ALBERT

Description: Submitted in partial fulfillment of the requirements for the degree of Doctor of Philosophy at the Massachusetts Institute of Technology and the Woods Hole Oceanographic Institution June, 1979

Description: A major goal in the study of plate tectonics is the acquisition of a knowledge of the history of relative motion among the rigid plates of the earth's lithosphere. The three papers of this thesis contribute to this effort and demonstrate that studies of the stability and evolution of triple junctions and of the finite rotations of systems of three plates can yield significantly more accurate tectonic histories than can studies of the relative motions between two plates alone. Topographic and magnetic investigation of the Southwest Indian Ridge and reconstruction of the plate system of the Indian Ocean shows that both Africa and Antarctica are rigid plates and their pole of relative rotation has remained fixed near 8°N, 42°W since the Eocene. A detailed survey of the Indian Ocean triple junction reveals that the Indian Ocean plate motions have remained constant since 10 Ma. The stability conditions of the junction show that the general morphology of the Southwest Indian Ridge results from the evolution of the Indian Ocean triple junction. A method is presented for determining the finite rotations best reconstructing the past relative positions of three plates around a triple junction. The method is illustrated by reconstructions of the plates around the Labrador Sea triple junction at the times of anomalies 24 (56 Ma) and 21 (50 Ma). The region of uncertainty of the Greenland-North America finite pole is mapped for each reconstruction, and it demonstrates that consideration of the three plate system yields more well-constrained results than does a treatment of the two plates alone.

Description: This work was supported by the Office of Naval Research contract N00014-75-C-0291 with the Massachusetts Institute of Technology.

Description: Submitted in partial fulfillment of the requirements for the degree of Doctor of Philosophy at the Massachusetts Institute of Technology and the Woods Hole Oceanographic Institution February 1999

Description: Changes in morphology of the Marquesas Fracture Zone are correlated with small changes in Pacific-Farallon relative motion. The simple flexural signal of a locked fracture zone may be obscured by tectonic effects, and there is no evidence for the release of shear stress on the fracture zone by vertical slip after leaving the active transform. One such small change in plate motion is documented in the South ern Austral Island region of the South Pacific. A twelve degree clock wise change in Pacific-Farallon relative motion occurred around fifty million years ago. This Eocene change in spreading direction and rate is locally constrained with observations of magnetic anomalies and spreading fabric orientation. At the southeastern end of the Cook-Austral Island chain, multiple episodes of volcanism have left a diverse population of seamounts. Volume estimates from geophysical data and modeling show that one-half to two-thirds of the volcanic material is over thirty million years old, while the remainder is less than five million years old. Seismic and bathymetric data imply the presence of abyssal basalt flows in the flexural moat of the Austral Islands, probably associated with Austral Islands volcanism, which may contribute a significant amount of material to the archipelagic apron.

Description: The research presented in Chapter 2 was supported by National Science Founda tion grants OCE-9012949 and OCE-9012529. Chapters 3, 4 and 5 were supported by National Science Foundation grant OCE-9415930. A National Science Foundation graduate fellowship supported my first three years of graduate study.

Description: Submitted in partial fulfillment of the requirements for the degree of Doctor of Philosophy at the Massachusetts Institute of Technology and the Woods Hole Oceanographic Institution December 1996

