Tectonic Stacking of HP/LT Metamorphic Rocks in Accretionary Wedges and the Role of Shallowing Slab‐Mantle Decoupling

Abstract

High‐pressure/low‐temperature (HP/LT) chloritoid‐bearing micaschists crop out widely in the central part of northern Turkey and represent deep‐seated subduction‐accretionary complexes. Three peak metamorphic assemblages are identified in the area studied: (1) garnet‐chloritoid‐glaucophane with pseudomorphs after lawsonite; (2) chloritoid with pseudomorphs after glaucophane; and (3) chloritoid with pseudomorphs after jadeite in addition to phengite, paragonite, quartz, chlorite, rutile, and apatite. The latter is interpreted as transformation of a chloritoid + glaucophane assemblage to chloritoid + jadeite with increasing pressure; PT modeling indicates ~17 and 22–25 kbars for the two peak parageneses. The diversity of peak metamorphic assemblages and the PT estimates suggest that basal accretion occurred at different depths within the wedge. The depth of the basal accretion is possibly controlled by the slab‐mantle decoupling depth. Stretching and thinning of the lithospheric fore arc induced by the slab rollback possibly caused shallowing of the slab‐mantle decoupling depth which limited depth of the basal accretion from 70–80 km to ~55 km within the subduction channel. A slab‐mantle coupling depth‐controlled basal accretion may also explain the scarcity of eclogite and high‐grade blueschist facies metamorphic rocks in active intraoceanic subduction zones. Because the overriding plate is young and hot in intraoceanic subductions, the slab and mantle are coupled at a relatively shallow depth before eclogitization of the oceanic crust. This prevents accretion and exhumation of eclogite along the subduction channel.

Multiple Exhumation Phases in the Central Pontides (N Turkey): New Temporal Constraints on Major Geodynamic Changes Associated With the Closure of the Neo‐Tethys Ocean

“…Simplified late Jurassic to Quaternary stratigraphy for the northern and southern slope of the Sinop range (with the thickness of each formation in parenthesis; Derman & İztan, ; Görür & Tüysüz, ; Hippolyte et al, , ; Ilgar, ; Janbu et al, ; Leren et al, ; Okay et al, , , ; Tüysüz et al, ). Major regional and local tectonomagmatic, metamorphic, and geodynamic events are reported with a thin black line (from: 1, Okay et al, ; 2, Aygül et al, ; 3, Meijers et al, ; 4, Aygül & Oberhansli, ; 5, Okay et al, ; 6, Kaymakçı et al, ; 7 Van Hinsbergen et al, ; 8, Özcan et al, ; 9, Dinçer, ; 10, Topuz et al, ; 11, Göçmengil et al, ; 12, Lefebvre et al, ; 13, Gürer et al, ; 14, Boztuğ & Jonckheere, ; 15, Okay et al, ; 16, Şengör et al, ), while the timing and magnitude of cooling obtained in this study is shown with a thicker black line. Gray transparent polygons show major cooling events depicted by our modeling.…”

“…It should be noted, however, that such a collision seems to postdate the Late Cretaceous (83 to 65 Ma) oroclinal bending in the Central Pontides (Meijers et al, ). A possible explanation could be that oroclinal bending processes took place during the collision between the intraoceanic Kosdağ magmatic arc (Aygül & Oberhansli, ) and the active Southern Eurasian margin. This scenario would imply that the Paleocene collision with the Kırşehir Block did not produce significant indentation and rotations along a vertical axis as previously thought (Kaymakçı et al, ; Meijers et al, ), but rather only fault‐related exhumation.…”

“…The Central Pontide mountains are characterized by a suite of metamorphic, magmatic, sedimentary, and mantle rocks, which recorded a prolonged and complex Late Paleozoic to Early Cenozoic history of subduction, microcontinent collision, and backarc extension (Figure ; e.g., Aygül & Oberhansli, ; Aygül et al, ; Çimen et al, , ; Okay et al, , , ; Okay & Nikishin, ). The Late Paleozoic episodes include the northward subduction of the Paleo‐Tethys oceanic lithosphere beneath southern Laurasia, in association with arc magmatism at ~350 to 300 Ma and the Carboniferous to Permian collision with Armorica type terrains (Variscan orogeny) that led to the development of Pangea (e.g., Okay & Nikishin, ; Stampfli & Borel, , and references therein).…”

“…This episode has been linked to tectonic accretion and exhumation of an imbricated, south verging system of metamorphosed (high pressure and low temperature) oceanic and mantle rocks (Domuzdağ Complex) and Lower Cretaceous metasediments (blueschist and greenschist of the Esenler and the Martin complexes) between 110 and 92 Ma (Central Pontides Supercomplex, Figure ; Aygül et al, ; Okay et al, , ). Farther south, the occurrence of ~95 Ma volcanic and sedimentary rocks that recorded low‐grade metamorphism at ~70 Ma suggests the development of a magmatic arc, most likely associated with a northward dipping, intraoceanic subduction zone between the Central Pontides Supercomplex and Gondwana‐derived blocks of Central Anatolia (Figure ; Kösdağ paleoarc, Aygül & Oberhansli, ; see also Gürer et al, ; Van Hinsbergen et al, ).…”

“…(a) Geologic map of Turkey redrawn after Şenel (), Okay et al (), and Aygül and Oberhansli (). (b) Cross section of the Central Pontides based on seismic lines (redrawn after Espurt et al, ).…”