Geodynamics of the Tavşanlı zone, western Turkey: Insights into subduction/obduction processes

Highlights

• Accretionary complex underplating during subduction: detachment of km-sized slices.

• Dynamics of obduction/subduction processes with similar PT conditions (Turkey, Oman).

• Processes occurring at the plate interface during oceanic and continental subduction.

Abstract

The tectono-metamorphic evolution of the high-pressure low-temperature (HP–LT) Tavşanlı zone (Western Anatolia, İzmir–Ankara suture zone) is herein reappraised to highlight processes occurring along a fossil subduction interface, from initial obduction stages to continental subduction. Structural and petrological data allow in particular to constrain the nature, internal structure and PT conditions of the oceanic accretionary complex sandwiched between the subducted continental margin of the Anatolide–Tauride Block (Orhaneli unit) and the non-metamorphic obducted ophiolite on top. Two distinct oceanic units (termed complexes 1 and 2) are recognised on top of one another, with metamorphic conditions ranging from incipient HP-LT imprint (complex 1) to blueschist facies conditions (complex 2). Based on the first occurrence of Fe–Mg carpholite and on pseudosection calculations, PT estimates of 250–350 °C and 11–13 kbar are inferred for complex 2. The internal structure of the accretionary complex points to the underplating of kilometre-scale units at different depths along the plate interface and to contrasting dynamics with respect to both the underlying continental unit and the ophiolite. Inter-plate mechanical coupling within the Tavşanlı zone is compared to the Oman case-study. The variable HP–LT overprint of the metamorphic sole places further constraints on regional scale tectonics, the accretionary dynamics and on the rapid thermal reequilibration of the subduction interface.

Introduction

High-pressure low-temperature (HP–LT) rocks/terranes returned from subduction zones convey essential information on deep processes such as exhumation mechanisms (Ernst, 1973, Jolivet et al., 2003, Platt, 1993), long-term mechanical coupling (Agard et al., 2009, Angiboust et al., 2012a, Angiboust et al., 2012b) or mass and fluid transfer (Marschall and Schumacher, 2012). They may either correspond to pieces of ocean-floor (Angiboust and Agard, 2010, Kienast, 1983), accreted sedimentary material (Agard et al., 2002, Cloos and Shreve, 1988a, Plunder et al., 2012) or rocks derived from continental crust (Chopin et al., 1991).

The İzmir–Ankara suture zone (İAZS; Fig. 1) is a major witness of the Mesozoic convergence between Gondwana-derived terranes (Anatolides, Taurides) and Eurasia. South of the İAZS, the predominantly continental-derived HP–LT Tavşanlı terrane underwent incipient blueschist to blueschist–eclogite facies conditions during the late Cretaceous (Çetinkaplan et al., 2008, Davis and Whitney, 2006, Davis and Whitney, 2008, Okay, 1978, Okay, 1980a, Okay, 1980b, Okay, 1982, Okay, 2002, Okay and Kelley, 1994, Whitney and Davis, 2006) and is overlain by a fragment of unmetamorphosed oceanic lithosphere, the Anatolian ophiolite. The Tavşanlı zone is thus generally regarded as the (cover of the) northern continental passive margin of the Anatolide–Tauride Block dragged into subduction as a result of obduction (Gautier, 1984, Okay et al., 1998). Similar settings with obducted ophiolites atop HP–LT continental rocks are found in Oman (Bailey, 1981, Michard et al., 1981, Goffé et al., 1988), Zagros (Angiboust et al., 2013) or New Caledonia (Agard and Vitale-Brovarone, 2013, Black et al., 1993).

The Tavşanlı zone represents an ideal target to study obduction/subduction processes as (1) it represents an extensive HP–LT province (i.e., > Oman) beneath one of the world largest obducted ophiolites (since all Anatolian ophiolites derive from the same ocean; Okay and Whitney, 2010, Pourteau et al., 2010, Van Hinsbergen et al., 2010), (2) it is well-preserved, thanks to later mild collision, and (3) it corresponds to the onset of an unusually long-lived subduction of continental material to depth of 15–80 km (> 30 My, from ~ 85–80 to 50–35 Ma). Subducted continental material comprises the Tavşanlı zone (which reached 50–80 km depth), the Afyon zone (15–45 km depth) and part of the Menderes Massif (15–45 km depth), hence ~ 450 km, from north to south, of subducted Anatolide–Tauride block (Fig. 1; Okay and Kelley, 1994, Okay et al., 1998, Sherlock et al., 1999, Seaton et al., 2009, Van Hinsbergen et al., 2010, Van Hinsbergen and Schmid, 2012, Gessner et al., 2013, Pourteau et al., 2013).

Unfortunately, the deformation patterns, large-scale metamorphic trends and tectonic setting of the Tavşanlı zone remain poorly known. The present field-based contribution therefore attempts to (a) provide critical structural data, cross-sections, petrological observations and PT estimates on these extensive exposures of subduction interface rocks, (b) set back their evolution within the frame of regional geodynamics (i.e., convergence between Eurasia and Anatolides–Taurides), and (c) characterize mechanisms of burial, accretion and exhumation and compare them to the classical Oman case study.