Description: Two-thirds of the surface of the Earth is created at mid-ocean ridges where magmas rise from the mantle and cool to form the oceanic crust. The objective of this Thesis is to examine the influence of magma supply and eruptive processes on axial morphology, crustal construction, and the properties of crustal magma chambers at intermediate and fast spreading ridges. Variations in magma supply on time scales of ~100 Kyr generate along-axis changes in crustal thickness and temperature. Magma sill properties and hydrothermal activity are closely linked to spreading events which occur on much shorter time scales (ca. 10-100 yr) than the longer-term variations in magma supply reflected in along-axis changes in ridge morphology. The seismically constrained depths of ridge crest magma sills (〉1-2 km) are considerably deeper than the level of neutral buoyancy (100-400 m). The apparent inverse relationship between magma sill depth and spreading rate suggests that a thermally controlled permeability boundary, such as the solidus horizon, controls the depth at which magma ponds beneath mid-ocean ridges. Recent thermo-mechanical models predict that, at intermediate spreading rates, rift valley and magma sill formation are sensitive to small changes in crustal thickness and mantle temperature. Analysis of gravity at an intermediate spreading ridge shows that small differences in crustal thickness (300-700 m) and mantle temperature (10-15°C) are indeed sufficient to produce major changes in lithospheric strength and axial morphology. A stochastic model for the emplacement of dikes and lava flows with a bimodal distribution of lava flows is required to satisfy geological and geophysical constraints on the construction of the extrusive section. Most dikes are intruded within a narrow zone at the ridge axis. Short flows build up approximately half the extrusive volume. Occasional flows that pond at a considerable distance off-axis build up the remainder of the extrusive section. This Thesis underlines the importance of eruption dynamics in the emplacement of the uppermost volcanic layer of the crust and of the crustal thermal structure in controlling local variations in magma sill depth and ridge morphology.

Description: Submitted in partial fulfillment of the requirements for the degree of Doctor of Philosophy at the Massachusetts Institute of Technology and the Woods Hole Oceanographic Institution September 1976

Description: This thesis is a collection and analysis of seafloor magnetic anomalies, bathymetry, and the paleomagnetism of DSDP sediments and basalt in the West Philippine Basin, in an attempt to resolve questions about its origin as a marginal basin. Our results suggest that this basin was formed in an Eocene pulse of rapid spreading (v1/2 = 41-44 mm/yr) in a direction (N 21°E) significantly different from later pulses which opened the more eastern basins of the Philippine Sea. The Central Basin Fault appears to be intimately associated with this spreading by nature of its structure and trend, and it may be a remanent of a former ridge system. Our preliminary calculation of paleopole positions also suggests that there was a large amount (60°) of clockwise rotation between this basin and the magnetic pole. This is consistent with rotations of the Pacific plate with respect to the magnetic pole and current directions of Philippine- Pacific'relative rotations. Basement depths of 6 km in the West philippine Basin imply that its crustal and/or lithospheric structure is different from Pacific structure of the same age.

Description: Submitted in partial fulfillment of the requirements for the degree of Doctor of Philosophy at the Massachusetts Institute of Technology and the Woods Hole Oceanographic Institution January, 1977

Description: This thes is is a collection of papers on the paleomagnetics of samples from several Deep Sea Drilling Project (DSDP) sites in the Indian Ocean. These papers present the basic paleomagnetic data, discuss the statistical methods for analyzing such data from DSDP cores, and examine the implications of the paleolatitudes for the origin of the Ninetyeast Ridge and the northward motion of India. Rarely do DSDP paleolatitudes approach the reliability of good continental pole positions. However, the reliability of such paleolatitudes can be markedly improved by using comparisons with paleolatitudes of different ages from the same site, paleolatitudes of similar ages from different sites on the same plate, estimates of paleolatitude from the skewness of marine magnetic anomalies, and continental paleopole. positions. Using such comparisons, a new paleomagnetic pole of upper Cretaceous age has been defined for the Pacific plate. A middle Cretaceous pole has been defined for the Wharton Basin plate, and it suggests that there may have been left lateral motion between Australia and the Wharton Basin. Paleolatitudes from the Ninetyeast Ridge are consistent with the pole position for the Deccan Traps. These data indicate that India and the Ninetyeast Ridge moved northwards with respect to the South Pole at 14.9 ± 4.5 cm/yr from 70 to 40 mybp and at 5.2 ± .8 cm/yr from 40 mybp until the present. However, when this paleomotion is compared to the Australian paleomagnetic data (by removing the relative motion components), a major inconsistency appears between 40 and 50 mybp. The Australian data indicate that India should be 13° further north than the positions implied by the Ninetyeast Ridge data. Basal paleolatitudes on the Ninetyeast Ridge indicate that its volcanic source was approximately fixed in latitude near 50°S, supporting the hypothesis that the ridge is the trace of the Kerguelen hotspot on the northward moving Indian plate. There is considerable geologic evidence in favor of such an hypothesis, and there is none to contradict it.

