Precollisional, multistage exhumation of subducted continental crust: The Sesia Zone, western Alps

Abstract

The Sesia Zone within the Tertiary arc of the western Alps is a relic of the subducted part of the Adriatic continental margin along the SE border of the Tethyan ocean. The Sesia Zone comprises three basement nappes which individuated during Late Cretaceous (65–80 Ma) subduction to different depths at high‐pressure (HP, blueschist, eclogite facies) conditions (peak pressures of 1.0–1.2, 1.0–1.5, and 1.5–2.0 GPa). The thrusts bounding these nappes developed where the crust was previously thinned during Jurassic rifting. Crustal‐scale shear zones partly overprinted these early thrusts and exhumed coherent slices of crust containing HP rocks. Initial exhumation of the internal part of the accreted margin involved thrusting (D1) and transpressional shearing (D2) along a subvertical, E‐W trending mylonitic shear zone under retrograde blueschist‐ to greenschist‐facies conditions. This exhumation was nearly isothermal to a depth of about 25 km, where the basement nappes were juxtaposed. Subsequent exhumation of these nappes to a common depth of about 15–20 km occurred in the footwall of a greenschist‐facies, top‐SE extensional shear zone (D3) preserved in some of the highest mountain peaks of the Sesia Zone. New Rb‐Sr mineral ages constrain D2 to have occurred at about 60–65 Ma and D3 at about 45–55 Ma. Thus top‐SE extensional exhumation was broadly coeval with Eocene, SE directed subduction of the Liguro‐Piemont oceanic lithosphere beneath the Adriatic margin. Slow cooling and erosional denudation of the Sesia Zone from 45 to 30 Ma occurred in the hanging wall of the Gressoney extensional shear zone (D4), which itself contributed to the exhumation of Eocene HP and ultra‐HP oceanic rocks in its footwall. By 30 Ma, HP rocks of the Sesia Zone were intruded by shallow granitic plutons which were eroded and redeposited within volcanoclastic sediments. Oligo‐Miocene Insubric backfolding and thrusting (D5) only exhumed northeastern parts of the Sesia Zone, where HP metamorphism is absent or was overprinted by Tertiary temperature‐dominated metamorphism. Most exhumation of continental HP rocks in the Sesia Zone therefore preceded Tertiary Alpine collision and coincided with Late Cretaceous to Early Tertiary subduction of the Adriatic and Tethyan lithosphere. The transition from D2 to D3 in the Sesia Zone is interpreted to mark a change from high‐stress, oblique SE directed subduction and accretion of the distal Adriatic continental margin to NW retreating, low‐stress subduction of the Liguro‐Piemont oceanic lithosphere.

Deeply subducted continental fragments – Part 1: Fracturing, dissolution–precipitation, and diffusion processes recorded by garnet textures of the central Sesia Zone (western Italian Alps)

“…Except for Rim1 in FG1249, all of the garnet rims modelled are found to be stable at eclogite facies conditions that are similar to those recently determined for the central Sesia Zone (e.g. KonradSchmolke et al, 2006;Konrad-Schmolke and Halama, 2014;Regis et al, 2014;Rubatto et al, 2011;. In the following, the main textures recognized in garnet are discussed and related to specific processes.…”

“…The fourth zone was modelled as stable at 550 • C and 1.7-2.2 GPa; hence it is certainly Alpine in age, as pre-Alpine metamorphism had not reached eclogite facies conditions. Similar garnet zoning was interpreted differently by KonradSchmolke et al (2006), who concluded that the zones do not reflect two orogenic cycles but are related to different water contents of the protoliths during Alpine HP subduction. In water-saturated rocks garnet would then display typical prograde zoning, whereas in water-undersaturated rocks garnet would produce more complex zoning, including abrupt compositional changes from core to rim.…”

“…It is thought to derive from the distal portion of the Adriatic passive margin that was involved in the Alpine subduction (Dal Piaz, 1999;Beltrando et al, 2014). The Sesia Zone is bounded to the west by blueschist to eclogite facies subunits derived from the Piemonte-Liguria Ocean (Bearth, 1967;Dal Piaz and Ernst, 1978;Martin et al, 1994;Cartwright and Barnicoat, 2002;Bucher et al, 2005;Groppo et al, 2009;Rebay et al, 2012;Negro et al, 2013) and to the east by the Insubric Line. This dextral brittle fault separates the Sesia Zone from the southern Alps, which show a weak Alpine imprint at sub-greenschist facies (Bertolani, 1959;Zingg, 1983).…”

A new petro-structural map of the Monte Mucrone metagranitoids (Sesia-Lanzo Zone, Western Alps)

“…More recently, another tectonic unit has been identified in the southern SLZ: the Rocca Canavese Thrust Sheet (RC of Pognante 1989a, 1989b. A redefinition of these metamorphic complexes has been proposed by Babist, Handy, Konrad-Schmolke, and Hammerschmidt 2006.…”

Arc and Slab-Failure Magmatism in Cordilleran Batholiths II – The Cretaceous Peninsular Ranges Batholith of Southern and Baja California

“…The effects of slab failure are important, diverse, and may be responsible for features such as rapid uplift (Chatelain et al 1992), syncollisional magmatism (Davies and von Blanckenburg 1995;Keskin 2003;Macera et al 2008), tomographic gaps in the descending slab (Wortel and Spakman 1992), thick-skinned foreland deformation (Cloos et al 2005), seismic discontinuities (Wortel and Spakman 1992), crustal recycling (Hildebrand and Bowring 1999), transitory pulses of mafic magmatism (Ferrari 2004) doubly vergent orogens (Regard et al 2008), plateau uplift (Rodgers et al 2002), ultra-high pressure exhumation Babist et al 2006;Xu et al 2010), changes in plate motion (Austerman et al 2011); subhorizontal swarms of deep earthquakes (Chen and Brudzinski 2011), lateral shifts in foredeep sedimentation (van der Meulen et al 1998), opening of small ocean basins (Carminati et al 1998), switchover from foredeep flysch to orogenic molasse (Sinclair 1997;Wilmsen et al 2009), and porphyry copper and other mineralization (Solomon 1990;de Boorder et al 1998;Cloos et al 2005;Hildebrand 2009). …”