Jill L. Karsten

Evolution of porosity and seismic structure of upper oceanic crust: Importance of aspect ratios

Seismic properties of the uppermost igneous crust of the oceans are dominated by porosity effects, that is, the size, concentration, and shape of void spaces. Porosity is initially determined by the physics of extrusion (does an eruption form breccia, pillows, or massive flows?) but is very rapidly modified by alteration and hydrothermal deposition. Laboratory data provide insight into compressional wave velocity-porosity behavior of basalts at a hand sample scale, while well logs provide data at outcrop scale. Relating observations at all scales to porosity structure and extrapolating to seismic scale requires application of rock physics theory. Using information from ophiolites and deep ocean cores, we have defined rock physics parameters for two simple models of upper oceanic crust. The models approximate different levels of void filling by alteration products by differing in the amount of crack (low aspect ratio) porosity they contain. From the models we compute theoretical compressional wave velocity and porosity profiles. Calculated profiles agree well with both well logs and seismic data and illustrate that the increase in seismic velocities measured seismically in the upper crust need not be accompanied by large changes in total porosity.

On the cause of the asymmetric distribution of seamounts about the Juan de Fuca ridge: ridge-crest migration over a heterogeneous asthenosphere

Abstract The distribution of non-hotspot seamounts in the northeast Pacific is highly asymmetric; small seamount chains and isolated edifices are numerous on the Pacific plate, but nearly absent on the Juan de Fuca plate. We propose a hypothesis for the asymmetric generation of seamounts near a ridge axis in which upwelling and early melting of upper mantle heterogeneities occurin advance of a spreading centre which migrates with respect to the asthenospheric frame of reference. Plate motion solutions indicate that the Juan de Fuca ridge is migrating to the west at a ridge-perpendicular rate of 20 mm a−1, which is large compared to the half-spreading rate of 30 mm a−1. Migration of the ridge axis will result in the initiation of upwelling in the upper mantle in advance of the spreading centre. If slightly enriched, lower melting temperature heterogeneities are present in the upper mantle, they will intersect their solidus at a greater depth and will begin to melt earlier than the host peridotite. Seamount volcanism will occur preferentially on the Pacific plate because of two factors: Firstly, more asthenosphere and hence more early-melt heterogeneities must ascend to supply the Pacific plate which is translating more rapidly than the relatively stationary Juan de Fuca plate. Secondly, the asthenosphere that is required to ascend to supply the thickening of the Juan de Fuca plate will have been flushed of its significant shallow-level heterogeneities by the previous advance and passing of the ridge; few should remain to cause volcanism on the Juan de Fuca plate. Testable physical and petrologic predictions of this model can be identified. For example, within a small seamount chain, the age difference between individual seamounts and the crust on which they lie will decrease toward the ridge axis. Variation in lava chemistry along a small seamount chain, which is interpreted to represent magmatism from a single large heterogeneity, will provide the major discriminant of this hypothesis. Systematic variations should be observed, with earlier formed edifices being constructed of more enriched lavas, and later (younger) ones of more depleted lavas. Isotope ratios should be unaffected by varying degrees of partial melting, but could display increased dilution towards the ridge axis as melting of the host mantle and magma mixing become significant.

Detailed geomorphology and neotectonics of the Endeavour Segment, Juan de Fuca Ridge: New results from Seabeam swath mapping

A 36-km-wide corridor of Seabeam bathymetry was collected on the Juan de Fuca Ridge, north of 48°00′N, in May 1983. These data, merged with previous Seabeam bathymetric compilations for the Cobb Offset, as well as with Sea MARC I and Sea MARC II side-scan sonar, seismic-reflection, magnetometer, and deep-tow photography data, have been used to identify morphological variations along the spreading center of the Endeavour Segment of the Juan de Fuca Ridge (between the Cobb Offset and Sovanco Fracture Zone). The Endeavour Segment consists of two ridge sections, separated by the 13-km-wide Endeavour Offset, which formed when spreading jumped from Middle Valley to West Valley < 200,000 yr ago. The locus of spreading occurs along the entire Endeavour Segment as a narrow (1–2 km wide), shallow (10–30 m deep) inner rift, which is superimposed upon larger structures that range from broad (5–10 km wide), deep (as much as 3,000 m) median valleys to narrow (5 km wide), shallow (2,100 m) volcanic ridges. An abrupt discontinuity in the tectonic fabric occurs at about 48°05′N. South of this latitude, spreading occurs within an axial high (Endeavour Ridge), which gradually deepens to the south (South Endeavour Valley) as it approaches the zone of overlap with the propagating rift at the Cobb Offset. North of 48°05′N, the axis is characterized by broad, deep, median valleys (North Endeavour Valley and West Valley). These large-scale morphological variations reflect a complex interplay of the spreading center with seamount volcanism, waning magma supply at the distal ends of ridge axes, thermal contrasts across ridge offsets, propagating rifts, and an unstable triple junction. The pronounced axial deep of West Valley is interpreted as reflecting the youth of the structure (< 200,000 yr) and may be primarily due to collapse, with little extension and magmatism. The rift axis jump to West Valley fortuitously isolated Endeavour Seamount, the youngest edifice in the Heck Seamount chain, from the Pacific plate and trapped it between a pair of overlapping spreading centers at the newly formed Endeavour Offset. Development of a remarkably simple overlapping conjugate rift pair at Endeavour Offset has resulted, in spite of the complex and highly variable crustal thickness, magma supply, and pre-existing structural grain. Rifting in South West Valley has caused pre-existing topography generated at the Endeavour Ridge to be destroyed by subsidence and burial, thereby creating the apparent discontinuity in morphology north of 48°05′N.

