27 research outputs found
Tectonics of the Isua Supracrustal Belt 2: Microstructures Reveal Distributed Strain in the Absence of Major Fault Structures
No full textArchean geological records are increasingly interpreted to indicate a ≤3.2 Ga initiation of plate tectonics on Earth. This hypothesis contrasts with dominant plate tectonic interpretations for the Eoarchean (ca. 4.0–3.6 Ga) Isua supracrustal belt (southwest Greenland). Alternatively, recent work shows the belt could have formed via heat-pipe tectonics. Predicted strain distributions across the belt vary between models. Plate tectonic models predict a dominant unidirectional shear sense, corresponding to subduction vergence, and strain localization within ∼10-m-scale shear zones. In contrast, the proposed heat-pipe model predicts two opposing shear senses, corresponding to opposite limbs of 0.1-m to km-scale a-type folds (i.e., sheath and curtain folds), with relatively equal strain distributed across the belt. Here, we present the first microstructure study using thin-section petrography and electron backscatter diffraction analysis on quartz of oriented samples from throughout the Isua supracrustal belt. Key findings are: (1) the Eoarchean Isua supracrustal belt was deformed at ∼500°C–650°C, with potential postdeformational recovery at similar or lower temperatures, (2) the spatial distribution of the two opposing shear senses which dominate the belt (top-to-southeast and top-to-northwest) appears to be random, and (3) the strain intensity across the belt appears to be quasiuniform as evidenced by the uniformly low (mostly <0.1) M-indexes of quartz fabrics, such that no ≤ 100 -m-scale shear zones can be detected. Our findings are only consistent with the predictions of the heat-pipe model and do not require plate tectonics, so the geology of the belt is compatible with a ≤3.2 Ga initiation of plate tectonics
Tectonics of the Isua Supracrustal Belt 1: P‐T‐X‐d Constraints of a Poly‐Metamorphic Terrane
No full textThe Eoarchean Isua supracrustal belt (ISB) has been interpreted as one of the earliest records of subduction processes, leading to the conclusion that a plate tectonic geodynamic system was likely operating since the early Archean. However, proposed tectonic models remain difficult to evaluate as our understanding of the metamorphic and structural evolution remains fragmentary. Here, we present a metamorphic study of the supracrustal rocks of the ISB. We used petrographic and microstructural observations, phase equilibria, isopleth geothermobarometry, and conventional thermometry to explore the prograde, peak, and retrograde metamorphic evolution of the northeastern ISB. Our results show that the ISB records a syn‐tectonic, amphibolite facies metamorphic event (M1) with peak conditions of 550°C–600°C and 0.5–0.7 GPa. M1 was followed by a static, lower amphibolite facies metamorphic event (M2; 3.5 Ga) and the Neoarchean (<2.9 Ga), respectively. These events are partially overprinted by late low temperature (<500°C) retrogression (M3) that is most intensely developed in the northeastern part of the belt; it typically overprints some peak mineral phases while preserving the peak fabric. Our findings are consistent with spatially homogeneous syn‐tectonic amphibolite facies metamorphism and macroscale folding. Such features are predicted by a heat‐pipe tectonic model. Therefore, our findings permit the interpretation of the ISB as a record of early nonuniformitarian tectonic processes
Reply to Comment by A.P. Nutman et al. on “Tectonics of the Isua Supracrustal Belt 1: P-T-X-d Constraints of a Poly-Metamorphic Terrane” by A. Ramírez-Salazar et al. and “Tectonics of the Isua Supracrustal Belt 2: Microstructures Reveal Distributed Strain in the Absence of Major Fault Structures” by J. Zuo et al.