Description: National Science Foundation (Grant DES-74-22552).

Description: Submitted in the partial fulfillment of the requirements for the degree of Doctor of Philosophy at the Massachusetts Institute of Technology and the Woods Hole Oceanographic Institution August, 1996

Description: Two-thirds of the Earth's surface is oceanic crust formed by magmatic and tectonic processes along mid-ocean ridges. Slow-spreading ridges, such as the Mid-Atlantic Ridge, are discontinuous and composed of ridge segments. Segments are thus fundamental units of magmatic accretion and tectonic deformation that control the evolution of the crust. The objective of this Thesis is to constrain the tectonic processes that occur at the scale of slowspreading segments, to identify the factors controlling segment propagation, and to provide constraints on lithospheric strength with laboratory deformation experiments. In chapter 2, bathymetry and gravity from various areas along the global mid-ocean ridge system are analyzed to quantify systematic variations at the scale of individual segments. There is a marked asymmetry in bathymetry and gravity in the vicinity of segment offsets. We develop a model of faulting to explain these observations. Low-angle faults appear to accommodate tectonic extension at the inside corners of ridge-offset intersections, and result in substantially uplifted terrain with thin crust with respect to that at the outside corners or centers of segments. Results from Chapter 3 indicate that the crust magmatically emplaced on axis is not maintained off-axis. This transition is revealed by both statistical and spectral analyses of bathymetry and gravity. Tectonic extension varies along the length of a segment, resulting in thinning and uplift of the crust at ridge-offset inside corners, and a decorrelation between bathymetry and gravity patterns. Tectonic deformation substantially reshapes the oceanic crust that is magmatically emplaced on-axis, and strongly controls the crustal structure and seafloor morphology off-axis. Satellite gravity data over the Atlantic shown in Chapter 4 reveal a complex history of ridge segmentation, and provides constraints on the processes driving the propagation of segments. The pattern of segmentation is controlled mainly by the geometry of the ridge axis, and secondarily by hot spots. Segments migrate primarily down regional gradients associated with hot spot swells. However, the lack of correlation between gradients and propagation rate, and the propagation up gradient of some offsets, suggest that additional factors control propagation (e.g., variations in lithospheric strength). Most non-transform offsets are short-lived and migrating, while transform offsets are long-lived and stable. Both the propagation of segments (Chapter 4) tectonism along a segment (Chapters 2 and 3) are controlled by the lithospheric rheology. In Chapter 5 I present results from laboratory deformation experiments on serpentinite. These experiments demonstrate that serpentinites are considerably weaker than peridotites or gabbros, display a non-dilatant style of brittle deformation, and strain is accommodated by shear cracking. Serpentinites may weaken the lithosphere, enhance strain localization along faults, and control the style of faulting.

Description: A fellowship from Caixa de Pensions "La Caixa" in Barcelona provided me with all the required financial support to come to WHOI. The work presented in this thesis was also supported by the National Science Foundation grants OCE-90l2576, OCE-930078, OCE-9313812, and Office of Naval Research grant N00014-9l-J-1433.

Description: Submitted in partial fulfillment of the requirements for the degree of Doctor of Philosophy at the Massachusetts Institute of Technology and the Woods Hole Oceanographic Institution September, 1975

