Hot off the presses! Apr 01 Geosphere

The Apr 01 issue of the Geosphere is now up on Pubget (About Geosphere): if you're at a subscribing institution, just click the link in the latest link at the home page. (Note you'll only be able to get all the PDFs in the issue if your institution subscribes to Pubget.)

Latest Articles Include:

• Growth of folds in a deep-water setting
  - Geosphere 5(2):59-89 (2009)
  Regional 3D seismic data of the deep-water area offshore NW Borneo provide a detailed picture of the interaction between sedimentary processes on a continental slope and the growth of major folds over a time period of ca. 3.5-5 Ma. In the deep-water area, the estimated rates of fold propagation of tens of mm/yr-1, shortening rates of mm/yr-1, and fold segment lengths of tens of kilometers indicate the studied folds are similar in scale and deformation rate to folds in orogenic belts such as the Zagros Mountains and Himalayas. Feedbacks between sediment dispersal patterns, anticline growth, and structural style are manifested in many ways, and are enhanced by the presence of weak, poorly lithified, synkinematic sediments at fold crests that undergo mass wasting as the fold grows. As folds tighten they range in geometry through simple folds--folds affected by crestal normal faults, folds with crestal normal faults and rotational slides, and folds with forelimb degradatio! n complexes and pronounced erosional unconformities. The unconformity surfaces are either elongate parallel to the fold axes (related to local mass wasting) or perpendicular (related to flows crossing the anticlines). Mass wasting processes in the study area that are large in scale compared with folds (i.e., giant landslides) have modified anticline shape by erosion, and are little affected by anticline topography. More commonly, gravity flows are relatively small compared with the anticlines, and transport pathways are influenced by anticline surface topography. The factors influencing sediment pathway changes during anticline growth include: proximal to distal propagation of folds, lateral propagation of folds, and changing locations of fold growth with time. Assuming an overall consistency in sediment supply, local relative changes in sediment supply to individual piggyback basins are defined by relative changes in sediment supply (generally increasing with time) with re! spect to growth in anticline amplitude. Such changes may be du! e to sediment infilling of depressions farther updip and prograding downslope with time or to changes in rate of deformation with time. Initial sediment pathways were predominantly subparallel to growing faults. As folds matured, canyons and channels exploited low points (e.g., fold linkage or intersection points) or weak points (e.g., mud pipes) on folds to initiate transverse channel systems. At a late stage, transverse channels carved up the once long and linear surface ridges into isolated segments.
• Significance of serpentinization of wedge mantle peridotites beneath Mariana forearc, western Pacific
  - Geosphere 5(2):90-104 (2009)
  In the Mariana forearc, horst and graben structures are well developed in the outer forearc basement, which is composed of both island arc and oceanic crust-mantle rocks. A zone of dome-shaped diapiric seamounts, which are composed mainly of serpentinized peridotites, formed on the basement in the outer forearc regions. Serpentine minerals in peridotites from both diapiric seamounts and basement are mostly chrysotile and/or lizardite. Antigorite, however, is rarely found in peridotites recovered from Conical, Big Blue, Celestial, and South Chamorro Seamounts. Antigorite-bearing peridotites always contain secondary iron-rich olivine and metamorphic clinopyroxene, and antigorite seems to coexist stably with them. Iron-rich secondary olivine (Fo86-90) occurs as overgrowth on the rim or along the cleavage traces of primary olivine (Fo90-92). The assemblage shows high-temperature conditions of serpentinization at ~450-550 {degrees}C, whereas chrysotile- and/or lizardite-bea! ring assemblages occur at ~200-300 {degrees}C. In antigorite-bearing samples, chrysotile and/or lizardite veins both predating and postdating antigorite formation are recognized. This may reflect a complex process of tectonic cycling of shallow mantle wedge serpentinized peridotites to depth and then back again to the surface.
• Saddle Mountain fault deformation zone, Olympic Peninsula, Washington: Western boundary of the Seattle uplift
  - Geosphere 5(2):105-125 (2009)