Subduction zone geochemical characteristics in ocean ridge basalts from the southern Chile Ridge: Implications of modern ridge subduction systems for the Archean

The southern portion of the Chile Ridge is one of few sites where active subduction of a spreading center and its consequences for ridge axis magmatism can be investigated. New major element, trace element, and isotopic data for lavas recovered from the ridge axis between 43 °S and 46 °20′S of the southern Chile Ridge have revealed a suite of mid-ocean ridge basalts which possess typical major element variations, but diverse and sometimes unusual trace element characteristics. For several Chile Ridge lavas, key trace element ratios, such as RbCs, CePb, NbU, LaTa, HfTh and NbLa, extend well outside the fields for normal MORB or ocean island basalts and have values more commonly associated with arc volcanics and continental crust. This hybrid mixture between MORB-like major elements and arc-like trace element signatures has only previously been seen in back-arc basins, and is considered to primarily reflect contamination of a depleted MORB source mantle with slab-derived components. Along the southern Chile Ridge, contamination with slab components is occurring in advance of the subduction zone, possibly as a result of slab break-up or shearing in conjunction with subduction of young, buoyant lithosphere, and subsequent entrainment of these slab components into the sub-ridge mantle. Interestingly, many Archean greenstone basalts share the unusual hybrid MORB-arc geochemical characteristics found along the southern Chile Ridge. On the basis of theoretical modeling, it has been suggested that the mantle was hotter, plate motions were more rapid and ridge-trench interactions were more frequent during the Archean. Although use of geochemical signatures to discriminate tectonic setting must be approached with caution, the observed geochemical affinity of modern lavas from the southern Chile Ridge and some Archean greenstone lavas lends support to the idea that ridge subduction may have been an important mechanism in the formation of Archean greenstone basalts.

Age constraints on crustal recycling to the mantle beneath the southern Chile Ridge: He‐Pb‐Sr‐Nd isotope systematics

Basalts from the four southernmost segments of the subducting Chile Ridge (numbered 1-4 stepping away from the trench) display large variations in Sr, Nd, Pb, and He isotope and trace element compositions. Klein and Karsten [1995] showed that segments 1 and 3 display clear trace element evidence for recycled material in their source (e.g., low Ce/Pb). The uniformly mid-ocean ridge basalt (MORB)-like 3 He/ 4 He and modest variations in Pb, Sr, and Nd isotopes of segment I (nearest the trench) suggest recent (<20 Ma) introduction of a contaminant into its source, consistent with recycling of material from the adjacent subduction zone. In contrast, segment 3 lavas display a dramatic southward increase in enrichment, extending to highly radiogenic Pb and Sr isotopic compositions (e.g., 206 Pb/ 204 Pb = 19.5) and the lowest 3 He/ 4 He yet measured in MORB (3.5RA). The segment 3 variations are most readily explained by ancient (∼2 Ga) recycling of terrigenous sediment and altered crust, but we cannot rule out more recent recycling of material derived from a distant continental source. The similarity in isotopic signatures of segment 4 lavas to Indian Ocean MORB extends the Dupal anomaly to the Chile Ridge. Like Indian Ocean MORB, the segment 4 isotopic variations are consistent with contamination by anciently recycled pelagic sediment and altered crust and require a complex history involving at least three stages of evolution and possibly a more recent enrichment event. Southern Chile Ridge MORB reflect the extensive degree of heterogeneity that is introduced into the depleted upper mantle by diverse processes associated with recycling. These heterogeneities occur on a scale of ∼50-100 km, corresponding to transform- and propagating-rift-bounded segmentation, and attest to the presence of distinct chemical domains in the mantle often bounded by surficial tectonic features that maintain their integrity on the scale sampled by melting.

