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The Makran accretionary prism in SE Iran and SW Pakistan is one of the most extensive subduction accretions on Earth. It is characterized by intense folding, thrust faulting and dislocation of the Cenozoic units that consist of sedimentary, igneous and metamorphic rocks. Rock units forming the northern Makran ophiolites are amalgamated as a melange. Metamorphic rocks, including greenschist, amphibolite and blueschist, resulted from metamorphism of mafic rocks and serpentinites. In spite of the geodynamic significance of blueschist in this area, it has been rarely studied. Peak metamorphic phases of the northern Makran mafic blueschist in the Iranshahr area are glaucophane, phengite, quartz +/- omphacite+epidote. Post peak minerals are chlorite, albite and calcic amphibole. Blueschist facies metasedimentary rocks contain garnet, phengite, albite and epidote in the matrix and as inclusions in glaucophane. The calculated P-T pseudosection for a representative metabasic glaucophane schist yields peak pressure and temperature of 11.5-15kbar at 400-510 degrees C. These rocks experienced retrograde metamorphism from blueschist to greenschist facies (350-450 degrees C and 7-8kbar) during exhumation. A back arc basin was formed due to northward subduction of Neotethys under Eurasia (Lut block). Exhumation of the high-pressure metamorphic rocks in northern Makran occurred contemporarily with subduction. Several reverse faults played an important role in exhumation of the ophiolitic and HP-LT rocks. The presence of serpentinite shows the possible role of a serpentinite diapir for exhumation of the blueschist. A tectonic model is proposed here for metamorphism and exhumation of oceanic crust and accretionary sedimentary rocks of the Makran area. Vast accretion of subducted materials caused southward migration of the shore.
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 similar to 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 70-80km to similar to 55km 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.
To address one of the central questions of plate tectonics-How do large transform systems work and what are their typical features?-seismic investigations across the Dead Sea Transform (DST), the boundary between the African and Arabian plates in the Middle East, were conducted for the first time. A major component of these investigations was a combined reflection/ refraction survey across the territories of Palestine, Israel and Jordan. The main results of this study are: (1) The seismic basement is offset by 3-5 km under the DST, (2) The DST cuts through the entire crust, broadening in the lower crust, (3) Strong lower crustal reflectors are imaged only on one side of the DST, (4) The seismic velocity sections show a steady increase in the depth of the crust-mantle transition (Moho) from 26 km at the Mediterranean to 39 km under the Jordan highlands, with only a small but visible, asymmetric topography of the Moho under the DST. These observations can be linked to the left-lateral movement of 105 km of the two plates in the last 17 Myr, accompanied by strong deformation within a narrow zone cutting through the entire crust. Comparing the DST and the San Andreas Fault (SAF) system, a strong asymmetry in subhorizontal lower crustal reflectors and a deep reaching deformation zone both occur around the DST and the SAF. The fact that such lower crustal reflectors and deep deformation zones are observed in such different transform systems suggests that these structures are possibly fundamental features of large transform plate boundaries
The Northern Zagros Suture Zone (NZSZ), formed as a result of the collision between Arabian and Sanandaj-Sirjan microplate, is considered as part of the Zagros orogenic belt. NZSZ is marked by two allochthonous thrust sheets in upward stacking order: lower and upper allochthon. The Bulfat complex is a part of the upper allochthon or "Ophiolite-bearing terrane" of Albian-Cenomenion age (97-105 Ma). Voluminous highly sheared serpentinites associated with ophiolites occur within this upper allochthon. In addition, the Gemo-Qandil Group is characterized by gabbroic to dioritic Bulfat intrusion with a crystallization age spanning from similar to 45 to similar to 40 Ma, as well as extensive metapelites with contact to the Walash-Naupurdam metavolcanic rocks. Due to the deformation in the Sanandaj-Sirjan Zone along the eastern side of the Iraqi segment of NZSZ, the Gemo-Qandil Group was regionally metamorphosed during late Cretaceous (similar to 80 Ma). This tectono-compressional dynamics ultimately caused an oscillatory deformation against Arabian continental margin deposits as well. During these events, gabbro-diorite intrusion with high-grade contact metamorphic aureoles occurred near Bulfat. Thus, there is an overlap between regional and contact metamorphic conditions in the area. The earlier metamorphic characteristic can be seen only in places where the latter contact influence was insignificant. Generally, this can only observed at a distance of more than 2.5 km from the contact. According to petrographic details and field observations, the thermally metamorphosed metapelitic units of the metasediment have been completely assimilated, with only some streaks of biotite and relicts of initial foliation. They strongly resemble amphibolite-grade slices from the regional metamorphic rocks in the region. Metapelitic samples far from the intrusion give similar biotite cooling ages as the intrusive rocks. Thus, they may be affected by the same thermal event. Ar-40/Ar-39 dating of biotite in metapelite rocks of Bulfat by step-wise heating with laser gave average weighted isotopic ages of 34.78 +/- 0.06 Ma. This is interpreted as crystallization/recrystallization age of biotite possibly representing the time of cooling and uplift history of the Bulfat intrusion. Cooling and exhumation rates for the Bulfat gabbro-diorite rocks were estimated as similar to 400 A degrees C/Ma and similar to 3.3 mm/year respectively. According to petrographic details, field observations and Ar/Ar dating concerning the contact metamorphism near Bulfat due to the gabbro-diorite intrusion, no significant deformation is visible during exhumation processes after the Paleogene tectono-thermal event, indicating that isotopic ages of 34.78 +/- 0.06 Ma could mark the timing of termination of the island arc activity in the Ophiolite-bearing terrane (upper allochthon).
