High-precision array element localization for vertical line arraysin the Arctic Ocean

Array element localization (AEL) surveys are often required to accurately localize acoustic instruments (transponders or sensors) in the ocean. These are typically based on transmitting or recording acoustic signals from or at a set of well-known positions. A significant limiting factor in many AEL surveys is the uncertainty inherent in these “known” positions. In this paper, an inversion algorithm is developed which properly treats both transponder and sensor positions as unknowns, subject to available a priori information in the form of position estimates and uncertainties. The algorithm essentially consists of an iterative linearized inversion of the raytracing equations employing the method of regularization. The approach is applied to independently localize transponders and vertical line array (VLA) sensors that form part of a three-dimensional sensor array in the Arctic Ocean. Confidence limits estimated via Monte Carlo simulation indicate that transponders and sensors are localized to less than 1 m in three dimensions. The VLA sensor motion, monitored over a seven-week period, appears to be predominately driven by tidal currents and is consistent with historical current measurements for the region     

Long-lived transpression in the Archean Bird River greenstone belt, western Superior Province, Southeastern Manitoba

The Archean Bird River greenstone belt (BRGB) is located on the southwestern edge of the Superior Province between the 3.2 Ga old Winnipeg River subprovince to the south and the metasedimentary belt of the English River subprovince (ERSP) to the north. This position between two major subprovinces makes the BRGB a primary target for investigating the geodynamic and kinematic evolution of a major structural boundary. New structural and geochronological data have allowed us to present an evolutionary framework for the southern boundary of the North Caribou superterrane. The BRGB underwent 3 main deformation phases. The D1 event took place ca. 2698 Ma and displays a north-side-up shearing. The D2 event, occurring at ca. 2684 Ma in a transpressive context, presents a complex structural pattern mixing vertical tectonics in the BRGB and strike-slip tectonics along the boundaries of the greenstone belt with other subprovinces. Between the BRGB and the ERSP, the 2832–2858 Ma old Maskwa batholith acted as a rigid passive block during the collision and marks the boundary between pure dextral strike-slip tectonics along his northern boundary with the ERSP and vertical south-side-up motion in the BRGB. The BRGB can be considered as a pop-up structure with anastomosed shear zones displaying different horizontal offset according to the orientation of the shear zones. The southern boundary with the Winnipeg River subprovince is represented by a sinistral south-side-up shear zone. The same pattern is found at the regional scale where major shear zones acted as a conjugate set in the horizontal plane. At ca. 2640 Ma, the D3 event occurred in a general dextral transpressive tectonic regime coeval with the emplacement of rare-elements pegmatitic plutons in a still hot (400–500 °C) country rock. The geodynamical and mechanical significance of the partitioning between pure strike-slip tectonics in the English River subprovince and vertical motion in the BRGB can be explained by the rheological behaviour of a hot and weak lithosphere undergoing transpressive strain. The structural framework of the BRGB is the result of strong interactions between hot and weak domains, coeval with widespread plutonism, and a rigid older domain (Maskwa batholith) during the D2 transpressive event.     

Evolution of West Rota Volcano, an extinct submarine volcano in the southern Mariana Arc: Evidence from sea floor morphology, remotely operated vehicle observations and 40Ar–39Ar geochronological studies

Abstract West Rota Volcano (WRV) is a recently discovered extinct submarine volcano in the southern Mariana Arc. It is large (25 km diameter base), shallow (up to 300 m below sealevel), and contains a large caldera (6 × 10 km, with up to 1 km relief). The WRV lies near the northern termination of a major NNE‐trending normal fault. This and a second, parallel fault just west of the volcano separate uplifted, thick frontal arc crust to the east from subsiding, thin back‐arc basin crust to the west. The WRV is distinct from other Mariana Arc volcanoes: (i) it consists of a lower, predominantly andesite section overlain by a bimodal rhyolite‐basalt layered sequence; (ii) andesitic rocks are locally intensely altered and mineralized; (iii) it has a large caldera; and (iv) WRV is built on a major fault. Submarine felsic calderas are common in the Izu and Kermadec Arcs but are otherwise unknown from the Marianas and other primitive, intraoceanic arcs. ⁴⁰Ar–³⁹Ar dating indicates that andesitic volcanism comprising the lower volcanic section occurred 0.33–0.55 my ago, whereas eruption of the upper rhyolites and basalts occurred 37–51 thousand years ago. Four sequences of rhyolite pyroclastics each are 20–75 m thick, unwelded and show reverse grading, indicating submarine eruption. The youngest unit consists of 1–2 m diameter spheroids of rhyolite pumice, interpreted as magmatic balloons, formed by relatively quiet effusion and inflation of rhyolite into the overlying seawater. Geochemical studies indicate that felsic magmas were generated by anatexis of amphibolite‐facies meta‐andesites, perhaps in the middle arc crust. The presence of a large felsic volcano and caldera in the southern Marianas might indicate interaction of large normal faults with a mid‐crustal magma body at depth, providing a way for viscous felsic melts to reach the surface.     

