Flac3d to model passive margins wedge
The formation of Jurassic OCTs is manifested by exhumed subcontinental mantle, thinned continental crust, and sparse mid-ocean-ridge basalts and gabbros (e.g., Mohn et al., 2011 McCarthy and Müntener, 2015).
Pre-rift magmatism is related to the formation of (post-)Variscan volcano-sedimentary basins during a prolonged period of post-collisional extension in Europe. Major tectonic events in the European Alps are recorded in the magmatic, sedimentary, and metamorphic record extending back to 300 Ma, and are summarized in Figure 3. The complex architectures of these basins are thought to play a major role in the mechanisms controlling the closure of the Piemont-Liguria ocean (e.g., Tugend et al., 2014). Crucially, unlike Neotethyan ophiolites, preserved alpine ophiolites indicate that the Piemont-Liguria ocean was not floored by mature ocean crust but formed predominantly of basins floored by exhumed subcontinental mantle, similar to magma-poor Iberia-Newfoundland ocean-continent transition zones (OCTs) (e.g., Manatschal and Müntener, 2009 Mohn et al., 2011 Picazo et al., 2016). 85–100 Ma ( Rosenbaum and Lister, 2005 Handy et al., 2010 Zanchetta et al., 2012), lacks the distinctive characteristics of subduction initiation recorded in Neotethyan ophiolites ( Fig. The Alpine orogen, however, upon subduction initiation at ca. 1) has led to models implying the subduction of a 500–1000-km-wide Jurassic Piemont-Liguria ocean during Cretaceous–Cenozoic convergence (e.g., Stampfli et al., 1998 Handy et al., 2010).
Therefore, calc-alkaline magmatism and low-temperature–high-pressure metamorphic rocks are interpreted as strong evidence of paleo–subduction zones (e.g., Stern, 2005).Įvidence of (ultra)high-pressure continental and oceanic fragments in the European Alpine orogen (e.g., Chopin, 1984 Reinecke, 1998) ( Fig. In addition, dehydration of the oceanic crust drives flux melting of the overlying mantle wedge, resulting in predominantly “calc-alkaline” (cf. Once initiated, partial eclogitization and densification of the subducting oceanic lithosphere results in a slab-pull mechanism representing a driving force for self-sustaining subduction and ocean closure (e.g., Cloos, 1993). Well-documented examples of subduction initiation in mature oceanic settings (Neotethys supra–subduction zone ophiolites and Izu-Bonin-Mariana arc) have revealed that subduction initiation is characterized by an initial stage of upper-plate extension and tholeiitic to boninitic magmatism (e.g., Shervais, 2001 Maffione et al., 2017). The mechanisms allowing for the initiation of new subduction zones are related to either spontaneous or induced subduction and are thought to occur predominantly within weaknesses in oceanic lithosphere such as transform faults and oceanic detachments (e.g., Stern and Gerya, 2017). Recycling of oceanic lithosphere through subduction is a fundamental process governing plate tectonics, continental collision, and arc magmatism ( Cloos, 1993). Instead, subduction initiation at passive margins allowed for the accretion of the hydrated portion of the subducting plate within an orogenic wedge as subduction of dry subcontinental lithosphere inhibited magmatism during subduction initiation and ocean closure. Here, we review the metamorphic, igneous, and sedimentary record of the past 300 Ma of the Alpine orogen to show that there is no evidence of igneous activity during subduction initiation and prograde high-pressure metamorphism, leading to an ∼50 Ma hiatus in magmatism, or “arc gap.” The closure of rift basins forming the Piemont-Liguria ocean did not follow a classical Wadati-Benioff–type subduction. Such models, based inter alia on thermobarometric and geochronological evidence preserved in high-pressure metamorphic rocks and subduction-related magmatism, have been used to explain the convergence of Europe and Adria in the Cretaceous–Cenozoic and the subsequent Alpine orogen. Models of orogens identify subduction of oceanic crust as the key mechanism leading to continental collision.