A tectonic slice of an arc sequence consisting of low-grade metavolcanic rocks and overlying metasedimentary succession is exposed in the Central Pontides north of the Izmir-Ankara-Erzincan suture separating Laurasia from Gondwana-derived terranes. The metavolcanic rocks mainly consist of basaltic andesite/andesite and mafic cognate xenolith-bearing rhyolite with their pyroclastic equivalents, which are interbedded with recrystallized pelagic limestone and chert. The metasedimentary succession comprises recrystallized micritic limestone with rare volcanogenic metaclastic rocks and stratigraphically overlies the metavolcanic rocks. The geochemistry of the metavolcanic rocks indicates an arc setting evidenced by depletion of HFSE (Ti, P and Nb) and enrichment of fluid mobile LILE. Identical trace and rare earth elements compositions of basaltic andesites/andesites and rhyolites suggest that they are cogenetic and derived from a common parental magma. The arc sequence crops out between an Albian-Turonian subduction-accretionary complex representing the Laurasian active margin and an ophiolitic melange. Absence of continent derived detritus in the arc sequence and its tectonic setting in a wide Cretaceous accretionary complex suggest that the Kosdag Arc was intra-oceanic. Zircons from two metarhyolite samples give Late Cretaceous (93.8 +/- 1.9 and 94.4 +/- 1.9 Ma) U/Pb ages. These ages are the same as the age of the supra-subduction ophiolites in western Turkey, which implies that that the Kosdag Arc may represent part of the incipient arc formed during the generation of the supra-subduction ophiolites. The low-grade regional metamorphism in the Kosdag Arc is constrained to 69.9 +/- 0.4 Ma by Ar-40/Ar-39 muscovite dating indicating that the arc sequence became part of a wide Tethyan Cretaceous accretionary complex by the latest Cretaceous. Non-collisional cessation of the arc volcanism is possibly associated with southward migration of the magmatism as in the Izu-Bonin-Mariana arc system. (c) 2015 Elsevier Ltd. All rights reserved.
Cenozoic uplift of the East African Plateau has been associated with fundamental climatic and environmental changes in East Africa and adjacent regions. While this influence is widely accepted, the timing and the magnitude of plateau uplift have remained unclear. This uncertainty stems from the lack of datable, geomorphically meaningful reference horizons that could record surface uplift. Here, we document the existence of significant relief along the East African Plateau prior to rifting, as inferred from modeling the emplacement history of one of the longest terrestrial lava flows, the similar to 300-km-long Yatta phonolite flow in Kenya. This 13.5 Ma lava flow originated on the present-day eastern Kenya Rift flank, and utilized a riverbed that once routed runoff from the eastern rim of the plateau. Combining an empirical viscosity model with subsequent cooling and using the Yatta lava flow geometry and underlying paleotopography (slope angle), we found that the prerift slope was at least 0.2 degrees, suggesting that the lava flow originated at a minimum elevation of 1400 m. Hence, high paleotopography in the Kenya Rift region must have existed by at least 13.5 Ma. We infer from this that middle Miocene uplift occurred, which coincides with the two-step expansion of grasslands, as well as important radiation and speciation events in tropical Africa.