The seedbed microclimate and active revegetation of disturbed lands in the Mojave Desert

One of the greatest challenges to the U.S. Department of Defense is maintaining the sustainability of military lands while maximizing their use for training activities. Restoration of disturbed lands in the Mojave Desert has proven to be a challenge, particularly revegetation from seed. This study presents soil moisture dynamics from three restoration sites at the National Training Center, Ft. Irwin, CA. The soil microclimate (matric potential, water balance, and temperature) was monitored beneath various surface treatments to better assess their effects on seed germination and establishment. Results show that under the typically hot and dry climatic conditions that prevail, the soil microclimate is a hostile environment. Irrigation had the most positive effect on the seedbed. Surface mulches including gravel, straw, emulsion, and bark had limited benefits often detrimental. Although seedlings germinated, survival after three months was negligible. Results presented here suggest that successful seeding with broadcast sprinkler irrigation in the central Mojave Desert is not possible unless the seedbed area is limited or advantageous environmental conditions prevail. Improved knowledge of the germination requirements and applied numerical modeling would result in more successful seedbed management.     

Structural, tectonic and glaciological controls on the evolution of fjord landscapes

The fjord landscape of South America, stretching ~ 1500 km between Golfo Corcovado (~ 43°S) and Tierra del Fuego (~ 56°S), is the largest continuous fjord landscape on Earth. This paper presents the results of new structural geological and geomorphological mapping of this landscape using optical satellite images and digital elevation models. First-order geological structures are represented by strike-slip faults forming lineaments up to hundreds of kilometres long. The strike-slip faulting has been active since Late Cretaceous times and is responsible for the presence of a conspicuous structural cleavage visible as lineaments up to ~ 10 km long. A detailed analysis of these second-order lineaments from digital image data was carried out in three sectors. In Sector 1, located northwest of the North Patagonian Icefield, there are three distinct mean orientations, characterized by a main nearly orogen-parallel orientation (az. ~ 145°) and two orogen-oblique secondary orientations (az. ~ 20° and az. ~ 65°). In Sector 2, located west of the South Patagonian Icefield, there are also three separate mean orientations, with most of the lineaments concentrated between azimuths 0° and 80° (mean at ~ 36°); and two other orogen-oblique means at azimuth ~ 122° and ~ 163°. In Sector 3, around the Cordillera Darwin, there is a single main orogen-parallel mean at ~ 100–115°. In all three sectors, mapped fjord orientations bear a striking similarity to the structural data, with fjords orientated preferentially in the same direction as structural lineaments. We infer that successive glaciations followed the same ice-discharge routes, widening and deepening pre-existing geological structures at the expense of the surrounding terrain to create the fjord landscape. This study has broader implications for ice sheet reconstructions and landscape evolution beneath ice sheets because we demonstrate that the primary control on fjord development in glaciated areas is geological and not glaciological.     

The impact of upland land management on flooding: results from an improved pasture hillslope

In response to growing concern about impacts of upland agricultural land management on flood risk, an intensely instrumented experimental catchment has been established at Pontbren, a sheep‐farmed headwater catchment of the River Severn, UK. Primary aims are to develop understanding of the processes governing flood generation and the associated impacts of land management practices, and to bridge the gap between process understanding and ability to predict effects on downstream flooding. To achieve this, the experiment is designed to operate at plot (～100 m²), hillslope (～0·1 km²) and small catchment scale (～10 km²). Hillslope‐scale data, from an under‐drained, agriculturally ‘improved’ pasture, show that drain flow is a dominant runoff process. However, depending on antecedent moisture conditions, overland flow may exceed drain flow rates and can be an important contributor to peak flow runoff at the hillslope‐scale. Flow, soil tension data and tracer tests confirm the importance of macropores and presence of perched water tables under ‘normal’ wet conditions. Comparisons of pasture runoff with that from within a 10 year‐old tree shelterbelt show significantly reduced overland flow due to the presence of trees and/or absence of sheep. Comparisons of soil hydraulic properties show significant increases in hydraulic conductivity and saturated moisture content of soil under trees compared to adjacent improved pasture. Copyright © 2008 John Wiley & Sons, Ltd.     

法理地震学与全面禁止核试验条约

法理地震学的用途之一就是帮助核查“全面禁止核试验条约”的执行情况。法理地震学家面临的一个重要挑战是将每年发生的成千上万次天然地震与可能违反条约的地下爆破区别开来。识别方法主要有4种：（a）体波与面波的震级比,（b）高频P波与S波能量比,（c）基于模型的方法,（d）震源深度。方法（a）和（b）有经验基础。方法（a）～（c）的不足是对全球地质学介质范围内激发的地下爆破缺少一个等效的弹性震源。可靠的常规震源深度的确定已被证明非常困难。然而,过去的十几年获得的在识别可疑震源方面的经验说明,尽管没有单一的方法能够长期奏效,但巧妙地独创性地应用各种互补方法通常能够解决可疑震源识别问题。