Mantle-driven dynamic uplift of the Rocky Mountains and Colorado Plateau and its surface response: Toward a unified hypothesis

The correspondence between seismic velocity anomalies in the crust and mantle and the differential incision of the continental-scale Colorado River system suggests that significant mantle-to-surface interactions can take place deep within continental interiors. The Colorado Rocky Mountain region exhibits low-seismic-velocity crust and mantle associated with atypically high (and rough) topography, steep normalized river segments, and areas of greatest differential river incision. Thermochronologic and geologic data show that regional exhumation accelerated starting ca. 6-10 Ma, especially in regions underlain by low-velocity mantle. Integration and synthesis of diverse geologic and geophysical data sets support the provocative hypothesis that Neogene mantle convection has driven long-wavelength surface deformation and tilting over the past 10 Ma. Attendant surface uplift on the order of 500-1000 m may account for ~25%-50% of the current elevation of the region, with the rest achieved during Laramide and mid-Tertiary uplift episodes. This hypothesis highlights the importance of continued multidisciplinary tests of the nature and magnitude of surface responses to mantle dynamics in intraplate settings

Structural and thermochronometric evidence for multi-stage exhumation of southern Syros, Cycladic islands, Greece

The Attic-Cycladic metamorphic-core complex province was formed during Miocene to recent regional back-arc extension in response to slab-rollback and gravitational collapse. The development of several metamorphic-core complexes along extensional low-angle detachment faults resulted in the exhumation of lower-plate rocks and the tectonic denudation of the upper plate that is only sparsely exposed throughout the Aegean Sea. One of these areas is in southern Syros where the tectonic configuration includes three tectonometamorphic units separated by two detachment faults. The lower-plate Cycladic Blueschist Unit underwent late Cretaceous-Eocene HP/LT metamorphism and subsequent greenschist-facies overprint during Oligo-Miocene times. The tectonically overlying greenschist-facies Upper Unit is a tectonic sliver between the Cycladic Blueschist Unit and the structurally highest unit, the Vari Unit, which is dominated by quartzofeldspathic gneisses that are affected by four distinct deformation events (D1-D4). The late Cretaceous upper greenschist-facies D1 episode is related to late Cretaceous-Eocene subduction while D2-D4 fabrics were formed in response to late Cenozoic extension and exhumation. Field relations and structural data suggest that the late Miocene brittle Vari Detachment juxtaposes both Vari Unit and Upper Unit against the Cycladic Blueschist Unit. Mylonitic rocks observed at the base of Vari Unit along the contact with Upper Unit and zircon and apatite (U-Th)/He thermochronometry corroborate this scenario with lower-plate Cycladic Blueschist Unit rocks recording rapid cooling at ~. 8-10. Ma. In contrast, cooling ages from the Vari Unit cluster at ~. 13-15. Ma, indicating rapid exhumation coeval with activity along the Livadi Detachment on the neighboring island of Tinos. These data document new evidence that Vari Unit was affected by a major middle Miocene phase of exhumation and an older semi-brittle episode, both unrelated to late Miocene detachment faulting, revealing that crustal extension occurred in multiple stages and in a protracted episodic fashion similar to other parts of the Cyclades. © 2012 Elsevier B.V

Zircon and Apatite U-Pb Constraints on the Tectonic Affinity and Metamorphic History of the Blueschist-Facies Ambelakia Unit, Mt. Ossa, Greece

In the Aegean region, the Cycladic Blueschist Unit (CBU) and the Ambelakia unit (Mt. Ossa, Thessaly, Greece) represent early Cenozoic subduction-related HP-LT metamorphic complexes exhumed in the back-arc of the Hellenic subduction zone. The Ambelakia unit has been linked to the classic CBU in the Cyclades; however, the tectonic affinity, structural position, nature of the protolith, or timing of metamorphism for these rocks in eastern Thessaly remains largely unresolved. This study provides detailed new insights into both the provenance, protolith age, and tectonic affinity of the Pelagonian, Ambelakia, and Olympos-Ossa tectonic units by integrating U-Pb detrital zircon (DZ) and detrital apatite (DA) data in a structural context. DZ results suggest the existence of distinct metasedimentary units, spanning in depositional age from Carboniferous to Late Cretaceous, with provenance signatures that support a correlation with the classic CBU and strengthen the argument for a similar pre-subduction tectonic relationship. Depth-profiling analysis reveals metamorphic rims that record HP-LT metamorphism in the Ossa Ambelakia during the Paleocene-Eocene. Apatite U-Pb data from the metasedimentary units preserve a detrital signature similar to the DZ signatures, indicating that apatites were not reset (<450 C) nor recrystallized during subduction metamorphism. These new data suggest that the Ambelakia unit is a lateral equivalent of the CBU subduction complex; however, this unit experienced distinctly lower pressure and temperature conditions during metamorphism than the along-strike classic CBU. © 2023. American Geophysical Union. All Rights Reserved