Description: The Mid-Atlantic Ridge is one of the most well known and yet poorly understood spreading centers in the world. A detailed investigation of the Mid-Atlantic Ridge crest near 37°N (FAMOUS) was conducted using a deeply towed instrument package. The objective was to study the detailed structure and spreading history of the Mid-Atlantic Ridge median valley, to explore the roles of volcanism and faulting in the evolution of oceanic crust, and to study the morphologic expression and structural history of the zone of crustal accretion. In addition, microearthquake surveys were conducted using arrays of free-floating hydrophones. The most recent expression of the accreting plate boundary in the Famous Rift is an alternating series of linear central volcanoes and depressions 1.5 km wide which lie within the inner floor. This lineament is marked by a sharp maximum in crustal magnetization only 2-3 km wide. Magnetic studies indicate that over 90% of the extrusive volcanism occurs within the rift inner floor, a zone 1 to 12 km wide, while volcanism is extremely rare in the rift mountains. Volcanoes created in the inner floor are transported out on, block faults, becoming a lasting part of the topography. Magnetic anomaly transition widths vary from 1 km to 8 km with time and appear to reflect a bi-stable median valley structure. The valley has either a wide inner floor and narrow terraces, in which case the volcanic zone is wide and magnetic anomalies are poorly recorded (wide transition widths); or it has a narrow inner floor and wide terraces, the volcanic zone is then narrow and anomalies are clearly recorded (narrow transition widths). The median valley of any ridge segment varies between these two structures with time. At present the. Famous Rift has a narrow inner floor and volcanic zone (1-3 km) while the south Famous Rift is at the opposite end of the cycle with a wide inner floor and volcanic zone (10-12 km). Over 95% of the large scale (〉2 km) relief of the median valley is accounted for by normal faults dipping toward the valley axis. Normal faulting along fault planes dipping away from the valley begins just past the outer walls of the valley. Outward facing normal faulting accounts for most of the decay of median valley relief in the rift mountains while crustal tilting accounts for less than 20%. The pattern of normal faulting creates a broad, undulating horst and graben relief. Volcanic features contribute little to the large scale relief, but contribute to the short wavelength (〈2km) roughness of the topography. Spreading in the Famous area is highly asymmetric with rates twice as high to the east as to the west. At 1.7 m.y.b.p. the sense of asymmetry reverses in direction with spreading faster to the west, resulting in a gross symmetry when averaged through time. The change in spreading asymmetry occurred in less than 0.15 m.y. Structural studies indicate that the asymmetric spreading is accomplished through asymmetric crustal extension as well as asymmetric crustal accretion. Spreading in the Famous area is 17° oblique. Even on a fine scale there is no indication of readjustment to an orthogonal plate boundary system. Spreading has been stably oblique for at least 6 m.y., even through a change in spreading direction. Magnetic studies reveal that the deep DSDP hole at site 332 was drilled into a magnetic polarity transition, and may have sampled rocks which recorded the earth i s field behavior during a reversal. The presence of negative polarity crust within the Brunhes normal epoch in the inner floor has been determined, and may be due to old crust left behind or recording of a geomagnetic field event. Crustal magnetization decays to lie of its initial value in less than 0.6 m.y. The rapid decay may be facillitated by very intense crustal fracturing observed in the inner floor. Microearthquake, magnetic and structural studies indicate that both the spreading and transform plate boundaries are very narrow (1-2 km) and well-defined for short periods, but migrate over zones 10-20 km wide through time.

Description: Submitted in partial fulfillment of the requirements for the degree of Doctor of Philosophy at the Massachusetts Institute of Technology and the Woods Hole Oceanographic Institution September 1978

Description: This thesis consists of three papers examining problems related to the crustal structure, isostasy and subsidence history of aseismic ridges and mid-plate island chains. Analysis of gravity and bathymetry data across the Ninetyeast and eastern Walvis Ridges indicates these features are locally compensated by an over thickening of the oceanic crust. Maximum crustal thicknesses are 15-30 km. The western Walvis Ridge is also compensated by crustal thickening; however, the isostasy of this part of the ridge is best explained by a plate model of compensation with elastic plate thicknesses of 5-8 km. These results are consistent with the formation of the Ninetyeast and Walvis Ridges near spreading centers on young lithosphere with flexural rigidities at least an order of magnitude less than those typically determined from flexural studies in older parts of the ocean basins. As the lithosphere cools and thickens, its rigidity increases, explaining the differences in isostasy between aseismic ridges and mid-plate island chains. The long-term subsidence of aseismic ridges and island/ seamount chains can also be explained entirely by lithospheric cooling. Aseismic ridges form near ridge crests and subside at nearly the same rate as normal oceanic crust Mid-plate island chains subside at slower rates because they are built on older crust. However, some island chains have subsided faster than expected based on the age of the surrounding sea floor, probably because of lithospheric thinning over midplate hot spots, like Hawaii. This lithospheric thinning model has major implications both for lithospheric and mantle convection studies as well as the origin of continental rift systems.

