John A. Madsen

The regional tectonic fabric of the East Pacific Rise from 12°50′N to 15°10′N

SeaMARC II backscatter data for the East Pacific Rise are used to create structural maps of ridge-parallel fabric on the crest and flanks of the rise from 12°50′N to 15°10′N. The structural data are statistically analyzed to describe the spacing, density, azimuth, facing direction, and length of faults. Results of the statistical studies are compared with predictions for the width of the zone of active fault formation, models for the generation of abyssal hills, plate kinematic predictions, models of along-strike segmentation, and observations of the asymmetric subsidence of oceanic crust on the Cocos and Pacific plates. Comparisons of the number of faults with distance from the rise crest and examination of the stratigraphic relationship between seamounts and the ridge-parallel tectonic fabric illustrate that the zone where new faults are created is located within a few kilometers of the ridge crest. Fault density data reveal that fault distributions do not resemble periodic processes. Examination of fault density and spacing data along-strike indicates that there is a high probability that the mean number of faults per crustal block differs significantly from north to south. Changes in along-strike statistics correlate well with the occurrence of three overlapping spreading centers located within the survey area. Additionally, fault spacing s determined from SeaMARC I and SeaMARC II backscatter imagery demonstrate that quantitative analyses of seafloor fabric are dependent upon the instrument used to collect the data. Analysis of inward and outward facing faults indicates that especially in the northern portion of the survey area, half-graben models are better predictors of abyssal hill morphology than full-graben models. Within the survey area, inward facing faults are more abundant and affect more of the oceanic crust than outward facing faults. Although the Cocos and Pacific plates subside at different rates, this asymmetry is not reflected in the tectonic component of morphology. Fault azimuth is observed to vary as a function of crustal age. Although the overall trend of the change in azimuthal values agrees with the trend predicted by relative poles of opening for the Pacific and Cocos plates, the variability in azimuthal data suggests that other processes contribute to the orientation of lineations formed near the axis of the East Pacific Rise. Finally, the statistical analyses demonstrate that all fault parameters are better correlated about the ridge axis than along-strike of the axis. The differences between the northern and southern portions of the survey area are reflected in the SeaMARC II bathymetric data which depict a continuous narrow ridge crest in the southern region, and an irregularly shaped ridge crest in the northern region that shoals and deepens every 30 to 40 km. This along-strike variability, observed over distances of less than 100 km, suggests that large-scale plate stresses are not the only processes responsible for generating the tectonic fabric observed on the flanks of the East Pacific Rise.

Structure and topography of the Siqueiros transform fault system: Evidence for the development of intra-transform spreading centers

The Siqueiros transform fault system, which offsets the East Pacific Rise between 8°20′N–8°30′N, has been mapped with the Sea MARC II sonar system and is found to consist of four intra-transform spreading centers and five strike-slip faults. The bathymetric and side-looking sonar data define the total width of the transform domain to be ≈20km. The transform domain includes prominent topographic features that are related to either seafloor spreading processes at the short spreading centers or shearing along the bounding faults. The spreading axes and the seafloor on the flanks of each small spreading center comprise morphological and structural features which suggest that the two western spreading centers are older than the eastern spreading centers. Structural data for the Clipperton, Orozco and Siqueiros transforms, indicate that the relative plate motion geometry of the Pacific-Cocos plate boundary has been stable for the past ≈1.5 Ma. Because the seafloor spreading fabric on the flanks of the western spreading centers is ≈500 000 years old and parallels the present EPR abyssal hill trend (350°) we conclude that a small change in plate motion was not the cause for intra-transform spreading center development in Siqueiros. We suggest that the impetus for the development of intra-transform spreading centers along the Siqueiros transform system was provided by the interaction of small melt anomalies in the mantle (SMAM) with deepseated, throughgoing lithospheric fractures within the shear zone. Initially, eruption sites may have been preferentially located along strike-slip faults and/or along cross-faults that eventually developed into pull-apart basins. Spreading centers C and D in the eastern portion of Siqueiros are in this initial pull-apart stage. Continued intrusion and volcanism along a short ridge within a pull-apart basin may lead to the formation of a stable, small intra-transform spreading center that creates a narrow swath of ridge-parallel structures within the transform domain. The morphology and structure of the axes and flanks of spreading centers A and B in the western and central portion of Siqueiros reflect this type of evolution and suggest that magmatism associated with these intra-transform spreading centers has been active for the past ≈0.5–1.0 Ma.

A different pattern of ridge segmentation and mantle Bouguer gravity anomalies along the ultra-slow spreading Southwest Indian Ridge (15°30′E to 25°E)

Abstract The results of a recent bathymetric and geophysical investigation of a ∼650 km-long portion of the very slowly opening (16 mm/yr full rate) Southwest Indian Ridge (SWIR) between 15°30′E and 25°E are presented. Bathymetry and mantle Bouguer gravity anomalies (MBA), caused by variations in crustal thickness and/or crustal and upper mantle densities, show different characteristics from those observed at faster spreading centers like the Mid-Atlantic Ridge (MAR) (20–30 mm/yr full rate). With the exception of the Du Toit Transform, none of the ridge-axis discontinuities have offsets greater than 10 km and few of the discontinuities have clearly defined off-axis traces. The MBA patterns associated with individual segments are much more complex than the simple circular bulls eyes lows reported along the MAR. While the short wavelength ridge segment length is comparable to that of the MAR, there is little correlation with MBA amplitude and segment length and axial relief. Furthermore, an eastward propagating magma source and an ∼84 km-long zone of oblique spreading appears to define a fundamental boundary along the SWIR between two 250–300 km-long sections characterized by distinctly different axial morphology and gravity signatures. We interpret these results to indicate a long-wavelength segmentation pattern of the underlying upwelling mantle. Melt separates from the upwelling mantle at the base of the lithosphere and is channeled to the surface along dikes. Fissure eruptions within the rift valley build linear ridges defining a short-wavelength spatial pattern of ridge segmentation that is not directly related to the segmentation pattern of the upwelling mantle. Our results and interpretation are quite different than that predicted by extending current models of the faster spreading MAR to these ultra-slow spreading rates.

