Researchers reveal new rock formation in Colorado

(Press-News.org) Boulder, Colo., USA - An astonishing new rock formation has been revealed in the Colorado Rockies, and it exists in a deeply perplexing relationship with older rocks. Named the Tava sandstone, this sedimentary rock forms intrusions within the ancient granites and gneisses that form the backbone of the Front Range. The relationship is fascinating because it is backward: ordinarily, it is igneous rocks such as granite that would that intrude into sedimentary rocks.

According to authors Christine Smith Siddoway and George E. Gehrels, to find sandstone injected into granite is utterly uncommon -- the extensive system that is found in Colorado may be unique in the world. There is evidence that the process of formation involved very large earthquakes, or possibly another type of catastrophic event, causing liquefaction of sediment, what they call "'natural fracking' in a certain sense!" Equally astonishing is the time of formation of the Tava sandstone, determined from detrital zircon analysis: the Tava proves to be from a time period ~750 million years ago, which was not known to be represented in the Colorado Rockies: the Cryogenian Period.

FEATURED ARTICLE Basement-hosted sandstone injectites of Colorado: A vestige of the Neoproterozoic revealed through detrital zircon provenance analysis Christine Smith Siddoway, Dept. of Geology, The Colorado College, Colorado Springs, Colorado 80903, USA; and George E. Gehrels, Dept. of Geosciences, University of Arizona, Tucson, Arizona 85712, USA. Published online ahead of print on 22 Aug. 2014; http://dx.doi.org/10.1130/L369.1.

Other recently posted LITHOSPHERE articles are listed below.

Abstracts are online at http://lithosphere.gsapubs.org/content/early/recent. Representatives of the media may obtain complimentary copies of LITHOSPHERE articles by contacting Kea Giles at the address above.

Please discuss articles of interest with the authors before publishing stories on their work, and please make reference to LITHOSPHERE in articles published. Contact Kea Giles for additional information or assistance. Non-media requests for articles may be directed to GSA Sales and Service, gsaservice@geosociety.org.

Metamorphism and geochronology of the exhumed Himalayan mid-crust Likhu Khola region, east-central Nepal: Recognition of a tectonometamorphic discontinuity Richard From, Dept. of Geological Sciences, University of Saskatchewan, 114 Science Place, Saskatoon, SK, S7N 5E2, Canada, Present Address: Dept. of Geological Sciences, University of Manitoba, 125 Dysart Road, Winnipeg, MB, R3T 2N2, Canada. Published online ahead of print on 27 Aug. 2014; http://dx.doi.org/10.1130/L365.1.

This study outlines the first temperature, pressure, and time constraints for rocks in the Likhu Khola region of east central Nepal. Monazite dates outline a history of metamorphism and partial melting spanning ca. 27 to 15 Ma. An abrupt break in metamorphic pressure estimates helps define a cryptic tectonometamorphic discontinuity within the exhumed Himalayan mid-crust that is interpreted to separate rocks above and below that record different tectonometamorphic histories. This discontinuity, which can be traced laterally into the adjacent valley system to the west, is one of a growing number of such structures recently identified along the orogen. The widespread recognition of these structures indicates that not only are they regionally extensive, but also that they played a significant role in the evolution of the Himalayan orogen.

Detrital zircons from Cretaceous midcontinent strata reveal an Appalachian Mountains-Cordilleran foreland basin connection Emily S. Finzel, Earth and Environmental Sciences Dept., University of Iowa, Iowa City, Iowa 52242, USA. Published online ahead of print on 27 Aug. 2014; http://dx.doi.org/10.1130/L367.1.

In this manuscript, Emily S. Finzel presents new detrital zircon U-Pb geochronologic data from Cretaceous strata exposed in western Iowa and eastern Nebraska provide evidence for westward-flowing river systems that stretched >2000 km from the Appalachian highlands to the western United States approximately 100 million years ago. Widespread deposition of the Dakota Formation in the midcontinent is interpreted in this study as the sedimentary record of a mantle plume beneath the Appalachians that drove uplift and erosion of the inactive mountain belt.

