Anjana K. Shah

Stochastic analysis of seafloor morphology on the flank of the Southeast Indian Ridge: The influence of ridge morphology on the formation of abyssal hills

In this study we estimate the statistical properties of abyssal hill morphology adjacent to the Southeast Indian Ridge in a region where the axial morphology changes from axial high to axial valley without a corresponding change in spreading rate. We explore the influence of axial morphology on abyssal hills and place these results within the context of response to spreading rate. Two cruises aboard the R/V Melville collected Sea Beam 2000 multibeam data along the Southeast Indian Ridge, providing continuous multibeam coverage of the axis from ∼89°W to ∼118°W, and ∼100% coverage within four survey regions extending out to ∼45 km (∼1.2 Ma) from the axis [Sempere et al., 1997; Cochran et al., 1997]. We apply the statistical modeling method of Goff and Jordan [1988] to gridded data from the four survey areas, examining in particular estimates of abyssal hill rms height, characteristic width and length, aspect ratio, and skewness. Two analyses are performed: (1) comparison of the along-axis variation in abyssal hill characteristics to ridge segmentation, and (2) a calculation of population statistics within axial high, intermediate, and axial valley data populations of this study, and comparison of these results to population statistics derived from studies adjacent to the Mid-Atlantic Ridge and East Pacific Rise. We find that abyssal hills generated along axial high mid-ocean ridges are very different from those generated along axial valley mid-ocean ridges, not only with respect to size and shape, but also in their response to such factors as spreading rate and segmentation.

Waxing and waning volcanism along the East Pacific Rise on a millennium time scale

Microbathymetric maps of the southern East Pacific Rise reveal subtle field relations between volcanic features and provide new insight on seafloor spreading processes. Along one of the shallowest and broadest sections of ridge at 17°28′S, lavas have erupted from a fissure system and flooded the axis through a network of lava tubes and lava channels. Along the neighboring ridge segment at 18°15′S, the axial area has subsided and formed a broad tectonized trough. A swath of newly accreted crust has since widened that trough; late-stage volcanism consists of small circular pillow mounds. We propose that these contrasting eruptive styles reflect the waxing and waning phases of a common magmatic evolution spanning a few millennia.

Causes for axial high topography at mid-ocean ridges and the role of crustal thermal structure

Mid-ocean ridge topography is modeled as the flexural response to loads using a thin plate approximation and setting thermal structure of the lithosphere to allow, but not require, a region of rapid cooling near the axis. Loads on the lithosphere arise from the presence of low-density melt, densification due to cooling with distance from the ridge axis, and thermal contraction stresses. We find two end-member classes of temperature and melt structure that can produce axial high topography and gravity observed at the East Pacific Rise (EPR). One class is very similar to previous models, requiring a narrow column of melt extending to at least 30 km depth within the mantle and lithosphere which cools and thickens very gradually with distance from the ridge axis. The other is a new class, predicting lithosphere which cools rapidly within a few kilometers of the axis and then slowly farther from the axis, with melt which is contained primarily within the crust. The latter solution is consistent with tomography and compliance studies at the EPR which predict rapid crustal cooling within a few kilometers of the axis that is attributed to hydrothermal circulation. This solution also allows the melt region to be coupled to crustal thermal structure and requires no melt anomaly within the mantle. Model fits predict 0–30% melt in the lower crust, depending on how temperatures are distributed within the lithosphere and the degree to which thermal contraction stresses are assumed to contribute to topography. The model generally predicts a wider axial high for lithosphere which is thin over a wider region near the axis. This is consistent with previous correlations between large cross-sectional area of the high and indicators of higher melt presence or a warmer crustal thermal regime. For a slightly slower rate of lithospheric cooling at distances more than ∼5 km from the axis the model predicts a trough at the base of the axial high. Such troughs have been previously observed at the base of the high on the western flank of the southern EPR, where subsidence rates are anomalously low. Finally, thick axial lithosphere reduces the amplitude of the high, making it sometimes difficult to distinguish from long-wavelength subsidence. This morphology is comparable to that of some intermediate spreading ridges, where topography is relatively flat, suggesting a transition from fast to intermediate style morphology.

