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The oceanic core complex comprising Atlantis Massif was formed within the past 1.5–2 Myr at the intersection of the Mid-Atlantic Ridge, 30° N, and the Atlantis Transform Fault. The corrugated, striated central dome prominently displays morphologic and geophysical characteristics representative of an ultramafic core complex exposed via long-lived detachment faulting. Sparse volcanic features on the massif's central dome indicate that minor volcanics have penetrated the inferred footwall, which geophysical data indicates is composed predominantly of variably serpentinized peridotite. In contrast, the hanging wall to the east of the central dome is comprised of volcanic rock. The southern part of the massif has experienced the greatest uplift, shoaling to less than 700 m below sea level, and the coarsely striated surface there extends eastward to the top of the median valley wall. Steep landslide embayments along the south face of the massif expose cross sections through the core complex. Almost all of the submersible and dredge samples from this area are deformed, altered peridotite and lesser gabbro. Intense serpentinization within the south wall has likely contributed to the uplift of the southern ridge and promoted the development of the Lost City Hydrothermal Field near the summit. Differences in the distribution with depth of brittle deformation observed in microstructural analyses of outcrop samples suggest that low-temperature strain, such as would be associated with a major detachment fault, is concentrated within several tens of meters of the domal surface. However, submersible and camera imagery show that deformation is widespread along the southern face of the massif, indicating that a series of faults, rather than a single detachment, accommodated the uplift and evolution of this oceanic core complex.

Approximately 600 km2 of Deep-Tow side-looking sonar data were collected at the eastern intersection of the Mid-Atlantic Ridge with the Kane Transform. The merged digital image mosaic provides a synoptic view of the oceanic crust exposed across major escarpments at this ridge-transform intersection (RTI). We characterized this large acoustic data set using textural attributes extracted from the backscatter image by means of a gray level co occurrence matrix method and Fourier fractal analysis. False-color texture attribute maps created from these methods aid in the interpretation and discrimination of surficial deposits and bedrock units on the seafloor. Principal components analysis performed on the textural attributes reduced the dimensionality of the textural feature vector and optimized imago texture discrimination. The textural attributes were calibrated to ground truth geologic data and extrapolated to regions with no ground data. The resultant texture and classification maps are consistent with the results of submersible studies but reveal more details of the seafloor geology than the conventional visual interpretation of the backscatter image and ground geologic observations. The inferred distribution of gabbros, basalts, surficial sediments, and rubble deposits shown in the classification map provides new constraints on seafloor geology at the eastern Kane RTI. Over 60% of the southern transform valley wall is covered by pelagic sediments and talus or rubble deposits, with intermittent exposures of coarse-grained gabbroic rocks and basaltic volcanic rocks. The western rift valley wall is classified as mainly massive outcrops of gabbroic rocks. The contact between the gabbroic outcrops on the western rift valley wall and the pillow basalt terrane on the rift valley floor can be traced for several kilometers along the base of the median valley wall. This new perspective supports the interpretation of the “inside-corner massif” as an “oceanic core complex” which is being degraded by mass wasting along tectonically active escarpments of the bounding median valley and transform valley walls.

We designed a series of case studies for Introductory Geology Laboratory courses using computer visualization techniques integrated with traditional laboratory materials. These consist of a comprehensive case study which requires three two-hour long laboratory periods to complete, and several shorter case studies requiring one or two, two-hour laboratory periods. Currently we have prototypes of the Yellowstone National Park, Hawaii volcanoes and the Mid-Atlantic Ridge case studies. The Yellowstone prototype can be used to learn about a wide variety of rocks and minerals, about geothermal activity and hydrology, about volcanic hazards and the hot-spot theory of plate tectonics. The Hawaiian exercise goes into more depth about volcanoes, volcanic rocks and their relationship to plate movements. The Mid-Atlantic Ridge project focuses on formation of new ocean crust and mineral-rich hydrothermal deposits at spreading centers. With new improvements in visualization technology that are making their way to personal computers, we are now closer to the ideal of a “virtual” field trip. We are currently making scenes of field areas in Hawaii and Yellowstone which allow the student to pan around the area and zoom in on interesting objects. Specific rocks in the scene will be able to be “picked up” and studied in three dimensions. This technology improves the ability of the computer to present a realistic simulation of the field area and allows the student to have more control over the presentation. This advanced interactive technology is intuitive to control, relatively cheap and easy to add to existing computer programs and documents.

