More Like this

1. Geochemical and geochronological records of tectonic changes along a flat-slab arc-transform junction: Circa 30 Ma to ca. 19 Ma Sonya Creek volcanic field, Wrangell Arc, Alaska

   https://doi.org/10.1130/GES02114.1  

   Berkelhammer, Samuel E. ; Brueseke, Matthew E. ; Benowitz, Jeffrey A. ; Trop, Jeffrey M. ; Davis, Kailyn ; Layer, Paul W. ; Weber, Maridee ( September 2019 , Geosphere)

   The Sonya Creek volcanic field (SCVF) contains the oldest in situ volcanic products in the ca. 30 Ma–modern Wrangell Arc (WA) in south-central Alaska, which commenced due to Yakutat microplate subduction initiation. The WA occurs within a transition zone between Aleutian subduction to the west and dextral strike-slip tectonics along the Queen Charlotte–Fairweather and Denali–Duke River fault systems to the east. New 40Ar/39Ar geochronology of bedrock shows that SCVF magmatism occurred from ca. 30–19 Ma. New field mapping, physical volcanology, and major- and trace-element geochemistry, coupled with the 40Ar/39Ar ages and prior reconnaissance work, allows for the reconstruction of SCVF magmatic evolution. Initial SCVF magmatism that commenced at ca. 30 Ma records hydrous, subduction-related, calc-alkaline magmatism and also an adakite-like component that we interpret to represent slab-edge melting of the Yakutat slab. A minor westward shift of volcanism within the SCVF at ca. 25 Ma was accompanied by continued subduction-related magmatism without the adakite-like component (i.e., mantle-wedge melting), represented by ca. 25–20 Ma basaltic-andesite to dacite domes and associated diorites. These eruptions were coeval with another westward shift to anhydrous, transitional-tholeiitic, basaltic-andesite to rhyolite lavas and tuffs of the ca. 23–19 Ma Sonya Creek shield volcano; we attribute these eruptionsmore »to intra-arc extension. SCVF activity was also marked by a small southward shift in volcanism at ca. 21 Ma, characterized by hydrous calc-alkaline lavas. SCVF geochemical compositions closely overlap those from the <13 Ma WA, and no alkaline lavas that characterize the ca. 18–10 Ma eastern Wrangell volcanic belt exposed in Yukon Territory are observed. Calc-alkaline, transitional-tholeiitic, and adakite-like SCVF volcanism from ca. 30–19 Ma reflects subduction of oceanic lithosphere of the Yakutat microplate beneath North America. We suggest that the increase in magmatic flux and adakitic eruptions at ca. 25 Ma, align with a recently documented change in Pacific plate direction and velocity at this time and regional deformation events in southern Alaska. By ca. 18 Ma, SCVF activity ceased, and the locus of WA magmatism shifted to the south and east. The change in relative plate motions would be expected to transfer stress to strike-slip faults above the inboard margin of the subducting Yakutat slab, a scenario consistent with increased transtensional-related melting recorded by the ca. 23–19 Ma transitional-tholeiitic Sonya Creek shield volcano between the Denali and Totschunda faults. Moreover, we infer the Totschunda fault accommodated more than ~85 km of horizontal offset since ca. 18 Ma, based on reconstructing the initial alignment of the early WA (i.e., 30–18 Ma SCVF) and temporally and chemically similar intrusions that crop out to the west on the opposite side of the Totschunda fault. Our results from the SCVF quantify spatial-temporal changes in deformation and magmatism that may typify arc-transform junctions over similar time scales (>10 m.y.).« less
2. Cretaceous to Miocene magmatism, sedimentation, and exhumation within the Alaska Range suture zone: A polyphase reactivated terrane boundary

   https://doi.org/10.1130/GES02014.1  

   Trop, J. ; Benowitz, J. ( June 2019 , Geosphere)