Description: Submitted in partial fulfillment of the requirements for the degree of Doctor of Philosophy at the Massachusetts Institute of Technology and the Woods Hole Oceanographic Institution September 1996

Description: We analyze bathymetric and gravity anomalies at five plume-ridge systems to constrain crustal and mantle density structure at these prominent oceanic features. Numerical models are then used to explore the physical mechanisms controlling plume-ridge interaction and to place theoretical constraints on the temperature anomalies, dimensions, and fluxes of the Icelandic and Galapagos plumes. In Chapter 1 we analyze bathymetric and gravity anomalies along the hotspot-influenced Galapagos Spreading Center. We find that the Galapagos plume generates along-axis bathymetric and mantle-Bouguer gravity anomalies (MBA) that extend 〉500 km east and west of the Galapagos Islands. The along-axis MBA becomes increasingly negative towards the plume center, reaching a minimum of ~-90 mGal near 91°W, and axial topography shallows by ~1.1 km toward the plume. These variations in MBA and bathymetry are attributed to the combined effects of crustal thickening and anomalously low mantle densities, both of which are due to a mantle temperature anomaly imposed beneath the ridge by the Galapagos plume. Passive mantle flow models predict a temperature anomaly of 50±25°C is sufficient to produce the 2-4 km excess crust required to explain the along-axis anomalies. 70-75% of the along-axis bathymetric and MBA variations are estimated to arise from the crust with the remaining 25-30% generated by the anomalously hot, thus low-density mantle. Along Cocos-plate isochrons, bathymetric and MBA variations increase with increasing isochron age, suggesting the subaxial mantle temperature anomaly was greater in the past when the plume was closer, to the ridge axis. In addition to the Galapagos plume-ridge system, in Chapter 2 we examine alongisochron bathymetric and MBA variations at four other plume-ridge systems associated with the Iceland, Azores, Easter and Tristan hotspots. We show that residual bathymetry (up to 4.7 km) and mantle-Bouguer gravity anomalies (up to -340 mGal) are greatest at on-axis plumes and decreases with increasing ridge-hotspot separation distance, until becoming insignificant at a plume-ridge separation of ~500 km. Along-isochron widths of bathymetric anomalies (up to 2700 km) decrease with increasing paleo-spreading rate, reflecting the extent to which plume material flows along-axis before being swept away by the spreading lithosphere. Scaling arguments suggest an average ridgeward plume flux of -2.2x106 km/my. Assuming that the amplitudes of the MBA and bathymetric anomalies reflect crustal thickness and mantle density variations, passive mantle flow models predict maximum subaxial mantle temperature anomalies to be 150-225°C for ridge-center plumes, which decrease as the ridges migrate away from the plumes. The dynamics of mantle flow and melting at ridge-centered plumes are investigated in Chapters 3 using three-dimensional, variable-viscosity, numerical models. Three buoyancy sources are examined: temperature, melt depletion, and melt retention. The width W to which a plume spreads along a ridge axis depends on plume volume flux Q, full spreading rate U, buoyancy number B = (QΔρg)/(48η0U2), and ambient/plume viscosity contrast ϒ according to W=2.37(Q/U)l/2(Bϒ)0.04. Thermal buoyancy is first order in controlling along-axis plume spreading while latent heat loss due to melting, and depletion and retention buoyancy forces contribute second order effects. Two end-member models of the Iceland-Mid-Atlantic Ridge (MAR) system are examined. The first endmember model has a broad plume source of radius 300 km, temperature anomaly of 75°C, and volume flux of 1.2xl07 km3/my. The second model has a narrower plume source of radius 60 km, temperature anomaly of l70°C, and flux of 2.1 x106 km3/my. The first model predicts successfully the observed crustal thickness, topographic, and MBA variations along the MAR, but the second model requires substantial along-axis melt transport in order to explain the observed along-axis variations in crustal thickness, bathymetry, and gravity. We favor this second model because it predicts a mantle P-wave velocity reduction in the plume of ~2% as consistent with recent seismic observations beneath Iceland. Finally in Chapter 4 we use three-dimensional numerical models to investigate the interaction of plumes and migrating midocean ridges. Scaling laws of axial plume spreading width Ware derived first for stationary ridges and off-axis plumes, which yield results consistent with those obtained from independent studies of Ribe [1996]. Wand the maximum plume-ridge interaction distance Xmax again scale with (Q/U)l/2 as in the case of ridge-centered plumes and increase with ϒ and buoyancy number. In the case of a migrating ridge, Xmax is reduced when a ridge migrates toward the plume due to excess drag of the faster-moving leading plate, and enhanced when a ridge migrates away from the plume due to reduced drag of the slower-moving trailing plate. Thermal erosion of the lithospheric boundary layer by the previously ridge-centered plume further enhances Wand Xmax but to a degree that is secondary to the differential migration rates of the two plates. Model predictions are compared with observed along-isochron bathymetric and MBA variations at the Galapagos plume-ridge system. The anomaly amplitudes and widths, as well as the increase in anomaly amplitude with age are predicted with a plume source temperature anomaly of 80-120°C, radius of 80-100 km, and volume flux of 4.5x106 km3/m.y. Our numerical models also predict crustal production rates of the Galapagos Islands consistent with those estimated independently using the observed island topography. Predictions of the geochemical signature of the plume along the present-day ridge suggest that mixing between the plume and ambient mantle sources is unlikely to occur in the asthenosphere or shallow crust, but most likely deeper in the mantle possibly by entrainment of ambient mantle as the plume ascends through the depleted portion of the mantle from its deep source reservoir.