The inter-graben zone between the Solea and Mitsero grabens in the northern part of the Troodos ophiolite (Cyprus) displays igneous, tectonic, and physiographic features that are characteristic of seamount volcanism at modern oceanic spreading centers. In this zone the 200-m-high Alestos hill volcanic edifice consisting of pillow lavas with preserved lava tubes and cone sheet intrusions is interpreted to have developed as a seamount fed by a late-stage, off-axis shallow-level gabbroic pluton that was intrusive into young oceanic crust. Concentric cone sheet intrusions and associated faults and shear zones in and around the Alestos hill seamount collectively form a seafloor caldera complex, which probably formed during the extrusion of lavas and subsequent cauldron subsidence over the gabbroic pluton. The ENE-trending elongated massive sulfide ore deposit formation in the Alestos–Memi area represents a focused upflow zone for hydrothermal fluids driven by the same off-axis gabbroic intrusion responsible for formation of the Alestos hill seamount. The ENE-trending and up to 150-m-wide Alestos–Memi fault zone dissects the elongated ore body in a dextral sense and might have provided a high permeability conduit in the upper crust for upwelling hydrothermal flow at the time of magmatism. The geometry of this fault zone and its kinematic relations with a N–S-striking Alestos Forest normal fault in the west are suggestive of its transfer fault origin, reminiscent of mineralized transfer faults found at the Mid-Atlantic Ridge. The active TAG hydrothermal mound with massive sulfide deposits that is located on a small volcanic dome 3 km off-axis and along a transfer fault in the Trans-Atlantic Geotraverse (TAG) area at 26°N provides a modern analogue for the Alestos hill seamount and associated faulting and mineralization in the ancient oceanic crust of the Troodos ophiolite. These findings indicate that off-axis magmatism played a major role in crustal accretion, hydrothermal alteration, and ore mineralization during the late-stage evolution of the Troodos ophiolite.

The first paleomagnetic results from oriented dike samples collected on the Mid-Atlantic Ridge shed new light on the complex interplay between magmatic accretion and mechanical extension at a slow spreading ridge segment. An upper crustal section about 1.5 km thick is exposed along a west-dipping normal fault zone that defines the eastern median valley wall of the southern segment of the Mid-Atlantic Ridge south of the Kane fracture zone (MARK area). Two distinct groups of dikes are differentiated on the basis of orientation and paleomagnetic characteristics. One group, on the basis of the paleomagnetic data, appears to be in its original intrusion orientation. This group includes both ridge-parallel, vertical dikes as well as dikes in other orientations, calling into question assumptions about uniform dike orientations at oceanic spreading centers. The second group consists of dikes that have paleomagnetic directions that are distinct from the predicted dipole direction, and we interpret them to have been tectonically rotated. These also occur in many orientations. The spatial relations between rotated and nonrotated dikes indicate that intrusion, faulting, and block rotation were contemporaneous beneath the median valley floor. Nonrotated dikes exposed on the eastern median valley wall indicate that there has been no net rotation of this upper crustal assemblage since magmatic construction ceased. Hence slip and associated uplift probably occurred in the fault zones' present orientation. These results provide the basis for a general model of mechanical extension and dike intrusion for this segment of the Mid-Atlantic Ridge. Initially, a portion of crust forms beneath the median valley by synkinematic dike intrusion into laterally discontinuous fault blocks. Slip and associated uplift along a cataclastic normal fault zone later exposes this crustal section on the valley margin. As spreading continues, this valley-bounding cataclastic normal fault zone is abandoned in favor of a new fault system thus passively moving the exposed crustal section away from the median valley.

Tilted dikes occur within the pillow lava, sheeted dike, and gabbroic sections of 1 million-year-old, fast spread oceanic crust exposed on the walls of the Hess Deep Rift. During recent Alvin dives, oriented samples from both tilted and vertical dikes were collected from several of the outcrops. These are the first oriented paleomagnetic samples ever collected directly from outcrops on the ocean floor and were obtained with the help of a specially constructed geological compass. Remanent magnetic inclinations measured on the dike samples are significantly different from the near-horizontal inclinations expected. The present attitudes of the dikes and their remanent magnetizations indicate that the dikes have been technically rotated since their intrusion. Submersible observations from other dive programs that examined parts of nearby escarpments suggest that the structural complexity observed in the Alvin study areas may be only a local phenomenon, or at least, not a pervasive characteristic of oceanic crust. Although tectonic rotations such as those observed may have taken place as a result of the opening of the Hess Deep Rift, crosscutting vertical dikes and other geologic considerations suggest that substantial tectonic rotations occurred at or very near the East Pacific Rise axis, probably as a result of intermittent axial collapse. The observed structures suggest that tectonic processes beneath fast spreading ridges and the internal structure of crust produced at them may be significantly more complex than previously assumed.