   The Alaska Range suture zone exposes Cretaceous to Quaternary marine and nonmarine sedimentary and volcanic rocks sandwiched between oceanic rocks of the accreted Wrangellia composite terrane to the south and older continental terranes to the north. New U-Pb zircon ages, 40Ar/39Ar, ZHe, and AFT cooling ages, geochemical compositions, and geological field observations from these rocks provide improved constraints on the timing of Cretaceous to Miocene magmatism, sedimentation, and deformation within the collisional suture zone. Our results bear on the unclear displacement history of the seismically active Denali fault, which bisects the suture zone. Newly identified tuffs north of the Denali fault in sedimentary strata of the Cantwell Formation yield ca. 72 to ca. 68 Ma U-Pb zircon ages. Lavas sampled south of the Denali fault yield ca. 69 Ma 40Ar/39Ar ages and geochemical compositions typical of arc assemblages, ranging from basalt-andesite-trachyte, relatively high-K, and high concentrations of incompatible elements attributed to slab contribution (e.g., high Cs, Ba, and Th). The Late Cretaceous lavas and bentonites, together with regionally extensive coeval calc-alkaline plutons, record arc magmatism during contractional deformation and metamorphism within the suture zone. Latest Cretaceous volcanic and sedimentary strata are locally overlain by Eocene Teklanika Formation volcanic rocks withmore »geochemical compositions transitional between arc and intraplate affinity. New detrital-zircon data from the modern Teklanika River indicate peak Teklanika volcanism at ca. 57 Ma, which is also reflected in zircon Pb loss in Cantwell Formation bentonites. Teklanika Formation volcanism may reflect hypothesized slab break-off and a Paleocene–Eocene period of a transform margin configuration. Mafic dike swarms were emplaced along the Denali fault from ca. 38 to ca. 25 Ma based on new 40Ar/39Ar ages. Diking along the Denali fault may have been localized by strike-slip extension following a change in direction of the subducting oceanic plate beneath southern Alaska from N-NE to NW at ca. 46–40 Ma. Diking represents the last recorded episode of significant magmatism in the central and eastern Alaska Range, including along the Denali fault. Two tectonic models may explain emplacement of more primitive and less extensive Eocene–Oligocene magmas: delamination of the Late Cretaceous–Paleocene arc root and/or thickened suture zone lithosphere, or a slab window created during possible Paleocene slab break-off. Fluvial strata exposed just south of the Denali fault in the central Alaska Range record synorogenic sedimentation coeval with diking and inferred strike-slip displacement. Deposition occurred ca. 29 Ma based on palynomorphs and the youngest detrital zircons. U-Pb detrital-zircon geochronology and clast compositional data indicate the fluvial strata were derived from sedimentary and igneous bedrock presently exposed within the Alaska Range, including Cretaceous sources presently exposed on the opposite (north) side of the fault. The provenance data may indicate ~150 km or more of dextral offset of the ca. 29 Ma strata from inferred sediment sources, but different amounts of slip are feasible. Together, the dike swarms and fluvial strata are interpreted to record Oligocene strike-slip movement along the Denali fault system, coeval with strike-slip basin development along other segments of the fault. Diking and sedimentation occurred just prior to the onset of rapid and persistent exhumation ca. 25 Ma across the Alaska Range. This phase of reactivation of the suture zone is interpreted to reflect the translation along and convergence of southern Alaska across the Denali fault driven by highly coupled flat-slab subduction of the Yakutat microplate, which continues to accrete to the southern margin of Alaska. Furthermore, a change in Pacific plate direction and velocity at ca. 25 Ma created a more convergent regime along the apex of the Denali fault curve, likely contributing to the shutting off of near-fault extension- facilitated arc magmatism along this section of the fault system and increased exhumation rates.« less
3. Cenozoic tectono-thermal history of the southern Talkeetna Mountains, Alaska: Insights into a potentially alternating convergent and transform plate margin

   https://doi.org/doi.org/10.1130/GES02008.1  

   Terhune, P. ; Benowitz, J. ; O'Sullivan, P. ; Gillis, R. ; Freymuller, J. ( January 2019 , Geosphere)

   The Mesozoic–Cenozoic convergent margin history of southern Alaska has been dominated by arc magmatism, terrane accretion, strike-slip fault systems, and possible spreading-ridge subduction. We apply 40Ar/39Ar, apatite fission-track (AFT), and apatite (U-Th)/He (AHe) geochronology and thermochronology to plutonic and volcanic rocks in the southern Talkeetna Mountains of Alaska to document regional magmatism, rock cooling, and inferred exhumation patterns as proxies for the region’s deformation history and to better delineate the overall tectonic history of southern Alaska. High-temperature 40Ar/39Ar thermochronology on muscovite, biotite, and K-feldspar from Jurassic granitoids indicates postemplacement (ca. 158–125 Ma) cooling and Paleocene (ca. 61 Ma) thermal resetting. 40Ar/39Ar whole-rock volcanic ages and 45 AFT cooling ages in the southern Talkeetna Mountains are predominantly Paleocene–Eocene, suggesting that the mountain range has a component of paleotopography that formed during an earlier tectonic setting. Miocene AHe cooling ages within ~10 km of the Castle Mountain fault suggest ~2–3 km of vertical displacement and that the Castle Mountain fault also contributed to topographic development in the Talkeetna Mountains, likely in response to the flat-slab subduction of the Yakutat microplate. Paleocene–Eocene volcanic and exhumation-related cooling ages across southern Alaska north of the Border Ranges fault system are similar and show no S-Nmore »or W-E progressions, suggesting a broadly synchronous and widespread volcanic and exhumation event that conflicts with the proposed diachronous subduction of an active west-east–sweeping spreading ridge beneath south-central Alaska. To reconcile this, we propose a new model for the Cenozoic tectonic evolution of southern Alaska. We infer that subparallel to the trench slab breakoff initiated at ca. 60 Ma and led to exhumation, and rock cooling synchronously across south-central Alaska, played a primary role in the development of the southern Talkeetna Mountains, and was potentially followed by a period of southern Alaska transform margin tectonics.« less
4. Evidence for regionally continuous Early Cretaceous sinistral shear zones along the western flank of the Coast Mountains, coastal British Columbia, Canada