  The Saddle Mountain fault, first recognized in the early 1970s, is now well mapped in the Hoodsport area, southeastern Olympic Peninsula (northwestern United States), on the basis of light detection and ranging (LIDAR) surveys, aerial photography, and trench excavations. Drowned trees and trench excavations demonstrate that the Saddle Mountain fault produced a MW 6.5-7.0 earthquake 1000-1300 yr ago, likely contemporaneous with the MW 7.5 Seattle fault earthquake 1100 yr ago and with a variety of other fault and landslide activity over a wide region of the Olympic Peninsula and Puget Lowland. This near synchroneity suggests that the Saddle Mountain and Seattle fault may be kinematically linked. Aeromagnetic anomalies and LIDAR topographic scarps define an en echelon sequence of faults along the southeastern Olympic Peninsula of Washington, all active in Holocene time. A detailed analysis of aeromagnetic data suggests that the Saddle Mountain fault extends at least 35 km! , from 6 km southwest of Lake Cushman northward to the latitude of the Seattle fault. A magnetic survey over Price Lake using a nonmagnetic canoe illuminated two east-dipping reverse faults with 20 m of vertical offset at 30 m depth associated with 2-4 m of vertical displacement at the topographic surface. Analysis of regional aeromagnetic data indicates that the Seattle fault may extend westward across Hood Canal and into the Olympic Mountains, where it terminates near the northward terminus of the Saddle Mountain fault. The en echelon alignment of the Saddle Mountain and nearby Frigid Creek and Canyon River faults, all active in late Holocene time, reflects a >45-km-long zone of deformation that may accommodate the northward shortening of Puget Lowland crust inboard of the Olympic massif. In this view, the Seattle fault and Saddle Mountain deformation zone form the boundaries of the northward-advancing Seattle uplift.
• Improving fractured carbonate-reservoir characterization with remote sensing of beds, fractures, and vugs
  - Geosphere 5(2):126-139 (2009)
  Many key aquifers and oil reservoirs are in carbonate rocks. Understanding the flow behavior within this commonly complex pore space requires new perspectives and technology in order to improve carbonate aquifer and reservoir characterization. Dissolution of carbonates is related to flow; hence, quantifying the size of dissolution vugs on carbonate outcrops can help characterize controls on flow, namely matrix permeability and fracture connectivity. LIDAR (light detection and ranging) scans, combined with high-resolution photography, enable us to both measure vugs' areas and assess spatial relationships between vugs, beds, and fractures. We developed a method of obtaining and interpreting necessary vug, bed, and fracture data on the basis of these technologies. Application of this method on a Cretaceous Edwards Group outcrop in Texas (United States) revealed a significant correlation between the relative vug area of beds obtained remotely and air permeability measured ! in plugs extracted from these beds (R2 = 0.94, P = 0.001). The total area of vugs intersected by fractures was used to establish a probability density function of fracture lengths that can improve flow modeling of the reservoir. These findings show the potential of using LIDAR and photo images in reservoir characterization by data analysis of geological features, in addition to their use for accurate mapping.
• Unconformity-bounded seismic reflection sequences define Grenville-age rift system and foreland basins beneath the Phanerozoic in Ohio
  - Geosphere 5(2):140-151 (2009)
  Interpretation of reprocessed Ohio Consortium for Continental Reflection Profiling (COCORP) OH-1 seismic reflection profiles indicates four structurally complex Precambrian unconformity-bounded stratigraphic sequences that clarify the relative timing of formation of the Fort Wayne Rift and East Continent Rift System with respect to the Grenville orogeny. Petrographic examination of sparse deep well samples in the region indicates or suggests sedimentary lithologies beneath the Paleozoic sedimentary cover. Other seismic profiles in the region, some with excellent well control, support our proposed model. A generalized model for the latter part of the Grenville orogeny suggests polyphase sedimentation and deformation with multiple episodes of crustal extension and compression. We propose the following events for Ohio and the surrounding region: (1) a major regional unconformity developed on the Eastern Granite-Rhyolite Province and accreted Grenville terranes; (2) wester! n Ohio became the site of extensive fault-bounded rift basins, beginning with the Fort Wayne Rift and extending into west-central Ohio as the East Continent Rift System; (3) westward-advancing thrust sheets followed with deposition of sediments into newly developed basins; (4) continued Grenville thrusting created foreland basins in a westward progression; and (5) a long period of Neoproterozoic to Middle Cambrian erosion removed much of the foreland basin sedimentary sequences prior to Paleozoic deposition. Erosion in the Ohio region did not remove the large volume of rock as in Canada north of Georgian Bay. Other seismic lines in the region suggest that Grenville-age sedimentary basins are preserved beneath the Phanerozoic from Georgian Bay southward. These new findings demonstrate the importance of using fault- and unconformity-bounded seismic sequences to enhance and clarify the relative timing of Proterozoic events in regions where Paleozoic sedimentary cover exists an! d core samples are sparse or lacking.