The provenance and internal structure of the Cycladic Blueschist Unit revealed by detrital zircon geochronology, Western Cyclades, Greece

This study presents detrital zircon U-Pb analyses of 23 samples of the Cycladic Blueschist Unit (CBU) from Kea, Kythnos, and Serifos islands, as well as the Lavrion Peninsula of SE Attica. The maximum depositional ages (MDA) and age distributions of detrital zircon U-Pb dates are used to correlate metasediments between the islands considered herein and infer their provenance. Two distinct detrital zircon U-Pb age distributions are found in CBU metasediments: “Proterozoic,” comprised of >40% Neoproterozoic zircons with Triassic-Early Jurassic maximum depositional ages and “Paleozoic,” containing >30% Paleozoic zircons and yielding Late Jurassic-Cretaceous MDAs. Proterozoic affinity metasediments are rift margin deposits derived from the northern Gondwanan margin. Paleozoic metasediments are flysch sediments most probably sourced from the Internal Hellenides. This metamorphosed flysch forms a distinct marker horizon found in a similar structural position in Lavrion, Kythnos, and Serifos. Based on lithologic correlation, sediment provenance, and MDA estimates, the CBU of Kythnos is correlative to the Lavrion Schists of Attica. On the islands of Serifos and Kythnos and within the Lavrion Schists only young-on-old relationships exist between rocks based on MDA estimates. ©2017. American Geophysical Union. All Rights Reserved

Tectono-magmatic and Stratigraphic Evolution of the Cycladic Basement, Ios Island, Greece

The Cycladic Basement (CB) and the overlying Cycladic Blueschist Unit (CBU) are part of the Paleogene Cycladic subduction complex exposed in Miocene metamorphic core complexes in the distended back-arc of the retreating Hellenic subduction zone of the southern Aegean. While the Cenozoic tectono-metamorphic evolutions of the CB and the CBU have been the foci of numerous studies, this study presents new laser ablation inductively coupled plasma mass spectrometry bedrock and detrital zircon (DZ) U-Pb ages that place robust constraints on the presubduction tectonic, magmatic, and paleogeographic evolution of the CB. Zircon U-Pb ages of crystalline CB are ~306-330 Ma, demonstrating local plutonism associated with regional voluminous, protracted Carboniferous magmatism related to Paleo-Tethys subduction. The plutons intruded the CB metasedimentary host-rock sequence, characterized by distinct Gondwanan DZ U-Pb provenance, Neoproterozoic to early Paleozoic maximum depositional ages, and synmagmatic, contact metamorphic zircon rims (~300-330 Ma). DZ U-Pb dating revealed postmagmatic Permian metasedimentary rocks (~270-295 Ma) that unconformably overlie the CB and have unimodal DZ spectra that indicate exhumation of the CB prior to Permian deposition within extensional basins, as well as mark the onset of CBU deposition prior to formation of the Pindos rift domain. These U-Pb results clarify the late Paleozoic-early Mesozoic evolution of the CB as a peri-Gondwanan terrane composed of Neoproterozoic and early Paleozoic metasedimentary rocks, intruded by voluminous Carboniferous arc magmatism, and exhumed in the Permian, prior to Triassic rifting and CBU deposition. Additionally, these data provide a chronostratigraphic framework and illuminate subduction-related juxtaposition within the CB metasedimentary sequence. ©2019. American Geophysical Union. All Rights Reserved

Miocene core complex development and coeval supradetachment basin evolution of Paros, Greece, insights from (U-Th)/He thermochronometry

