Heather Sloan

Contrasted styles of rifting in the eastern Gulf of Aden: A combined wide-angle, multichannel seismic, and heat flow survey

Continental rifts and passive continental margins show fundamental along-axis segmentation patterns that have been attributed to one or a number of different processes: extensional fault geometry, variable stretching along strike, preexisting lithospheric compositional and structural heterogeneities, oblique rifting, and the presence or absence of eruptive volcanic centers. The length and width scales of the rift stage fault-bounded basin systems change during the late evolution of the new plate boundary, and the role of magmatism may increase as rifting progresses to continental rupture. Along obliquely spreading ridges, first-order mid-ocean ridge geometries originate during the synrift stage, indicating an intimate relationship between magma production and transform fault spacing and location. The Gulf of Aden rift is a young ocean basin in which the earliest synrift to breakup structures are well exposed onshore and covered by thin sediment layers offshore. This obliquely spreading rift is considered magma-poor and has several large-offset transforms that originated during late stage rifting and control the first-order axial segmentation of the spreading ridge. Widely spaced geophysical transects of passive margins that produce only isolated 2-D images of crust and uppermost mantle structure are inadequate for evaluation of competing rift evolution models. Using closely spaced new geophysical and geological observations from the Gulf of Aden we show that rift sectors between transforms have a large internal variability over short distances (∼10 km): the ocean-continent transition (OCT) evolves from a narrow magmatic transition to wider zones where continental mantle is probably exhumed. We suggest that this small-scale variability may be explained (1) by the distribution of volcanism and (2) by the along-strike differences in time-averaged extension rate of the oblique rift system. The volcanism may be associated with (1) the long-offset Alula-Fartak Fracture Zone, which may enhance magma production on its younger side, or (2) channeled flow from the Afar plume material along the newly formed OCT and the spreading ridge. Oblique extension and/or hot spot interactions may thereby have a significant control on the styles of rifting and continental breakup and on the evolution of many magma-poor margins.

Direct evidence for the distribution and occurrence of hydrothermal activity between 27°N–30°N on the Mid-Atlantic Ridge

Abstract A survey along the axis of the slow-spreading Mid-Atlantic Ridge, between 27°N and 30°N, using a combination of geophysical imaging and geochemical sensing has assessed the regional extent of hydrothermal activity. As a result, three areas were identified as possible sources of hydrothermal activity: at 27°00′N, 29°10′N and 30°02′N. The location of the strongest signals of high-temperature activity, at 29°10′N, was examined with a combination of water column sensors and sea floor observations, including submersible studies. These studies confirmed the presence of a high-temperature hydrothermal vent field comprising three discrete ‘black smoker’ sources of fluid in excess of 350°C, as well as two weathered sulphide mounds with diffuse, low-temperature fluid seeps. This systematic regional survey of the Mid-Atlantic Ridge reinforces the earlier supposition that hydrothermal activity is spatially restricted on slow spreading ridges in comparison to fast spreading ridges.

From slow to ultraslow: A previously undetected event at the Southwest Indian Ridge at ca. 24 Ma

Changes in plate motion are thought to be recorded in the trend of fracture zones, even though fracture zones provide no information about the spreading rate. Using newly compiled published and unpublished magnetic data from the Southwest Indian Ridge, we calculated fi nite rotation poles for A13, A8, and A6, from which we determined a 50% decrease in spreading rate from slow to ultraslow at ca. 24 Ma not accompanied by a signifi cant change in spreading direction. This spreading rate decrease is concurrent with changes in plate motions at only two of the four adjoining plate boundaries. Finally, we discuss the possible relationships of this event with other absolute or relative plate motion events that occurred at ca. 24 Ma at the global scale.

From slow to ultra-slow: How does spreading rate affect seafloor roughness and crustal thickness?

