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Abstract Back‐arc basins frequently form within subduction zones, creating sources of lithospheric weakness that can accommodate subsequent compressional deformation. The crustal structure of these basins, including whether they contain extended preexisting crust and/or new crust formed by seafloor spreading, can thus exert a major influence on strain partitioning in orogenic belts. Here, we present field observations, petrographic analyses, and major/trace element geochemical data from the Caucasus Basin, a back‐arc basin that initiated in continental lithosphere in the Jurassic and subsequently localized deformation in the present‐day Greater Caucasus during the latter stages of Cenozoic Arabia‐Eurasia continent‐continent collision. Our results reveal distinct lithologic and geochemical domains separated by south‐vergent thrust faults within the North Georgia fault system (NGFS) in the Republic of Georgia. Along the Enguri River, shallow intrusive and volcanic rocks are thrust over dominantly volcaniclastic cover, whereas along the Tskhenistskali River, intrusions into metasedimentary rocks are juxtaposed against volcanic flows. The presence of a minor depleted mantle geochemical signature in intrusive rocks from the Tskhenistskali traverse supports an episode of Jurassic seafloor spreading in the Caucasus Basin, with the resulting lithosphere facilitating Cenozoic basin closure by north‐dipping subduction during Arabia‐Eurasia collision. The Khaishi fault along the Enguri River and the Lentekhi fault along the Tskhenistskali river mark major juxtapositions in back‐arc crustal structure and may be components of the terminal suture indicating Caucasus Basin closure. Our results highlight how magmatic rocks in relict basin rocks can yield key insights into basin structure and orogenesis, even when no ophiolite is present.
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A revised map of volcanic units in the Oman ophiolite: insights into the architecture of an oceanic proto-arc volcanic sequence
Abstract. Numerous studies have revealed genetic similarities between Tethyanophiolites and oceanic “proto-arc” sequences formed above nascent subductionzones. The Semail ophiolite (Oman–U.A.E.) in particular can be viewed as ananalogue for this proto-arc crust. Though proto-arc magmatism and themechanisms of subduction initiation are of great interest, insight isdifficult to gain from drilling and limited surface outcrops in marinesettings. In contrast, the 3–5 km thick upper-crustal succession of theSemail ophiolite, which is exposed in an oblique cross section, presents anopportunity to assess the architecture and volumes of different volcanicrocks that form during the proto-arc stage. To determine the distribution ofthe volcanic rocks and to aid exploration for the volcanogenic massivesulfide (VMS) deposits that they host, we have remapped the volcanic unitsof the Semail ophiolite by integrating new field observations, geochemicalanalyses, and geophysical interpretations with pre-existing geological maps.By linking the major-element compositions of the volcanic units to rockmagnetic properties, we were able to use aeromagnetic data to infer theextension of each outcropping unit below sedimentary cover, resulting ina new map showing 2100 km2 of upper-crustal bedrock. Whereas earlier maps distinguished two main volcanostratigraphic units, wehave distinguished four, recording the progression from early spreading-axisbasalts (Geotimes), through axial to off-axial depleted basalts (Lasail), topost-axial tholeiites (Tholeiitic Alley), and finally boninites (BoniniticAlley). Geotimes (“Phase 1”) axial dykes and lavas make up ∼55 vol % of the Semail upper crust, whereas post-axial (“Phase 2”) lavasconstitute the remaining ∼45 vol % and ubiquitously coverthe underlying axial crust. Highly depleted boninitic members of the Lasailunit locally occur within and directly atop the axial sequence, marking anearlier onset of boninitic magmatism than previously known for theophiolite. The vast majority of the Semail boninites, however, belong to theBoninitic Alley unit and occur as discontinuous accumulations up to 2 kmthick at the top of the ophiolite sequence and constitute ∼15 vol % of the upper crust. The new map provides a basis for targetedexploration of the gold-bearing VMS deposits hosted by these boninites. Thethickest boninite accumulations occur in the Fizh block, where magma ascentoccurred along crustal-scale faults that are connected to shear zones in theunderlying mantle rocks, which in turn are associated with economicchromitite deposits. Locating major boninite feeder zones may thus be anindirect means to explore for chromitites in the underlying mantle.
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- Award ID(s):
- 1642268
- PAR ID:
- 10129276
- Date Published:
- Journal Name:
- Solid Earth
- Volume:
- 10
- Issue:
- 4
- ISSN:
- 1869-9529
- Page Range / eLocation ID:
- 1181 to 1217
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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- Free Publicly Accessible Full Text
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Accepted Manuscript1.0
- Journal Article:
- https://doi.org/10.5194/se-10-1181-2019