No full textStructural and metamorphic analyses from the works under discussion (Ramírez-Salazar et al., 2021, https://doi.org/10.1029/2020tc006516; Zuo et al., 2021, https://doi.org/10.1029/2020tc006514) show that the Isua supracrustal rocks can be interpreted to record one single deformation and metamorphic event featuring quasi-homogeneous deformation and amphibolite facies metamorphism, followed by late static retrogression or thermal event(s). Observed deformation and metamorphic records are consistent with three hypotheses: (a) they represent Neoarchean plate tectonic overprints following Eoarchean plate tectonic evolution (e.g., Nutman et al., 2022, https://doi.org/10.1029/2021TC007036); (b) they represent Eoarchean heat-pipe and/or plate tectonic deformation that survived later tectonic event(s) (e.g., Ramírez-Salazar et al., 2021, https://doi.org/10.1029/2020tc006516; Zuo et al., 2021, https://doi.org/10.1029/2020tc006514), and; (c) they represent one major Neoarchean tectonic event, such that the Isua supracrustal belt (ISB) records Eoarchean protolith-related processes but does not record Eoarchean metamorphism nor deformation. While a heat-pipe model for crustal formation is central to hypothesis 2, it is also a viable crustal formation mechanism for hypothesis 3 where the ISB would still form in a heat-pipe setting in Eoarchean time, but the major deformation of the heat-pipe lithosphere happened during Neoarchean time, probably by (proto-)plate tectonic processes. If the data presented in Zuo et al. (2021), https://doi.org/10.1029/2020tc006514 and Ramírez-Salazar et al. (2021), https://doi.org/10.1029/2020tc006516 only reflect Neoarchean histories, then these cannot be used to refute or support any Eoarchean geodynamic background for the formation of the ISB
Earth’s earliest phaneritic ultramafic rocks: Mantle slices or crustal cumulates?
No full textWhen plate tectonics initiated remains uncertain, partly because many signals interpreted as diagnostic of plate tectonics can be alternatively explained via hot stagnant-lid tectonics. One such signal involves the petrogenesis of early Archean phaneritic ultramafic rocks. In the Eoarchean Isua supracrustal belt (Greenland), some phaneritic ultramafic rocks have been dominantly interpreted as subduction-related, tectonically-exhumed mantle slices or cumulates. Here, we compared Eoarchean phaneritic ultramafic rocks from the Isua supracrustal belt with mantle peridotites, cumulates, and phaneritic ultramafic samples from the Paleoarchean East Pilbara Terrane (Australia), which is widely interpreted to have formed in non-plate tectonic settings. Our findings show that Pilbara samples have cumulate and polygonal textures, melt-enriched trace element patterns, relative enrichment of Os, Ir, and Ru versus Pt and Pd, and chromite-spinel with variable TiO2 and Mg#, and relatively consistent Cr#. Both, new and existing data show that cumulates and mantle rocks potentially have similar whole-rock geochemical characteristics, deformation fabrics, and alteration features. Geochemical modeling results indicate that Isua and Pilbara ultramafic rocks have interacted with low-Pt and Pd melts generated by sequestration of Pd and Pt into sulphide and/or alloy during magmatism. Such melts cannot have interacted with a mantle wedge. Correspondingly, geochemical compositions and rock textures suggest that Isua and Pilbara ultramafic rocks are not tectonically-exhumed mantle peridotites, but are cumulates that experienced metasomatism by fluids and co-genetic melts. Because such rocks could have formed in either plate or non-plate tectonic settings, they cannot be used to differentiate early Earth tectonic settings
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Tectonic Evolution of the Easternmost Himalayan Collisional System