Morphology of the transition from an axial high to a rift valley at the Southeast Indian Ridge and the relation to variations in mantle temperature

The Southeast Indian Ridge exhibits a transition in axial morphology from an East Pacific Rise-like axial high near 100°E to a Mid-Atlantic Ridge-like rift valley near 116°E but spreads at a nearly constant rate of 74–76 mm/yr. Assuming that the source of this transition lies in variations in mantle temperature, we use shipboard gravity-derived crustal thickness and ridge flank depth to estimate the variations in temperature associated with the changes in morphological style. Within the transitional region, SeaBeam 2000 bathymetry shows scattered instances of highs, valleys, and split volcanic ridges at the axis. A comparison of axial morphology to abyssal hill shapes and symmetry properties suggests that this unorganized distribution is due to the ridge axis episodically alternating between an axial valley and a volcanic ridge. Axial morphology can then be divided into three classes, with distinct geographic borders: axial highs and rifted highs are observed west of a transform fault at 102°45′E; rift valleys are observed east of a transform fault at 114°E; and an intermediate-style morphology which alternates between a volcanic ridge and a shallow axial valley is observed between the two. One segment, between 107° and 108°30′E, forms an exception to the geographical boundaries. Gravity-derived crustal thickness and flank depth generally vary monotonically over the region, with the exception of the segment between 107°E and 108°30′E. The long-wavelength variations in these properties correlate with the above morphological classification. Gravity-derived crustal thickness varies on average ∼2 km between the axial high and rift valley regions. The application of previous models relating crustal thickness and mantle temperature places the corresponding temperature variation at 25°C–50°C, depending on the model used. The average depth of ridge flanks varies by ∼550 m over the study area. For a variation of 25°–50°C, thermal models of the mantle predict depth variations of 75–150 m. These values are consistent with observations when the combined contributions of crustal thickness and mantle density to ridge flank depth are considered, assuming Airy isostasy. Crustal thickness variations differ at the two transitions described above: A difference of 750 m in crustal thickness is observed at the rift valley/intermediate-style transition, suggesting small variations in crustal thickness and mantle temperature drive this transition. At the axial high-rifted high/intermediate-style transition, crustal thickness variations are not resolvable, suggesting that this transition is controlled by threshold values of crustal thickness and mantle temperature, and is perhaps related to the presence of a steady state magma chamber.

Episodic dike swarms inferred from near-bottom magnetic anomaly maps at the southern East Pacific Rise

fissure eruption at 1728 0 S. Similar lows are observed at three other drained lava lake troughs, including one which is at least 1800 years old, residing 400 m away from the present-day axis. We attribute these lows to the presence of shallow dike swarms. The degree to which other geologic features may contribute to the lows is constrained using geologic, geophysical, and geochemical observations and forward modeling. Compositional analyses of Alvin samples at 1728 0 S do not support Fe or Ti variations as a primary source. Hypotheses requiring hydrothermal alteration and porosity variations are both inconsistent with geologic observations and near-bottom gravity data analysis from similar areas. Previous mappings between paleointensity variations and the observed magnetic field over distances of several kilometers from the axis suggest that such variations do not create the field low. The dominant source of the magnetization low is most likely the presence of a 100–200 m wide region of shallow dikes which are poorly magnetized relative to extrusives, or a region heated above magnetic blocking or Curie temperatures by intrusions during the most recent eruption (though the latter interpretation cannot explain the low at the fossil trough). In the first case, this extrusive thinning implies a change in eruptive behavior over the last 750–1500 years given the local spreading rate. For the latter case, thermal models suggest the anomaly had to have been created by a dike swarms totaling at least 45 m width during the most recent eruption(s), corresponding to � 300 years of plate spreading. Models indicate that the source of the low is centered slightly east of the axial trough. This offset suggests that the axis has been progressively migrating westward over the past millennium, consistent with other studies covering greater length and timescales. Westward migration provides an explanation for the preferential emplacement of recent lavas flows west of the axis, evident in ABE bathymetry and submersible observations. INDEX TERMS: 3035 Marine Geology and Geophysics: Midocean ridge processes; 1517 Geomagnetism and Paleomagnetism: Magnetic anomaly modeling; 8499 Volcanology: General or miscellaneous; 3045 Marine Geology and Geophysics: Seafloor morphology and bottom photography; 3005 Marine Geology and Geophysics: Geomagnetism (1550);