We examine the relationship of seafloor roughness and gravity-derived crustal thickness to both spreading rate and inferred mantle temperature using statistical analysis of a multibeam bathymetry and gravity data compilation of the axis and flanks between 54°E and 67°E at the Southwest Indian Ridge (southwest Indian Ocean). Our findings indicate that root mean square values of abyssal hill heights increase from 220 ± 20 m to 300 ± 20 m along flow line corridors that transition a well-constrained full spreading rate change from slow (30 mm/yr) to ultra-slow (15 mm/yr). Mantle Bouguer gravity anomalies, however, indicate no significant change in inferred crustal thickness at the spreading rate transition. In the axis-parallel direction, roughness of both slow and ultra-slow seafloor increases from 54°E to 63°E while inferred crustal thickness and/or mantle temperature decrease. These findings have implications for the relationship between spreading rate and melt production: they suggest that mantle temperature at slow and ultra-slow ridges may play a more important role than spreading rate in determining seafloor morphology. The lack of evidence for significant crustal thinning accompanying a change from slow to ultra-slow spreading rate lends support to focused subaxial mantle upwelling models.

Improving Urban Earth Science Education: The TRUST Model

TRUST, or Teacher Renewal for Urban Science Teaching, is a National Science Foundation funded Earth science teacher preparation partnership between the American Museum of Natural History and Brooklyn and Lehman Colleges of the City University of New York. Our research and practice form a promising and replicable model for formal-informal partnership between teacher education programs and science-rich institutions such as museums, zoos, and botanical gardens. The model takes a problem-based approach to urban teacher shortages by focusing on the knowledge required for Earth science teacher certification. The initiative included two types of participants, teachers seeking Earth science certification and teacher leaders/school administrators seeking to improve science instruction in their schools. Key features of the model included new college-based courses that focus on Earth systems science and urban Earth science investigation and a two week museum-based summer institute emphasizing essential questions, interactions with scientists, and approaches to science teaching outside the classroom. The overall findings after four-years of implementation, action research, and external evaluation indicate that the model successfully provides timely and relevant approaches to partnership that complex urban situations require.

Abyssal Hills and Abyssal Plains

Abyssal hills and abyssal plains makeup the majority of the seafloor, and thus cover vast amount of the Earth’s surface. Abyssal hills form in the young oceanic lithosphere near mid-ocean ridges. These elongate, ridge-parallel hills and intervening valleys provide the characteristic fabric of the recently accreted and sparsely sedimented seafloor. Near-bottom investigations document that abyssal hills owe most of their morphology to extensional faulting. Their tectonically-driven growth continues as far as ~35 km from the spreading axis, thus defining a broader plate boundary zone within which the parting plates acquire their steady-state motion. Abyssal hill morphology is sensitive to key aspects of seafloor accretion, and thus preserves accurate records of changing spreading rate, lithospheric thermal structure, and plate boundary geometry. In general, the slower the spreading rate, the larger their dimensions are. This relationship is modulated by regional variations in the thermal structure of the lithosphere, such as may be produced by proximity to hot spots, cold spots, or transform faults and non-transform ridge offsets. As divergent plate motion rafts the aging, subsiding oceanic lithosphere away from the mid-ocean ridge, abyssal hills are generally slowly buried beneath layers of sediments. However, extreme variability in sedimentation rates means that the burial of abyssal hills by sediments is not predictably related to the age of the lithosphere. In fact, the rugged fabric of the abyssal hills is transformed into the remarkably flat surface of the abyssal plains only where oceanic basins are within reach of the fast-moving turbidity currents that originate along the continental margins.

The TRUST Partnership: Institutional Impacts at Lehman College

This work explores the institutional impacts at Lehman College, City University of New York, of the Teacher Renewal for Urban Science Teaching (TRUST) Project, a partnership between the American Museum of Natural History and Brooklyn and Lehman Colleges funded by the National Science Foundation. We examine the impact that TRUST had and continues to have at Lehman College through its external partnerships, the evolving natural sciences-education cultural changes within the college, changes within the departments involved, and student outcomes and reflections.