No full textThe Cenozoic India-Asia collision generated the Tibetan Plateau and the Himalayan collisional system, the latter consisting of the convergence-perpendicular Himalayan orogen and the convergence-parallel Eastern and Western Flanking Belts located along the margins of India. Studying the evolution of each of these tectonic domains is critical to understanding the collision process and differentiating the end-member models of indenter-induced continental deformation. Despite this importance, there is a notable lack of geologic investigations on the development of the flanking belts in comparison to the extensive research of the Tibetan Plateau and east-trending Himalayan orogen. To address this problem, the research of this dissertation is focused on the Mesozoic-Cenozoic tectonic evolution of the northernmost segment of the Eastern Flanking Belt, the northern Indo-Burma Ranges, which are located directly east to southeast of the eastern Himalayan syntaxis. In the following chapters, I integrate the results of geologic field mapping, balanced cross section construction and restoration, U-Pb zircon geochronology, whole-rock geochemistry, thermobarometry, and (U-Th)/He zircon thermochronology to examine the litho-structural framework of the northern Indo-Burma Ranges and tectonic relationships in time and space with the adjacent eastern Himalayan orogen, the southern Tibetan Plateau, and the Eastern Flanking Belt.The research of this dissertation shows that the study area exposes a southwest- to west-directed Cenozoic thrust belt cored by a hinterland-dipping duplex system. Thrust faults sole into a northeast- to east-dipping d�collement, which extends to >30 km depth. Southwestward forward propagation of the thrust belt in the foreland was coeval with out-of-sequence thrusting in the hinterland. This structural framework combined with the observed southward deflection in the trends of ductile stretching lineations within shear zones (northeast-trending in the north and east-trending in the south) suggest deformation around the eastern Himalayan syntaxis is best approximated by models of clockwise lithospheric flow accommodated by distributed thrusting. Major lithologic units involved in the northern Indo-Burma thrust belt from south to north include the easternmost continuations of the Tertiary Sub-Himalayan Sequence, Proterozoic-Cambrian Lesser Himalayan Sequence, and Indus-Yarlung suture zone of the Himalayan orogen and the Mesozoic northern Gangdese batholith belt and Mesoproterozoic basement of the Lhasa terrane. However, several Himalayan-Tibetan lithologic units are missing, including the Paleoproterozoic-Ordovician Greater Himalayan Crystalline Complex, Proterozoic-Eocene Tethyan Himalayan Sequence, Mesozoic-Cenozoic Xigaze forearc basin, and Cenozoic igneous rocks of the southern Gangdese batholith. Research suggests that these units were present in the study area at the onset of the Cenozoic India-Asia collision and their present-day absence is related to an eastward increase in post-collisional crustal shortening and continental underthrusting along the Himalayan collisional system. This interpretation is supported by a Cenozoic shortening strain estimate of ~81% (>156 km) across the northern Indo-Burma Ranges and a dramatic southward decrease in the width of the collisional system from ~200 km across the Himalayan orogen to ~5 km across the study area.Active deformation across the northern Indo-Burma Ranges and adjacent southeastern Tibetan Plateau is characterized by right-slip transpression partitioned between the range-bounding, oblique-slip Mishmi thrust in the southwest and right-slip Puqu and Parlung faults of Jiali fault zone in the northeast. The leading Mishmi thrust is kinematically-linked with the ~1000-km-long, right-slip Sagaing fault to the south via a previously-unmapped, southwest-trending restraining bend. This structural relationship of the Eastern Flanking Belt provides a key example of the spatial transition from transpressional deformation near the corner of an indenter to discrete right-slip motion along the side of an indenter during continental collision
Supplemental Material: Cenozoic kinematic histories of the Tidding and Lohit thrusts in the northern Indo-Burma Ranges: Implications for crustal thickening and exhumation of Gangdese lower arc crust along the Indus-Yarlung suture zone
No full textMethodological details, additional figures, and data tables.</p
SPATIALLY VARIABLE BURIAL AND EXHUMATION HISTORY OF THE CENTRAL APPALACHIAN MOUNTAINS FROM ZIRCON (U-TH)/HE THERMOCHRONOLOGY
No full textWest-directed thrusting south of the eastern Himalayan syntaxis indicates clockwise crustal flow at the indenter corner during the India-Asia collision
No full textCENOZOIC SLIP HISTORIES OF THE TIDDING AND LOHIT THRUSTS IN THE NORTHERN INDO-BURMA RANGES, AND IMPLICATIONS FOR CRUSTAL THICKENING AND EXHUMATION OF GANGDESE LOWER ARC CRUST ALONG THE INDIA-ASIA SUTURE ZONE
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