg 1995 • Model -> oceanic lith. detaches from continental lith. during continental collision • Opposing buoyancy leads to extension in the subducting slab – Rifting occurs with strain localization • Critical strain rates & lithosphere strength 30 !! Fun Fact !! • Comparison of mean life span –Continental crust • 2.2 Ga –Oceanic crust • 100 Ma • Continental crust does not subduct very well 31 Davies & Blanckenburg 1995 • Buoyant continental crust vs. cold dense oceanic crust – Extensional forces acting at transition region 32 Sacks & Secor, 1990 Delamination in Collisional Orogens • Continental crust buoyancy vs. slab pull force – Extensional stress may lead to extensionally rupture • A: simple shear • B: pure shear 33 Davies & Blanckenburg 1995 • Strain localization • Critical strain rates – Kusznir and Park (1987) – weakening of 50 m.y old lithosphere • 5 x 10-15 s-1 – Other strain rates • 0.12 x 10-15 s-1 – 0.37 x 10-15 s-1 Houseman & England (1996) • Critical strain rate may lower with depth* 34 Davies & Blanckenburg 1995 • Figure 4 – Integrated strength vs. depth 35 Consequences • Magmatism – Asthenospheric upwelling • Exhumation of high-pressure rocks • Orogen deformation and uplift 36 Discussion • Setting expected – Syn-collisional magmatism – Preceded by subduction of oceanic and continental lithosphere • Bird’s delamination model – Difficult to start and stop – Extensive crustal melt vs. localized/linear crustal melt 37 Ferrari 2004 fig 2 38 Slab Roll-Back • Subducting slab sweeps back through the mantle like a paddle • Hinge migrates away from the arc region • Upper plate is pulled along and may lead to back-arc spreading • Causes/Effects – Gravity – Forces – Asthenosphere upwelling 39 Heuret & Lallemand, 2005, Plate motions, slab dynamics and back-arc deformation • Where Δρ = density difference between slab and mantle, L = slab length, A = age of slab, K = constant 40 Heuret, Lallemand 2005 • In contrast to the theoretical rule where rollback increases with age. • Younger age subducting slabs roll-back faster 41 Heuret, Lallemand 2005 • Rules out slab roll-back related to slab pull –As many advancing trenches as retreating ones –Slab age is not correlated with trench retreat 42