The Aegean region of Greece hosts a series of crustal-scale extensional detachment systems that have accommodated the southward retreating Hellenic subduction zone. Extension has overprinted and dissected the Alpine nappe pile and locally exhumed Cordilleran-type metamorphic core complexes. On the island of Paros, a low-angle extensional detachment fault separates metamorphic footwall rocks from an unmetamorphosed sedimentary succession of the hanging wall. Basement orthogneisses were extensionally sheared in the footwall of the detachment until after 16. Ma (zircon U-Pb age of a slightly deformed granite), but pervasive ductile deformation had ceased by 7. Ma (zircon U-Pb age of an undeformed rhyolite dike that intrudes gneisses). Apatite and zircon (U-Th)/He ages from the gneisses confirm a period of cooling at rates >. 100. °C/Ma from 16 to 7. Ma. In the upper-plate, the basal sedimentary unit yields reset detrital apatite (U-Th)/He (DAHe) ages from 17 to 7. Ma and detrital zircon (U-Th)/He (DZHe) ages ranging from 270 to 18. Ma. DAHe ages from the stratigraphically higher fanglomerate units are reset to 10-7. Ma. The DZHe data have a primary thermal signature of 12-7. Ma, but preserve ages up to 113. Ma. The uppermost conglomerates exhibit completely reset DAHe ages of 15-9. Ma and reset DZHe ages from 10 to 8. Ma, with DZHe ages up to 104. Ma. Reset DAHe ages indicate late exposure of the footwall and constrain the depositional age of most sedimentary rocks on Paros to be from 14 to 7. Ma. Unreset DZHe ages preserve thermal signatures from the major Mesozoic-Tertiary tectonic events in the Aegean Region: [1] Cretaceous Pelagonian-type metamorphism; [2] Eocene peak HP metamorphism; and [3] Miocene Barrovian overprinting. Preservation of these signatures indicates long-term upper-plate recycling prior to syn-extensional deposition. The Paros supradetachment basin represents a classic inverted unroofing sequence deposited during progressive core complex exhumation in the Middle to Late Miocene. © 2012 Elsevier B.V

Zircon U-Pb Chronostratigraphy and Provenance of the Cycladic Blueschist Unit and the Nature of the Contact With the Cycladic Basement on Sikinos and Ios Islands, Greece

Sikinos and Ios Islands, located in the Southern Cyclades, represent part of a Cenozoic metamorphic core complex system that exposes subduction-related metamorphic rocks in the highly extended back-arc region of the Hellenic subduction zone. These exhumed HP-LT metamorphic units are composed of Mesozoic metasedimentary rocks of the Cycladic Blueschist Unit (CBU) and the Paleozoic Cycladic Basement (CB). The magmatic and stratigraphic evolution of these units, as well as the nature of the contact between the CBU and CB, have remained poorly understood. We used zircon U-Pb dating to determine crystallization ages of the CB on Sikinos and the maximum deposition ages and detrital provenance of the metasedimentary units to reconstruct the Mesozoic to early Cenozoic stratigraphic and tectonic evolution of the CBU on both islands. The results reveal that the CB in Sikinos is composed of Cambrian-Silurian metasedimentary rocks intruded by Carboniferous granites and is overlain by metasedimentary rocks of the CBU with depositional ages spanning from Permo-Triassic to Late Cretaceous. The provenance data from the CBU records a long-lived tectonic evolution from Paleo-Tethys subduction and rifting, to passive margin formation, and to subduction of the Neo-Tethyan Pindos basin. The continuous stratigraphic record and provenance evolution from the CB into the CBU imply a para-autochthonous relationship. On NE Sikinos and Ios, stratigraphic constraints suggest older-over-younger relationships along cryptic-thrusts, supporting premetamorphic or synmetamorphic structural repetition of the CBU by imbrication, likely during subduction underplating. ©2019. American Geophysical Union. All Rights Reserved

Structural and Thermal Evolution of an Infant Subduction Shear Zone: Insights From Sub-Ophiolite Metamorphic Rocks Recovered From Oman Drilling Project Site BT-1B

Subduction interface thermal structure changes drastically within the first few million years of underthrusting (i.e., subduction infancy). Metamorphic soles beneath ophiolites record snapshots of dynamic conditions and mechanical coupling during subduction infancy. Beneath the Samail Ophiolite (Oman), the sole comprises structurally higher high-temperature (HT) and lower low-temperature (LT) units. This inverted metamorphic gradient has been attributed to evolving metamorphic Pressure-Temperature (P-T) conditions during infancy; however, peak P-T and timing of LT sole subduction are poorly constrained. Oman Drilling Project core BT-1B sampled the base of the ophiolite in a location lacking the HT sole. Metasedimentary and meta-mafic samples collected from 104 m of core reveal that the LT sole subducted to similar peak P as HT rocks preserved elsewhere in Oman, but experienced ∼300°C lower peak T. Prograde fabrics record Si-in-phengite and amphibole chemistries consistent with peak P-T of ∼7–10 kbar and ∼450–550°C in the epidote-amphibolite facies. Retrograde fabrics record a transition from near-pervasive ductile to localized brittle strain under greenschist facies conditions. Titanite U-Pb ages (n = 2) constrain timing of peak LT sole subduction to ∼91 Ma (post-dating initial HT sole subduction by ∼12–13 Myr) and dynamic retrogression through ∼90 Ma. Combined with existing geo/thermo-chronology, our results support a model of protracted subduction and accretion while the infant subduction zone experienced multi-phase, slow-fast-slow cooling. Temporal overlap of HT sole cooling (rehydration?) and ophiolite formation suggests that cooling may lead to interface weakening, facilitating upper-plate extension and spreading. The LT sole formed in a rapidly-refrigerating forearc after ophiolite formation and may reflect the transition to self-sustaining subduction. © 2021. The Authors.Open access articleThis item from the UA Faculty Publications collection is made available by the University of Arizona with support from the University of Arizona Libraries. If you have questions, please contact us at [email protected]

