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Subduction, Underplating, and Return Flow Recorded in the Cycladic Blueschist Unit Exposed on Syros, Greece

Journal Article · · Tectonics
DOI:https://doi.org/10.1029/2020tc006528· OSTI ID:1886140
 [1];  [2];  [3];  [4];  [5];  [4];  [6]
  1. Univ. of Texas, Austin, TX (United States); McGill Univ., Montreal, QC (Canada); Utrecht University (Netherlands)
  2. Univ. of Texas, Austin, TX (United States); Eidgenoessische Technische Hochschule (ETH), Zurich (Switzerland); Lawrence Livermore National Lab. (LLNL), Livermore, CA (United States)
  3. Univ. of Texas, Austin, TX (United States); Eidgenoessische Technische Hochschule (ETH), Zurich (Switzerland)
  4. Univ. of Texas, Austin, TX (United States)
  5. National and Kapodistrian Univ. of Athens (Greece)
  6. Univ. of Texas, Austin, TX (United States); Massachusetts Inst. of Technology (MIT), Cambridge, MA (United States)

Exhumed high-pressure/low-temperature (HP/LT) metamorphic rocks provide insights into deep (~20–70 km) subduction interface dynamics. On Syros Island (Cyclades, Greece), the Cycladic Blueschist Unit preserves blueschist-to-eclogite facies oceanic- and continental-affinity rocks that record the structural and thermal evolution linked to Eocene subduction. Despite decades of research, the metamorphic and deformation history (P-T-D) and timing of subduction and exhumation are matters of ongoing discussion. We suggest that Syros comprises three coherent tectonic slices and that each slice underwent subduction, underplating, and syn-subduction return flow along similar P-T trajectories, but at progressively younger times. Subduction and exhumation are distinguished by lineations and ductile fold axis orientations, and are kinematically consistent with previous studies that document top-to-the-S-SW shear (prograde-to-peak subduction), top-to-the-NE shear (blueschist facies exhumation), and then E-W coaxial stretching (greenschist facies exhumation). Amphibole zonations record cooling during decompression, indicating return flow above a cold slab. Multi-mineral Rb-Sr isochrons and compiled metamorphic geochronology show that the three slices record distinct stages of peak subduction (53–52, ~50, and 45 Ma) that young with structural depth. Retrograde blueschist and greenschist facies fabrics span ~50–40 and ~43–20 Ma, respectively, and also young with structural depth. Synthesized data sets support a revised tectonic framework for Syros, involving subduction of structurally distinct coherent slices and simultaneous return flow of previously accreted tectonic slices in the subduction channel shear zone. Distributed, ductile, dominantly coaxial return flow in an Eocene-Oligocene subduction channel proceeded at rates of ~1.5–5 mm/yr and accommodated ~80% of the total exhumation of this HP/LT complex.

Research Organization:
Lawrence Livermore National Laboratory (LLNL), Livermore, CA (United States)
Sponsoring Organization:
USDOE National Nuclear Security Administration (NNSA); National Science Foundation (NSF)
Grant/Contract Number:
AC52-07NA27344
OSTI ID:
1886140
Report Number(s):
LLNL-JRNL-832897; 1049929
Journal Information:
Tectonics, Journal Name: Tectonics Journal Issue: 6 Vol. 41; ISSN 0278-7407
Publisher:
Wiley/American Geophysical Union (AGU)Copyright Statement
Country of Publication:
United States
Language:
English

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