Boulder, Colo., USA - The August issue of Lithosphere presents papers that provide insights into Tyrrhenian margin neotectonics in Italy; the Wrangellia composite terrane in Canada; fault-related fissures on Gower Peninsula, Wales; blueschist facies rocks and serpentinites from Kochi in Shikoku Island, Japan; the Shuswap metamorphic complex in southern British Columbia; and paleo-Pacific plate subduction along northeastern China. Highlights are below.
Open-access abstracts for LITHOSPHERE papers are online at http://lithosphere.
Tectonics, hydrothermalism, and paleoclimate recorded by Quaternary travertines and their spatio-temporal distribution in the Albegna basin, central Italy: Insights on Tyrrhenian margin neotectonics
Gianluca Vignaroli et al., Consiglio Nazionale Delle Ricerche (CNR), Istituto di Geologia Ambientale e Geoingegneria, Area della Ricerca di Roma 1, Via Salaria KM 29, 300-00015, Monterotondo Stazione, Rome, Italy. This paper is online at http://lithosphere.
This work deals with thermogene travertine deposition within the Plio-Quaternary geothermal-extensional province of Tuscany, Tyrrhenian margin, central Italy. This work provides original data coming from a multidisciplinary and multiscale approach, including geological-structural investigations, geochronological analyses (230Th/234U, Uranium-series disequilibrium), and stable isotope (d13C and d18O) systematics. This work is innovative as, for the first time, the spatio-temporal distribution of travertine deposits and other CaCO3 mineralization as well as their morphological, geological, structural, and geochemical attributes are used to reconstruct the tectonic-hydrothermal evolution in a Quaternary basin over a distance of about 30 km and a time-span of 300-400 ka. The same work is timely and of interest to a broad audience inasmuch it provides general implications on the relationships and feedbacks between travertine deposition, hydrothermalism, paleoclimate oscillations, and neotectonics within a region of positive geothermal anomaly all through the Quaternary time.
Upper-crustal cooling of the Wrangellia composite terrane in the northern St. Elias Mountains, western Canada
Sarah Falkowski, 1 University of Tübingen, Department of Geosciences, 72074 Tübingen, Germany; and Eva Enkelmann, University of Cincinnati, Department of Geology, Cincinnati, Ohio 45221-0037, USA. This paper is online at http://lithosphere.
This study presents the long-term exhumation history of the Wrangellia composite terrane of the remote and ice-covered northern St. Elias Mountains in southwest Yukon, northwest British Columbia, and adjacent Alaska. Detrital zircon and apatite fission-track age distributions are presented from 21 glacial catchments. The detrital sampling approach allows for a large spatial coverage (~30,000 km2) and access to material eroded beneath the ice. An additional five bedrock samples were dated by zircon fission-track analysis for a comparison with detrital results. Our new thermochronology data record the Late Jurassic-mid-Cretaceous accretion of the Wrangellia composite terrane to the former North American margin and magmatism, which reset the older thermal record. The good preservation of the Jurassic-Cretaceous record suggests that Cenozoic erosion must have been limited overall. Nonetheless, Eocene spreading-ridge subduction and Oligocene-Neogene cooling in response to the ongoing Yakutat flat-slab subduction are evident in the study area despite its inboard position from the active plate boundary. The results further indicate an area of rapid exhumation at the northern end of the Fairweather fault ca. 10 to 5 Ma; this area is bounded by discrete, unmapped structures. The area of rapid exhumation shifted southwest toward the plate boundary and the center of the St. Elias syntaxis after 5 Ma. Integrating the new data with published detrital thermochronology from the southern St. Elias Mountains reveals an evolving concentration of deformation and exhumation, possibly within a large-scale, transpressional structure providing important constraints for geodynamic models of syntaxes.
Record of paleofluid circulation in faults revealed by hematite (U-Th)/He and apatite fission-track dating: An example from Gower Peninsula fault fissures, Wales
Alexis K. Ault et al., Department of Geology, Utah State University, 4505 Old Main Hill, Logan, Utah 84322, USA. This article is online at http://lithosphere.
Fault zones transfer hydrothermal fluids and thus heat in the upper crust. Secondary minerals precipitating from fluids and entrained in faults provide a record of these processes. Low temperature thermochronometry specifically targeting fault materials constrains the timing, temperature, and significance of this flow. We demonstrate this with hematite (U-Th)/He dating integrated with detrital sandstone apatite fission-track and apatite and zircon (U-Th)/He thermochronometry from fault-related fissures on Gower Peninsula, Wales, that initially formed in the Carboniferous. When compared with the reconstructed long-term surface history, our results reveal Early Cretaceous paleofluid flow in these faults contemporaneous with the opening of the North Atlantic Ocean. These observations are significant because they reveal that faults serve as conduits for fluid and heat, even long after they initially form, thereby impacting local and regional geothermal gradients through time.
Proto-Japan and tectonic erosion: Evidence from zircon geochronology of blueschist and serpentinite
Qiong-Yan Yang et al., School of Earth Sciences and Resources, China University of Geosciences Beijing, 29 Xueyuan Road, Beijing 100083, China. This article is online at http://lithosphere.
From the early Magma Ocean stage, the Earth has gone through a series of dynamic changes over its 4.6 billion years of history, principally controlled by global plate tectonic processes. Among these, the growth and destruction of continental crust is of prime importance in understanding the building of Earth as a habitable planet. By analyzing the chronology of zircons in blueschist facies rocks and serpentinites from Kochi in Shikoku Island, the authors demonstrate extensive "tectonic erosion" that destroyed a part of the accretionary belt in western Japan. This study provides important insights on continental destruction through tectonic processes at convergent plate margins.
Corrugated architecture of the Okanagan Valley shear zone and the Shuswap metamorphic complex, Canadian Cordillera
Sarah R. Brown et al., Department of Geological Sciences, California State University, Bakersfield, 9001 Stockdale Highway, Bakersfield, California 93311, USA. This article is online at http://lithosphere.
The mechanisms by which rocks from deep within the crust are exhumed to the Earth's surface remain poorly understood. The Shuswap complex in southern British Columbia is an unusually large domain of mid-crustal rocks that was exhumed by uplift along the margin of the Okanagan Valley shear zone. However, the magnitude of crustal extension has been vigorously debated because mid-crustal rocks are not exposed everywhere along the shear zone. Compilation and review of numerous geological maps reveals that the shear zone is warped by previously unrecognized, large-scale, east-west trending folds. These folds repeatedly juxtapose middle crust rocks against non-metamorphosed rocks across the entire Shuswap and explains why extensive areas do not expose the deeper rocks: they are buried beneath the shear zone. This explains how belts of upper crustal rocks can be traced across the Shuswap, offering potential stratigraphic correlations between otherwise separate tectonic belts in the Canadian Cordillera.
Early Jurassic monzogranite-tonalite association from the southern Zhangguangcai Range: Implications for paleo-Pacific plate subduction along northeastern China
Jiang-Feng Qin et al., State Key Laboratory of Continental Dynamics, Dept. of Geology, Northwest University, Xi'an 710069, China. This article is online at http://lithosphere.
When was the Paleo-Pacific plate subducted beneath the Eurasian continents? Early-Jurassic Na-rich monzogranite-tonalite from the NE China can provide the answer.