Reconstructing the southern Pelagonian domain in the Aegean Sea: Insights from U-Pb detrital zircon analysis, lithostratigraphic and structural study, and zircon (U-Th)/He thermochronology on Amorgos Island (SE Cyclades, Greece)

In the Attic-Cycladic Crystalline Complex (Aegean Sea, Greece), following early syn-convergence exhumation processes, the extensional tectonics resulted in the extreme denudation of the upper plate, preserved today in a few places. The Cycladic Blueschist Unit (CBU) and the underlying Cycladic Basement and the Basal Units have been exhumed in the footwall of detachment systems, juxtaposed against the Pelagonian-derived hanging wall fragments. Traditional views accept that Amorgos Unit occupies a structurally lower position, correlated to the Basal Unit. This study presents lithostratigraphic and structural data, integrated with detrital zircon MDA, provenance analyses, and (U-Th)/He ages, showing a new investigation of the paleogeographic and structural position of Amorgos in the Cycladic archipelago. The low-grade metamorphosed Amorgos Unit presents a tripartite lithostratigraphy: a basal metaconglomerate, a middle carbonate sequence with shallow-water to pelagic facies, and an Eocene (meta)flysch. An HP/LT metabasite huge block is incorporated within the metaconglomerate either by gravitational movements as an isolated olistolith or tectonic processes as a slice during the early deformation stage. Detrital zircon U-Pb analysis on the meta-siliciclastic deposits revealed that the basal metaconglomerate shows a dominant Ediacaran input with Mid-Permian and Precambrian MDAs for the matrix and the high-grade clasts, respectively. The metaflysch shows Paleozoic affinity (e.g., mainly Pelagonian-sourced) and Triassic-Jurassic MDAs. The deformation history of Amorgos includes an early-stage top-to-NW thrusting in the retro-wedge setting of the Late Eocene–Oligocene subduction zone and a late-stage top-to-SE extensional low- and high-angle normal faulting. Zircon (U-Th)/He ages reveal an Early–Mid Miocene (18–14 Ma) exhumation below ∼ 200 °C. Similarities on lithostratigraphy, structural inventory, exhumation history, and structural position of Amorgos Unit with the Santorini Detachment System Pelagonian hanging wall imply their close spatial relationship. From all the above, we propose that Amorgos belongs to the upper plate of the CBU and is paleogeographically located at the southern Pelagonian margin. © 2022 International Association for Gondwana Researc

Insights from elastic thermobarometry into exhumation of high-pressure metamorphic rocks from Syros, Greece

Retrograde metamorphic rocks provide key insights into the pressureerature (P-T) evolution of exhumed material, and resultant P-T constraints have direct implications for the mechanical and thermal conditions of subduction interfaces. However, constraining P-T conditions of retrograde metamorphic rocks has historically been challenging and has resulted in debate about the conditions experienced by these rocks. In this work, we combine elastic thermobarometry with oxygen isotope thermometry to quantify the P-T evolution of retrograde metamorphic rocks of the Cycladic Blueschist Unit (CBU), an exhumed subduction complex exposed on Syros, Greece. We employ quartz-in-garnet and quartz-in-epidote barometry to constrain pressures of garnet and epidote growth near peak subduction conditions and during exhumation, respectively. Oxygen isotope thermometry of quartz and calcite within boudin necks was used to estimate temperatures during exhumation and to refine pressure estimates. Three distinct pressure groups are related to different metamorphic events and fabrics: high-pressure garnet growth at ∼1.4-1.7 GPa between 500-550 ° C, retrograde epidote growth at ∼1.3-1.5 GPa between 400-500 ° C, and a second stage of retrograde epidote growth at ∼1.0 GPa and 400 ° C. These results are consistent with different stages of deformation inferred from field and microstructural observations, recording prograde subduction to blueschist-eclogite facies and subsequent retrogression under blueschist-greenschist facies conditions. Our new results indicate that the CBU experienced cooling during decompression after reaching maximum high-pressure-lowerature conditions. These P-T conditions and structural observations are consistent with exhumation and cooling within the subduction channel in proximity to the refrigerating subducting plate, prior to Miocene core-complex formation. This study also illustrates the potential of using elastic thermobarometry in combination with structural and microstructural constraints, to better understand the P-T-deformation conditions of retrograde mineral growth in high-pressure-lowerature (HP/LT) metamorphic terranes. © 2021 Miguel Cisneros et al