Subduction and exhumation mechanisms of ultra‐high and high‐pressure oceanic and continental crust at Makbal (Tianshan, Kazakhstan and Kyrgyzstan)
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Abstract The Makbal Complex in the northern Tianshan of Kazakhstan and Kyrgyzstan consists of metasedimentary rocks, which host high‐ P ( HP ) mafic blocks and ultra‐HP Grt‐Cld‐Tlc schists ( UHP as indicated by coesite relicts in garnet). Whole rock major and trace element signatures of the Grt‐Cld‐Tlc schist suggest a metasomatized protolith from either hydrothermally altered oceanic crust in a back‐arc basin or arc‐related volcaniclastics. Peak metamorphic conditions of the Grt‐Cld‐Tlc schist reached ~580 °C and 2.85 GPa corresponding to a maximum burial depth of ~95 km. A Sm‐Nd garnet age of 475 ± 4 Ma is interpreted as an average growth age of garnet during prograde‐to‐peak metamorphism; the low initial εΝd value of −11 indicates a protolith with an ancient crustal component. The petrological evidence for deep subduction of oceanic crust poses questions with respect to an effective exhumation mechanism. Field relationships and the metamorphic evolution of other HP mafic oceanic rocks embedded in continentally derived metasedimentary rocks at the central Makbal Complex suggest that fragments of oceanic crust and clastic sedimentary rocks were exhumed from different depths in a subduction channel during ongoing subduction and are now exposed as a tectonic mélange. Furthermore, channel flow cannot only explain a tectonic mélange consisting of various rock types with different subduction histories as present at the central Makbal Complex, but also the presence of a structural ‘dome’ with UHP rocks in the core (central Makbal) surrounded by lower pressure nappes (including mafic dykes in continental crust) and voluminous metasedimentary rocks, mainly derived from the accretionary wedge.Keywords:
Protolith
Metamorphic core complex
We document field relationships, petrography, and geochemistry of a newly identified exposure of Orocopia Schist, a Laramide subduction complex, in the northern Plomosa Mountains metamorphic core complex of west-central Arizona (USA). This core complex is characterized by pervasive mylonitic fabrics associated with early Miocene intrusions. The quartzofeldspathic Orocopia Schist records top-to-the-NE mylonitization throughout its entire ∼2–3 km structural thickness and 10 km2 of exposure in the footwall of the top-to-the-NE Plomosa detachment fault. The schist of the northern Plomosa Mountains locally contains graphitic plagioclase poikiloblasts and scattered coarse-grained actinolitite pods, both of which are characteristic of the Orocopia and related schists. Actinolitite pods are high in Mg, Ni, and Cr, and are interpreted as metasomatized peridotite—an association observed in Orocopia Schist at nearby Cemetery Ridge. A 3.5-km-long unit of amphibolite with minor interlayered ferromanganiferous quartzite is localized along a SE-dipping contact between the Orocopia Schist and gneiss. Based on their lithologic and geochemical characteristics, we interpret the amphibolite and quartzite as metabasalt and metachert, respectively. The top of the Orocopia Schist is only ∼3–4 km below a ca. 21 Ma tuff in the footwall of the Plomosa detachment fault, suggesting that a major Paleogene exhumation event brought the schist to upper-crustal depths after it was subducted in the latest Cretaceous but before most Miocene core complex exhumation. The Orocopia Schist in the northern Plomosa Mountains is located near the center of the Maria fold-and-thrust belt, which likely represented a crustal welt in the Late Cretaceous. The keel of this crustal welt may have been sheared off by the shallowly dipping Farallon slab prior to underplating of rheologically weak Orocopia Schist. Paleogene exhumation of the Orocopia Schist in the northern Plomosa Mountains is consistent with extensional exhumation recorded in Orocopia Schist in the Gavilan Hills of southeasternmost California, which shortly postdated schist underplating, suggesting that subduction of schist may have triggered Paleogene extension in the region.
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Basement
Rodinia
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Adakite
Incompatible element
Planetary differentiation
Underplating
Primitive mantle
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Dike
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Section (typography)
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The stretching and thinning of the continental crust, which occurs during the formation of passive continental margins, may cause important changes in the velocity structure of such crust. Further, crust attenuated to a few kilometres' thickness, can be found underlying 'oceanic' water depths. This paper poses the question of whether thinned continental crust can be distinguished seismically from normal oceanic crust of about the same thickness. A single seismic refraction line shot over thinned continental crust as part of the North Biscay margin transect in 1979 was studied in detail. Tau-p inversion suggested that there are differences between oceanic and continental crust in the lower crustal structure. This was confirmed when synthetic seismograms were calculated. The thinned continental crust (α ≥ 7.0) exhibits a two-gradient structure in the non-sedimentary crust with velocities between 5.9 and 7.4 km s-1; an upper 0.8 s-1 layer overlies a 0.4 s-1 layer. No layer comparable to oceanic layer 3 was detected. The uppermost mantle also contains a low-velocity zone.
Continental Margin
Convergent boundary
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Adakite
Convergent boundary
Continental Margin
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We investigate how subduction may be triggered by continental crust extension at a continental margin. The large topography contrast between continental and oceanic domains drives the spreading of continental crust over oceanic basement. Subduction requires the oceanic plate to get submerged in mantle, so that negative buoyancy forces may take over and drive further descent. This is promoted by two mechanisms. Loading by continental crust bends the oceanic plate downwards. Extension in the continental domain induces crustal thinning, which acts to raise mantle above the oceanic plate. In this model, the width of the continental region undergoing extension is an important control parameter. The main physical controls are illustrated by laboratory experiments and simple theory for elastic flexure coupled to viscous crustal spreading. Three governing dimensionless parameters are identified. One involves the poorly constrained oceanic plate buoyancy. We find that the oceanic plate can be thrust to depths larger than 40 km even if it is buoyant, enabling metamorphic reactions and density increase in the oceanic crust. Another parameter is the ratio between the width of the continental extension region and the flexural parameter for the oceanic plate. Initiating subduction is easier if the continent thins over a short lateral distance or if the oceanic plate is strong. The third important parameter is the ratio of oceanic plate thickness to initial continental crust thickness, such that a weak plate and a thick crust do not favour subduction. Thus, the change from a passive to an active margin depends on the local characteristics of the continental crust and is not determined solely by the age and properties of the oceanic lithosphere. It is shown that the spreading of continental crust induces uplift of the margin as the adjacent seafloor subsides. Evidence for the emplacement of continental crust over oceanic basement at passive margins is reviewed.
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Eclogitization
Adakite
Passive margin
Underplating
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Abstract Field subdivision of schist using textural appearance is valuable as a means to readily identify post‐metamorphic faults and to subdivide monotonous schist, at 1:50 000 scale or smaller. Recent regional mapping in the Haast Schist of South Island, New Zealand, has revealed ambiguities and shortcomings in the existing field‐based systems of textural subdivision. We propose a revised textural zonation scheme that is broadly compatible with the previous Hutton‐Turner and Bishop systems, yet overcomes their deficiencies. The main criteria for identifying textural zones (TZ) are white mica grain size and foliation development. For field use, the main features are: (1) restriction to first generation penetrative textures and fabrics; (2) clarification of the definition and usage of segregation to better distinguish TZIIB from III schist; and (3) grouping of TZIIIB and IV rocks because of problems of protolith identification and quartz veining. The revised system is applicable to both sandstone and mudstone protoliths.
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