Description: Submitted in partial fulfillment of the requirements for the degree of Doctor of Philosophy at the Massachusetts Institute of Technology and the Woods Hole Oceanographic Institution March 1997

Description: The formation of new oceanic crust is the result of a complex geodynamic system in which mantle rises beneath spreading centers and undergoes decompression melting. The melt segregates from the matrix and is focused to the rise axis, where it is eventually intruded and/or erupted to form the oceanic crust. This thesis combines surface observations with laboratory studies and geodynamic modeling to study this crustal-production system. Quantitative modeling of the crustal and mantle contributions to the axial gravity and topography observed at the East Pacific Rise shows that the retained melt fraction in the mantle is small (〈3%) and is focused into a narrow column extending up to 70 km beneath the ridge axis. Consistent with geochemical constraints, the extraction of melt from the mantle therefore appears to be efficiently focus melt toward the ridge axis. A combination of laboratory and numerical studies are used to constrain the pattern of mantle flow beneath highly-segmented ridges. Even when the buoyant component of mantle flow is constrained to be two-dimensional, laboratory studies show that a segmented ridge will drive three-dimensional mantle upwelling. However, using reasonable mantle parameters in numerical models, it is difficult to induce large-amplitude three-dimensional mantle upwelling at the relatively short wavelengths of individual segments (~50 km). Instead, a simple model of three-dimensional melt migration shows that the observed segment-scale variations in crustal thickness can be explained by focusing of melt as it upwells through a more two-dimensional mantle flow field. At the Reykjanes Ridge, the melt appears to accumulate in small crustal magma chambers, before erupting in small batches to form numerous overlapping hummocky lava flows and small volcanoes. This suggests that crustal accretion, particularly at slow-spreading centers, may be a highly discontinuous process. Long-wavelength variations in crustal accretion may be dominated by variations in mantle upwelling while short-wavelength, segment-scale variations are more likely controlled by a complex three-dimensional processes of melt extraction and magma eruption.

Description: During my first three years in the Joint Program, I was supported by an National Science Foundation Graduate Student Fellowship. Other support has been derived from National Science Foundation grants OCE-9296017, OCE-9224738, OCE-9215544, and EAR grant 93-07400.