Boulder, CO, USA - This month's GEOSPHERE papers cover the geologic and neotectonic evolution of Savu Island, Indonesia; a digital archive of active tectonic maps for the Tibet-Himalaya region; syntectonic zoning in biotite as a diagnostic indicator of deformation style during metamorphism; the potential for explosive eruptions of silicic volcanoes; the Hortavaer intrusive complex, Norway; evidence of an low-oxygen deep sea at the former Jones Hill copper mine northwest of Pecos, New Mexico; and the SLIP hypothesis.
Transition from subduction to arc-continent collision: Geologic and neotectonic evolution of Savu Island, Indonesia
Ron Harris et al., Dept. of Geological Sciences, Brigham Young University, Provo, Utah 84602, USA. (Received NSF funding.)
The Savu region provides a rare glimpse of neotectonic associations of the transition from subduction to collision. Harris et al. document this transition, noting that the geology of the Savu region is separated into two very different domains divided by a developing suture zone between units of Australian and Asian affinity, and that complexities here result from lower plate structural and stratigraphic discontinuities that cause waves of crustal shortening and uplift followed by subsidence.
Active structures of the Himalayan-Tibetan orogen and their relationships to earthquake distribution, contemporary strain field, and Cenozoic volcanism
Michael Taylor, Dept. of Geology, University of Kansas, 1735 Jayhawk Blvd., Lawrence, Kansas 66045, USA; and An Yin. (Received NSF funding.)
A digital archive of active structures related to the Indo-Asian collision and its surrounding regions is now available for use by geoscientists. The digital active tectonic map allows a comparison to be made between the distribution, geometry, and kinematics of active structures and the distribution of earthquake focal mechanisms. The distribution of active structures is also compared with the GPS velocity field of Tibet and the Himalaya in order to assess the partitioning of the strain rate field across individual faults and folds. Evaluation of the Late Cenozoic volcanism in Tibet is also possible to determine whether syn-collisional volcanism was spatially related to active structures in the same area.
Growth, behavior, and textural sector zoning of biotite porphyroblasts during regional metamorphism and the implications for interpretation of inclusion trails: Insights from the Pequop Mountains and Wood Hills, Nevada, USA
Phyllis A. Camilleri, Geosciences, Austin Peay State University, Box 4418, Clarksville, Tennessee 37044, USA
Previous studies indicate that the growth of large biotite crystals (porphyroblasts) is a two-stage process in phyllite and schist during regional metamorphism. Stage 1 accommodates the bulk of the growth of a porphyroblast and proceeds by growth over matrix, which results in incorporation of inclusions. Stage 2 involves growth that is largely restricted to precipitation of biotite in fractures within and along the margins of the porphyroblast. On the basis of a detailed microstructural study of biotite porphyroblasts, Camilleri demonstrates that in a tectonic environment where rock deformation occurs by shortening perpendicular to foliation and extension parallel to foliation, early growth actually occurs through a combination of the processes in stage 1 and 2 and results in textural sector zoning of the inclusions. In two dimensions, textural sector zoning is characterized by an hourglass-shaped distribution of inclusions whereby the long axis of the hourglass is normal or at a high angle to foliation. Textural sector zoning has long been recognized in many other porphyroblast species such as chiastolite (andalusite), where it is defined by the characteristic cross-shaped distribution of inclusions. The zoning in other mineral species is thought to develop in porphyroblasts that grew before or between tectonic deformations (i.e., they are pretectonic or intertectonic, respectively). This paper shows that zoning in biotite is different because it is syntectonic. Furthermore, zoning in biotite is significant because it can be used as a diagnostic indicator of the style of deformation during metamorphism.
Determining relative magma and host rock xenolith rheology during magmatic fabric formation in plutons: Examples from the middle and upper crust
Aaron S. Yoshinobu et al., Dept. of Geosciences, Texas Tech University, Lubbock, Texas 79409-1053, USA. (Received NSF funding.)
This article utilizes foreign blocks of rock trapped in a magma chamber to evaluate the viscosity of magmas in subvolcanic settings. Such information may be useful in understanding the viscosity and thus potential for explosive eruptions of silicic volcanoes.
Growth and zoning of the Hortavaer intrusive complex, a layered alkaline pluton in the Norwegian Caledonides
Calvin G. Barnes et al., Dept. of Geosciences, Texas Tech University, Lubbock, Texas 79409, USA. (Received NSF funding.)
The Hortavaer intrusive complex in north-central Norway provides a remarkable example of the styles of growth of magma bodies in Earth's crust and of the ways in which magmas chemically and physically interact with host rocks rich in calcium carbonate (calcite). Magma emplacement involved injection of repeated pulses of hot, mantle-derived magmas into the middle crust, followed by differentiation to potassium-rich magmas. Ultimately, a layered intrusion was formed and subsequent deformation and erosion has exposed a paleo-vertical section through the complex. Magma interaction with calcite-rich rocks can be shown to cause the enrichment of potassium. This process also caused development of distinctive reaction zones (skarn rocks) and evidently released large amounts of carbon dioxide into overlying strata. Processes revealed in this study are similar to those proposed for active and dormant volcanic systems in Italy, such as Vesuvius and the Alban Hills.
Seafloor-hydrothermal Si-Fe-Mn exhalites in the Pecos greenstone belt, New Mexico, and the redox state of ca. 1720 Ma deep seawater
John F. Slack et al., U.S. Geological Survey, National Center, MS 954, Reston, Virginia 20192, USA
A study of the mineralogy and chemistry of Precambrian iron-rich and manganese-rich sedimentary rocks near the site of the former Jones Hill copper mine northwest of Pecos, New Mexico, reveals information on the oxidation state of deep seawater approximately 1.72 billion years ago. The results of this study indicate that the deep ocean at that time had very small amounts of dissolved oxygen, in contrast to a currently popular model in which Precambrian deep seawater of that age had no oxygen, like the deep waters of the modern Black Sea.
Climate forcing by iron fertilization from repeated ignimbrite eruptions: The icehouse-silicic large igneous province (SLIP) hypothesis
Steven M. Cather et al., New Mexico Bureau of Geology and Mineral Resources, New Mexico Institute of Mining and Technology, Socorro, New Mexico 87801, USA
The present-day glacial-interglacial climate on Earth was established about 34 million years ago during a prolonged episode of explosive silicic volcanism, the ignimbrite flare-up of southwestern North America. Our data suggest that fertilization of oceanic algae by great volumes of iron-bearing volcanic ash caused increased photosynthetic productivity, which decreased the CO2 content of the atmosphere and cooled the planet. Most cool-climate episodes during the past 0.5 billion years were accompanied by major explosive volcanism in silicic large igneous provinces (SLIPs), suggesting a common link between these phenomena.
Review abstracts for these articles at http://geosphere.gsapubs.org/.
Representatives of the media may obtain complimentary copies of articles by contacting Christa Stratton at cstratton@geosociety.org. Please discuss articles of interest with the authors before publishing stories on their work, and please make reference to GEOSPHERE in articles published. Contact Christa Stratton for additional information or other assistance.
Non-media requests for articles may be directed to GSA Sales and Service, gsaservice@geosociety.org.
Journal
Geosphere