Cyclic Brittle‐Ductile Oscillations Recorded in Exhumed High‐Pressure Continental Units: A Record of Deep Episodic Tremor and Slow Slip Events in the Northern Apennines

Abstract

Apennines and preserves brittle/ductile structures  During subduction local dehydration reactions triggered transiently high pore pressure in continental metasediments at ~1.1 GPa and 350 °C  Coeval carpholite dilational shear veins and mylonitic foliation cab be a geological record of deep episodic tremor and slow slip events Accepted ArticleThis article has been accepted for publication and undergone full peer review but has not been through the copyediting, typesetting, pagination and proofreading process, which may lead to differences between this version and the Version of Record. Please cite this article as

Alpine subduction zone metamorphism in the Palaeozoic successions of the Monti Romani (Northern Apennines, Italy)

“…Tectonic foliations across the MRC, ranging from subhorizontal to gently NE and SW dipping, define an antiformal structure, which is common in other exposures of the TMC (e.g., Carmignani et al, 1994; Carmignani & Kligfield, 1990; Molli & Vaselli, 2006; Papeschi et al, 2017). The observed HP–LT chloritoid‐bearing parageneses are compatible with similar Alpine assemblages that have been described in nearby TMC exposures, for example, in the Argentario (Theye et al, 1997), and the Mid‐Tuscan Ridge (Brogi & Giorgetti, 2012; Giorgetti et al, 1998; Giuntoli & Viola, 2021) (Figure 1a). The RSCM thermometry (average peak T of 382 ± 8°C) does not support a Variscan tectono‐metamorphic inheritance within the MRC, because its thermal structure is not compatible with the hotter environments (HT greenschist to amphibolite facies conditions) documented for the Variscan metamorphism in the region (e.g., Franceschelli et al, 2004; Lo Pò et al, 2016; Pandeli et al, 2005).…”

“… Pourteau et al (2014) showed that extensive, continuous, and discontinuous zoning patterns can develop in (Fe, Mg)‐chloritoid–carpholite parageneses due to fractionation of elements in the chloritoid cores from the bulk during the metamorphic evolution. Indeed, (Fe, Mg)‐carpholite commonly occurs in syn‐metamorphic quartz veins in the Verrucano Group metasediments of the TMC (e.g., Brogi & Giorgetti, 2012; Giorgetti et al, 1998; Giuntoli & Viola, 2021; Rossetti et al, 1999; Theye et al, 1997). We could, in principle, infer its former presence also in the MRC.…”

“…The subduction dynamics controlled the mode and the space–time distribution of the Alpine orogenic construction and collapse along convergence zones, during the transition from crustal thickening to thinning in back‐arc domains (e.g., Dewey, 1988; Faccenna et al, 2004; Jolivet & Faccenna, 2000; Malinverno & Ryan, 1986). Studies on the tectono‐metamorphic evolution of the exhumed high‐pressure‐low‐temperature (HP–LT) blueschist and eclogite facies metamorphic roots of the Alpine orogens have provided a wealth of key information on convergent plate margins structure and evolution (e.g., Agard et al, 2006, 2009, 2018; Chopin, 1984; Ernst, 1971; Ernst & Dal Piaz, 1978; Escher & Beaumont, 1997; Jolivet et al, 2003; Malusà et al, 2015; Penniston‐Dorland et al, 2015; Platt, 1986; Ring & Layer, 2003; Rubatto & Hermann, 2001; Yamato et al, 2007), deformation mechanisms (e.g., Agard et al, 2018; Angiboust et al, 2011; Giuntoli & Viola, 2021; Kotowski & Behr, 2019; Locatelli et al, 2018), and fluid transport in subduction zones (Bebout & Penniston‐Dorland, 2016; Cook‐Kollars et al, 2014; Epstein et al, 2020; Frezzotti et al, 2011; Lefeuvre et al, 2020; Maggi et al, 2014; Rubatto & Hermann, 2003; Scambelluri et al, 2015).…”

“…This is the case of the metamorphic complexes of the Northern Apennines belt (Figure 1), where the discovery of HP–LT metamorphism is relatively recent with respect to other circum‐Mediterranean belts. In the Northern Apennines, Al‐rich metapelitic/metapsammitic rocks and related syn‐metamorphic veins preserve chloritoid‐ and carpholite‐bearing parageneses that recorded HP–LT metamorphism (Brogi & Giorgetti, 2012; Giorgetti et al, 1998; Giuntoli & Viola, 2021; Jolivet, Faccenna, et al, 1998; Jolivet, Goffé, et al, 1998; Molli et al, 2000; Rossetti et al, 1999; Theye et al, 1997). This is typical of other Alpine segments like Calabria and the Southern Apennines (Iannace et al, 2007; Rossetti et al, 2004; Rossetti, Faccenna, Goffé, et al, 2001; Vitale et al, 2013), the Betic‐Rif (Azañón et al, 1998; Bouybaouene et al, 1995; Vidal et al, 1999), Anatolia (Oberhänsli et al, 2001; Plunder et al, 2015; Pourteau et al, 2014), and the Aegean region (Cao et al, 2018; Parra et al, 2002; Theye et al, 1992).…”

Embrittlement Within Viscous Shear Zones Across the Base of the Subduction Thrust Seismogenic Zone

“…Along subduction plate boundaries, localized embrittlement within otherwise aseismic regions may be triggered by either or a combination of locally reduced effective stresses (e.g., Fagereng, Diener, Meneghini, et al., 2018; Gao & Wang, 2017; Giuntoli & Viola, 2021; Shelly et al., 2006; Yardley, 1983), or locally increased shear stress (Beall et al., 2019a; Sibson, 1980). Direct constraints on the composition and internal geometry of plate boundary shear zones may provide insight into how and why variations in shear stress or effective stress occur.…”