The Solea graben, on the northern flank of the Troodos ophiolite, Cyprus, is a 15–20-km-wide asymmetric structure primarily defined by dike attitudes in the sheeted dike complex. The wider western portion of the graben consists mainly of gently dipping dikes while the eastern portion is narrow and consists of blocks of dikes with widely varying attitudes. Using a method which restores the sampled dikes to vertical and the primary remanent magnetizations to a previously known stable remanent magnetization direction for the Troodos complex, we have calculated rotation axes and amounts of rotation for the dikes. The rotation axes for dikes in the western portion of the Solea graben are subhorizontal and subparallel to the original strikes of the dikes. In the eastern portion the rotation axes have varying attitudes, including some which are very steep. Our results are consistent with field observations that suggest that the western Solea graben formed by listric normal faulting near the axis of a spreading center, with rotation of the dikes above a detachment surface. Across the graben axis on the eastern portion, more intense intrusive activity and minor normal faulting resulted in less rotation about subhorizontal axes and some rotations about vertical axes.

[1] Side-scan sonar data collected along the steep, faulted north wall of the Hess Deep Rift provide images of a cross section of the upper oceanic crust. These data are integrated with ground truth from the submersible Alvin and the Argo II remotely operated vehicle (ROV) to evaluate faulting and mass wasting associated with the opening of the rift and to help trace geologic contacts between widely spaced dives and ROV surveys. Initial shipboard and subsequently processed data show excellent backscatter returns with well-imaged textures and distinctive patterns corresponding to specific rock units and surficial deposits. Four backscatter textures are widely developed and found to correspond to outcrops of basaltic lavas, outcrops of sheeted dikes and gabbroic rocks, surficial talus and rubble deposits, and pelagic sedimentary material. Using test areas identified by Alvin dives, the side-scan sonar data were processed to automatically classify the backscatter returns in terms of major rock units. The resultant processed image is used for further manual and computer-aided classification of the scarp geology. These techniques are evaluated for mapping the geology in rugged terrains similar to the steep walls of Hess Deep Rift.

The uppermost 2 km of the oceanic crust created at the fast spreading (135 mm yr−1, full rate) equatorial East Pacific Rise (EPR) is exposed for tens of kilometers along escarpments bounding the Hess Deep Rift. Mosaics of large-scale digital images from the remotely operated vehicle (ROV) Argo II and direct observations from the submersible Alvin document a degree of geological complexity and variability that is not evident from most studies of ophiolites or prevailing models of seafloor spreading. Dramatic variations in the thickness and internal structure are documented in both the basaltic volcanic and sheeted dike rock units. These rock units are characterized by extensive faulting, fine-scale fracturing, and rotations of coherent crustal blocks meters to tens of meters across. The uppermost basaltic lavas are essentially undeformed and have overall gently inclined flow surfaces. Through most of the basaltic lava unit, however, lava flow contacts dip (20°–70°W) toward the EPR and generally increase in dip downward in the section. Dikes cutting the lavas and in the underlying sheeted dike unit generally dip (90°–40°E) away from the EPR. Deeper level gabbroic rocks show little evidence of the intense fracturing typical of the overlying units. We interpret this upper crustal structure as the result of subaxial subsidence within 1–2 km of the EPR that accommodated the thickening of the basaltic lava unit to ∼500 m. Variations in the thickness of lava and dike units and spatially related structures along the rift escarpments suggest temporal fluctuations in magma supply. These results indicate that substantial brittle deformation accompanied waxing and waning volcanism during the accretion of the crustal section exposed at the Hess Deep Rift. If this type of structure is typical of uppermost oceanic crust generated at the EPR, these processes may be common along fast spreading mid-ocean ridges.

Oceanic crust that initially formed at the fast-spreading (130 mm/yr) East Pacific Rise at ∼1 Ma is exposed in the walls of Hess Deep rift at the northern edge of the Galapagos microplate. Steep fault scarps expose a dismembered vertical sequence of pillow basalts, sheeted diabase dikes and gabbroic rocks. Diabase dikes in these units are dominantly vertical; however, extensive outcrops of steeply to moderatley dipping dikes exist in some places. The nonvertical attitude of the dikes may be the result of tectonic rotations attending sea-floor spreading at the East Pacific Rise or the result of complex rotations of fault blocks during the opening of the Hess Deep rift at ∼0.65 Ma.