   https://doi.org/10.1130/GES02502.1  

   Wang, Jordan W. ; Gehrels, George ; Kapp, Paul ; Sundell, Kurt ( December 2022 , Geosphere)

   Abstract The plate-boundary conditions of the Mesozoic North American Cordillera remain poorly constrained, but most studies support large (>800 km) southward motion of the Insular and Intermontane superterranes during Jurassic–Cretaceous time. An implicit feature in these models of large coastwise displacements is the presence of one or more continentalscale sinistral strike-slip faults that could have dismembered and displaced terrane fragments southward along the western margin of North America prior to the onset of mid-Cretaceous shortening and dextral strike-slip faulting. In this study, we documented a system of sinistral intra-arc shear zones within the Insular superterrane that may have accommodated large southward motion. Employment of a new large-n igneous zircon U-Pb method more than doubled the precision of measurements obtained by laser ablation–inductively coupled plasma–mass spectrometry (from ~1% to 0.5%) and allowed us to demonstrate the close temporal-spatial relationship between magmatism and deformation by dating comagmatic crosscutting phases. Crystallization ages of pre-, syn-, and postkinematic intrusions show that the intra-arc shear zones record an Early Cretaceous phase of sinistral oblique convergence that terminated between 107 and 101 Ma. Shear zone cessation coincided with: (1) collapse of the Gravina basin, (2) development of a single voluminous arc that stitched the Insular andmore »Intermontane superterranes together, and (3) initiation of eastwest contractional deformation throughout the Coast Mountains. We interpret these concurrent tectono-magmatic events to mark a shift in plate kinematics from a sinistral-oblique system involving separate terranes and intervening ocean basins to a strongly convergent two-plate margin involving a single oceanic plate and the newly assembled western margin of North America.« less
5. Raising the Resurrection plate from an unfolded-slab plate tectonic reconstruction of northwestern North America since early Cenozoic time

   https://doi.org/10.1130/B35677.1  

   Fuston, Spencer ; Wu, Jonny ( October 2020 , GSA Bulletin)

   The configuration of mid-ocean ridges subducted below North America prior to Oligocene time is unconstrained by seafloor isochrons and has been primarily inferred from upper-plate geology, including near-trench magmatism. However, many tectonic models are permitted from these constraints. We present a fully kinematic, plate tectonic reconstruction of the NW Cordillera since 60 Ma built by structurally unfolding subducted slabs, imaged by mantle tomography, back to Earth’s surface. We map in three-dimensions the attached Alaska and Cascadia slabs, and a detached slab below western Yukon (Canada) at 400−600 km depth that we call the “Yukon Slab.” Our restoration of these lower plates within a global plate model indicates the Alaska slab accounts for Pacific-Kula subduction since ca. 60 Ma below the Aleutian Islands whereas the Cascadia slab accounts for Farallon subduction since at least ca. 75 Ma below southern California, USA. However, intermediate areas show two reconstruction gaps that persist until 40 Ma. We show that these reconstruction gaps correlate spatiotemporally to published NW Cordillera near-trench magmatism, even considering possible terrane translation. We attribute these gaps to thermal erosion related to ridge subduction and model mid-ocean ridges within these reconstruction gap mid-points. Our reconstructions show two coeval ridge-trench intersections that boundmore »an additional “Resurrection”-like plate along the NW Cordillera prior to 40 Ma. In this model, the Yukon slab represents a thermally eroded remnant of the Resurrection plate. Our reconstructions support a “northern option” Farallon ridge geometry and allow up to ∼1200 km Chugach terrane translation since Paleocene time, providing a new “tomographic piercing point” for the Baja-British Columbia debate.« less