New surveys of the Chesapeake Bay impact structure suggest melt pockets and target-structure effect

We present high-resolution gravity and magnetic field survey results over the 85-km- diameter Chesapeake Bay impact structure. Whereas a continuous melt sheet is antici- pated at a crater this size, shallow-source magnetic field anomalies of ;100 nT instead suggest that impact melt pooled in kilometer-scaled pockets surrounding the base of a central peak. A central anomaly of ;300 nT may represent additional melt or rock that underwent shock-induced remagnetization. Models predict that the total volume of the melt ranges from ;0.4 to 10 km 3 , a quantity that is several orders of magnitude smaller than expected for an impact structure this size. However, this volume is within predictions given a transient crater of diameter of 20-40 km for a target covered with water and sedimentary deposits such that melt fragments were widely dispersed at the time of impact. Gravity data delineate a gently sloping inner basin and a central peak via a contrast between crystalline and sedimentary rock. Both features are ovoid, oriented parallel to larger preimpact basement structures. Conceptual models suggest how lateral differences in rock strength due to these preimpact structures helped to shape the craters morphology during transient-crater modification.

Plate bending stresses at axial highs, and implications for faulting behavior

Abstract We develop flexural models of axial high topography which include the effects of plastic yielding, and use the results to examine the contribution of plate bending stresses and strains to fault patterns at the East Pacific Rise (EPR). The lithosphere responds to buoyancy near the ridge axis created by the presence of elevated temperatures and melt within the crust. Parameterizing these quantities, we find topography at wide axial highs can be fit assuming the crust cools to lithosphere temperatures to 3–4 km depth within ∼2 km from the axis, and the presence of 20–30% melt in the lower crust. A trade-off in buoyancy implies the greater the amount of crustal cooling, the less melt is required. Significant crustal cooling leads to lithospheric strengthening, but plastic bending effects allow the plate to deflect over the distance needed to fit the observed axial relief. The plate develops at the axis with non-zero curvature, and ‘unbends’ as it moves away from the axis, placing a section of the upper surface or seafloor into extension. Seafloor stresses are extensional beginning a few kilometers from the axis, and continuing up to distances of ∼35 km from the axis for models that fit wide axial high topography. A compilation of previous faulting studies at the EPR reveals that distance from the axis over which fault slip remains active appears to depend on the width of the axial high: for narrow highs, this region is confined to at most 15 km from the axis; but at wider axial highs it reaches distances of 20–45 km, comparable to regions of extension predicted by the flexural models. The model strain can be as large as 1.9%, amounting to half the observed strain assuming a fault slip angle of 45°, but most strain assuming a slip angle of 60°. These results strongly suggest that bending stresses contribute significantly to normal faulting on the flanks of fast-spreading ridges, and may have a strong influence on faulting patterns there.

Buried Object Classification using a Sediment Volume Imaging SAS and Electromagnetic Gradiometer

To advance naval capabilities in identifying buried mines and unexploded ordnance, hybrid systems that fuse data from disparate sensors are being developed. This paper describes preliminary results of a classification engine that combines target features and classification parameters from a synthetic aperture Buried Object Scanning Sonar (BOSS) and an electromagnetic Real-time Tracking Gradiometer (RTG). The target characteristics that generate signals of interest for these sensors (acoustic backscatter and induced changes in local magnetic field) are sufficiently diverse that optimal combination should effectively increase the probability of correct target classification and reduce false alarm rates. Geometric and backscatter intensity features automatically extracted from three-dimensional acoustic imagery are combined with magnetic moment and associated parameters in a joint-Gaussian Bayesian classifier (JBC), which makes mine-like/non-mine-like decisions for each contact. The fused acoustic-magnetic classifier was evaluated using a combination of sea-trial and synthetic data sets. Nine data runs were processed to yield acoustic and magnetic features, supplemented by the synthetic data. An initially large variety of feature types were down-selected by a training process to a critical subset. With this limited dataset, initial results show probabilities of false classification (Pfc) from 1.6% to 6.3% when at high probability of correct classification (Pcc)