Petrology and geochemistry of the Valle de Santiago lower-crust xenoliths: Young tectonothermal processes beneath the central Trans-Mexican volcanic belt Fernando Ortega-Gutiérrez et al., Instituto de Geología, Universidad Nacional Autónoma de México, Avenida Universidad 3000, México D.F., 04510, México. Published online ahead of print on 27 Aug. 2014; http://dx.doi.org/10.1130/L317.1.

The Quaternary Valle de Santiago volcanic field represents the only known place along the Trans-Mexican Volcanic Belt where abundant and large granulite-facies xenoliths from the lower continental crust have been discovered. Combined petrologic and geochemical arguments suggest that granulite facies metamorphism is coeval and genetically related to juvenile alkaline basaltic magmatism in the region, and reveal the existence of a very high-temperature gabbroic plumbing system at nearly 35 km depth. Scarce charnockitic xenoliths from the wall or roof rocks of the granulitized gabbros, lifted together by the carrier alkaline basalts, yielded latest Cretaceous to Paleocene U-Pb zircon ages, probably indicating the absence of pre-Mesozoic continental crust beneath the western sector of the Mexican arc.

Local isobaric heating above an extensional detachment in the middle crust of a Variscan allochthonous terrane (Órdenes Complex, NW Spain) Antonio M. Álvarez-Valero et al., Departamento de Geología, Universidad de Salamanca, 37008 Salamanca, Spain. Published online ahead of print on 2 Sept. 2014; http://dx.doi.org/10.1130/L369.1.

Rocks in nature coming from high depths within the crust are exceptional witnesses of what processes are occurring in the Earth's interior. Antonio M. Álvarez-Valero and colleagues present a special example of high temperature rocks (that even underwent partial melting), at the NW Iberian Variscan belt (NW Spain), where many new theories have been advanced for the first time. These rocks from the deep crust within a particular detachment structure describe during their ascent a picture in the Earth's surface that needs an interdisciplinary study in detail of rocks and structures. Thereby, this paper focuses in the information given by the detachment rocks and how they transferred heat from the bottom to the top materials surrounding them.

Multisystem dating of modern river detritus from Tajikistan and China: Implications for crustal evolution and exhumation of the Pamir Barbara Carrapa et al., Dept. of Geosciences, University of Arizona, Tucson, Arizona 85721, USA. Published online ahead of print on 22 Aug. 2014; http://dx.doi.org/10.1130/L360.1.

This paper by Barbara Carrapa and colleagues presents the first orogen-scale geochemical dataset for the Pamir able to constrain the crustal evolution and exhumation history of this part of the Himalaya-Tibet-Pamir orogenic system. We present new zircon U-Pb and Hf determinations, apatite fission track and white mica 40Ar/39Ar thermochronological ages from sand grains collected from rivers that drain roughly 90% of the Pamir. U-Pb ages are able to test existing terrane correlation; our new thermochronological ages show that the Pamir Mountains have experienced widespread deep Miocene erosion and that this erosion has preferentially attacked the western flank of the range, which is exposed to relatively intense precipitation from westerly sources. We suggest that uplift and orographic barrier in the Miocene has facilitated exhumation of the Pamir domes. Lithospheric scale processes such as southward rollback of India under Asia can explain the observed trends of southward younging ages.

Pre- to post-Cordilleran transposition history of Joss Mountain: Insights into the exhumation of the Shuswap complex, southeastern Canadian Cordillera Davide Zanoni et al., Dipartimento di Scienze della Terra "A. Desio," Università degli Studi di Milano, Via Mangiagalli 34, 20133 Milano, Italy. Published online ahead of print on 2 Sept. 2014; http://dx.doi.org/10.1130/L346.1.

This paper by Davide Zanoni and colleagues presents the first multiscale structural analysis and P-T-d-t evolution history of the polydeformed rocks of the Joss Mountain domain of the Shuswap complex in the southeastern Canadian Cordillera (British Columbia). We applied an analytical method, commonly used in the European Alps to define tectono-metamorphic units, based on the integration of multiscale structural analysis and mineral chemical and radiometric age data. This work contributes to the knowledge of the intra-lithospheric trajectories of tectonic units within the North American Cordillera and gives new insights into the pre-Cordilleran and syn-Cordilleran paleo-tectonic setting of this part of the Shuswap complex.

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