Abstract
Vertical edges along subducted slabs have been recognized in the majority of subduction zones. Surprisingly, slab edges evolved into Subduction-Transform-Edge-Propagator (STEP) faults in only a few regions; the conditions under which STEPs form are special. It is relevant to constrain the conditions that facilitate STEP fault initiation because they leave a clear geological footprint in the overriding plate, whereas vertical tears generally do not. We therefore study a candidate region for STEP fault initiation in the western Hellenic Subduction Zone.We investigate the structure and seismicity of the shallow western Hellenic Subduction Zone using a recent full-waveform inversion model which both captures details of crustal and upper-mantle structure, yielding constraints in the depth interval from 10 to 200 km where lithosphere-mantle interactions have tectonic expressions. The western end of the Hellenic Subduction Zone is fragmented near the Kefalonia Transform Fault. We identify a separate Epirus lithospheric fragment that is roughly vertical below the southern Albanides. We also identify a new and major contrast within the lithospheric mantle of the Ionian ocean basin, which aligns with a gradient in free-air anomalies.In the overriding plate, the Kefalonia Transform Fault zone accommodates right lateral strike-slip deformation. We interpret this fault zone as a proto-STEP fault that formed simultaneously with Pliocene fragmentation of the Epirus fragment.Comparing the recent evolution of the NW Hellenic slab edge with currently active STEPs indicates that along-trench variations in convergence velocity are a prerequisite for STEP fault initiation. Such velocity variations may result from subduction of continental crust along part of the trench. Resistance to sea ward tear propagation by a mechanically strong subducting plate may prevent variations in convergence velocity to occur, and thus STEP fault initiation. The amount of time over which a velocity contrast persists will also be relevant for STEP fault initiation. Mechanical coupling between upper- and lower-plate, and the deformability of the upper-plate appear to also play a role in the initiation of the STEP fault once a slab is fragmented.
Highlights
IntroductionLateral edges have been recognized along or within various subducted slabs (Fig. 1; present-day, unless indicated otherwise); including Nazca slab (Gutscher et al, 1999; Pesicek et al, 2012), Izu-Bonin slab (Fryer et al, 2003; Gvirtzman and Stern, 2004; Miller et al, 2004, 2005, 2006a, 2006b; Gong et al, 2018), south Lesser Antilles slab (Russo et al, 1993; Clark et al, 2008; Van Benthem et al, 2013), various Mediterranean slabs (Spakman et al, 1988; Carminati et al, 1998; Wortel and Spakman, 2000; Biryol et al, 2011; Jolivet et al, 2013; de Lis Mancilla et al, 2018), Zagros/Makran slab (Agard et al, 2011), Pamir slab (Sippl et al, 2013; Sobel et al, 2013; Thiede et al, 2013), Burma slab (Kumar et al, 2016), Sikkim slab (Zhang et al, 2017; Sunilkumar et al, 2019), Philippine Sea slab near Taiwan (Lallemand et al, 1997), northern Scotia slab (Forsyth, 1975), eastern Sunda slab (Spakman and Hall, 2010), north Tonga slab (Isacks et al, 1969; Millen and Hamburger, 1998), Kermadec-Hikurangi slab (Reyners and Robertson, 2004), and Solomon slab (Neely and Furlong, 2018)
The western end of the Hellenic slab is fragmented near the Kefalonia Transform Fault
The Epirus fragment is laterally separated from the Hellenic slab by a low velocity zone (“gap”) that we interpret as a tear
Summary
Lateral edges have been recognized along or within various subducted slabs (Fig. 1; present-day, unless indicated otherwise); including Nazca slab (Gutscher et al, 1999; Pesicek et al, 2012), Izu-Bonin slab (Fryer et al, 2003; Gvirtzman and Stern, 2004; Miller et al, 2004, 2005, 2006a, 2006b; Gong et al, 2018), south Lesser Antilles slab (Russo et al, 1993; Clark et al, 2008; Van Benthem et al, 2013), various Mediterranean slabs (Spakman et al, 1988; Carminati et al, 1998; Wortel and Spakman, 2000; Biryol et al, 2011; Jolivet et al, 2013; de Lis Mancilla et al, 2018), Zagros/Makran slab (Agard et al, 2011), Pamir slab (Sippl et al, 2013; Sobel et al, 2013; Thiede et al, 2013), Burma slab (Kumar et al, 2016), Sikkim slab (Zhang et al, 2017; Sunilkumar et al, 2019), Philippine Sea slab near Taiwan (Lallemand et al, 1997), northern Scotia slab (Forsyth, 1975), eastern Sunda slab (Spakman and Hall, 2010), north Tonga slab (Isacks et al, 1969; Millen and Hamburger, 1998), Kermadec-Hikurangi slab (Reyners and Robertson, 2004), and Solomon slab (Neely and Furlong, 2018). We study this candidate region for STEP fault initiation in the western Hellenic Subduction Zone
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