Opinion: The time has come for offshore wind power in the United States

Offshore wind turbines have been successfully deployed in Europe since 1991, providing thousands of megawatts of clean energy for multiple nations. Ten years ago, it seemed that the United States would follow suit: The US Energy Policy Act of 2005 directed the Department of the Interior (DOI) to establish an offshore leasing regime in federal waters (generally oceanic waters 3–200 nautical miles from the coast). It appeared to be a crucial step in opening the door to the country’s vast offshore wind resource: turbine installations in the Mid-Atlantic Bight alone could power all United States electricity, automobile transport, and building heat needs (1).

The Science Semester: Cross-Disciplinary Inquiry for Prospective Elementary Teachers.

We describe the Science Semester, a semester-long course block that integrates three science courses and a science education methods course for elementary teacher education majors, and examine prospective elementary teachers’ developing conceptions about inquiry, science teaching efficacy, and reflections on learning through inquiry. The Science Semester was designed to provide inquiry-oriented and problem-based learning experiences, opportunities to examine socially relevant issues through cross-disciplinary perspectives, and align with content found in elementary curricula and standards. By the end of the semester, prospective elementary teachers moved from naïve to intermediate understandings of inquiry and significantly increased self-efficacy for science teaching as measured on one subscore of the STEBI-B. Reflecting on the semester, prospective teachers understood and appreciated the goals of the course and the PBL format, but struggled with the open-ended and student-directed elements of the course.

The application of ground-penetrating radar to a coastal prehistoric archaeological site, Cape Henlopen, Delaware, USA

Cape Henlopen, Delaware is a coastal spit complex located at the confluence of Delaware Bay and the Atlantic Ocean. This region was occupied by prehistoric peoples throughout the evolution of ancestral Cape Henlopen. A ground-penetrating radar (GPR) survey was conducted at one of the prehistoric archaeological sites (7S-D-30B) located within the Cape Henlopen Archaeological District. The site was in a remote location in the center of a tide dominated back-barrier marsh. Ground-penetrating radar waves penetrated to depths of 7 m, and four major sets of reflections were observed. Three sets were interpreted to be GPR images of geomorphic units associated with the spit complex, and the fourth was identified as the GPR image of a shell midden deposit. The GPR survey was used to determine the approximate dimensions of the shell midden, including its depth below ground surface (up to 2.1 m) and horzontal extent (∼250 m2), and to establish the paleoenvironmental setting and antecedent topography of the site prior to occupation. The GPR data suggests that the shell midden was initially deposited upon an aeolian dune surface and the antecedent topography at the site included an up to 1 m deep trough located 5 m to the north of, and trending parallel to, the axis of a present-day topographic high. This survey illustrates that GPR is a useful, noninvasive, tool that may be implemented at archaeological sites in coastal areas. It provides constraints on the environmental setting and topography of the terrain which prehistoric peoples inhabited, and it can be used in planning excavations at sites in coastal geomorphic settings.

High Resolution Seismic Reflection Images of New Jersey Coastal Aquifers

A high‐resolution seismic reflection experiment was conducted on the barrier islands of New Jersey to study the stratigraphy and physical properties of four regionally important aquifers. Five multichannel profiles, totaling 5.4 km in length, were collected from Island Beach State Park to Shipbottom. Careful selection of acquisition and processing parameters produced very high resolution profiles with penetration depths to 186 m. The average wavelet frequency of 225 Hz provided average quarter‐wavelength resolution of 1.9 m; in some places, recorded frequencies of up to 400–425 Hz allowed individual sand and clay layers less than a meter thick to be resolved. Synthetic seismograms were generated from geophysical logs from nearby wells for comparison with the seismic data and to confirm interpretations.All aquifers and confining units of interest are resolved in detail on the profiles. Typical aquifer responses include strong, continuous reflection peaks at the tops of sand bodies and a less distinguishabl...

Use of Geospatial Data in Planning for Offshore Wind Development

Offshore wind projects currently provide over two gigawatts of power to the global energy market (Global Wind Energy Council, 2010). Nearly all of this is generated in the North Sea dominated by projects in the United Kingdom and Denmark (Global Wind Energy Council, 2010). The development of offshore wind energy is moving forward in the United States (US) with the recently approved Cape Wind project in federal waters off Massachusetts, the continued planning for offshore wind projects in the Great Lakes and the granting of limited leases for study and calls for requests for interest in leasing selected portions of the eastern outer continental shelf (e.g., Minerals Management Service, 2009; Bureau of Ocean Energy Management, Regulation and Enforcement, 2010; Great Lakes Wind Council, 2010). In addition to the United Kingdom in Europe, large-scale offshore wind projects are projected for France, Belgium and the Netherlands, as well as, in China (Global Wind Energy Council, 2010). The development of offshore wind projects further contributes to the existing pressures on the marine environment increasing the potential benefits of comprehensive, integrated, ecosystem-based planning. This type of marine spatial planning (MSP) is based on sound science and considers current and anticipated uses of the ocean and coastal environment (Ehler and Douvere, 2009). Geospatial techniques provide the framework by which data can be manipulated to aid in implementation of MSP.