A geological and geophysical investigation of Baby Bare, locus of a ridge flank hydrothermal system in the Cascadia Basin

Baby Bare is one of three small basement outcrops on the eastern, sedimentburied Juan de Fuca Ridge flank that have localized heat loss and fluid movement within 3.5 Ma oceanic crust. Low-temperature (25°C) hydrothermal vents near the summit of Baby Bare represent the highest-temperature occurrence of off-axis hydrothermal activity found in oceanic crust older than 1 million years. This site has been investigated with seismic reflection profiling, towed-camera surveys, and an Alvin dive series that included heat flow measurements to document the detailed geological setting of these off-axis vents. A new geologic map based on visual observations of Baby Bare shows that the distribution of rock, sediment, and biota appears to be controlled by seafloor slope and elevation, while specific vent locations are controlled by faulting and occur only in areas of thin or no sediment cover. Alvin heat flow data indicate that conductive heat loss from the edifice is ∼4.5 times greater than that from the sediment-blanketed area around the outcrops. Although the outcrop is generally conical, seismic reflection profiles reveal that the sediment-buried portions of the edifice have an asymmetric morphology, strongly suggesting that Baby Bare is a volcano built upon a preexisting, fault-generated abyssal hill. This evidence, combined with previously published petrologic data and results of Ocean Drilling Program Leg 168 drilling, is consistent with the hypothesis that Baby Bare formed by off-axis volcanism rather than at the adjacent ridge axis; sediment thickness and fossil assemblages indicate that it could be as young as 2.7 Ma. Off-axis volcanoes such as Baby Bare increase the overall roughness of basement topography and thus delay complete sediment burial during normal lithospheric aging, particularly in areas where near-axis sediment accumulation is rapid. Partially buried seamounts play an important role in focusing hydrothermal exchange between the oceans and young oceanic crust and, if Baby Bare is representative, may contribute as much as 85% of the heat flux from a sedimented ridge flank.

Petrogenesis of axial lavas from the southern Chile Ridge: Major element constraints

We present major element glass data for 163 rock samples collected from four ridge segments of the southern Chile Ridge between the Chiloe Fracture Zone and the Chile Margin Triple Junction, including the segment currently being subducted at the Chile Trench (segment 1). The subridge mantle is heterogeneous at small spatial scales. Normal mid-ocean ridge basalts (N-MORB), recovered from all four ridge segments, have experienced variable extents of low pressure fractionation but have been generated by relatively uniform extents (F) and initial pressures (Po) of melting of a slightly heterogeneous depleted source. Type 1 E-MORB, found only on segment 4, have trace element affinities to some ocean island basalts, display a large range of major element variations at constant and high MgO, and are spatially associated with N-MORB. Type 2 E-MORB have trace element affinities with suprasubduction zone settings. They are found at two segment 1 sites and along most of segment 3. In order to minimize fractionation and source heterogeneity effects and assess melting conditions, E-MORB compositions were double-backtracked to 8 wt % MgO and a K/Ti ratio of 0.1. Although the magnitudes of F and Po are model-dependent, we find that N-MORB and both types of E-MORB were generated under similar melting conditions. These observations indicate that spreading rate and mantle temperature exert primary control on the southern Chile Ridge thermal regime. We see no influence of ridge subduction on the major element systematics and melting conditions of segments closest to the trench.

Constraints on rift propagation history at the cobb offset, juan de Fuca Ridge, from numerical modeling of tectonic fabric

Abstract Rift propagation has been proposed as the mechanism for the reorganization of the Juan de Fuca Ridge during the Cenozoic. In particular, magnetic anomalies in the Cobb Offset region of this ridge have been interpreted in terms of a complex “dueling propagator” history, in which rift propagation has occurred alternately in opposing directions on adjacent ridge segments. Although the overall propagation history has been inferred from the magnetic data, these data are limited in their resolution for the detailed history by the length of time during which magnetic polarity remains constant. We thus examine whether more detailed resolution within polarity chrons can be obtained using seafloor tectonic fabric from acoustic imagery. By modeling lineations as isochrons, synthetic fabric can be generated for a given rift propagation history and the fit of the model to the data can be compared numerically. For the Cobb, we used a propagation history model with a given timing and direction of propagation, and generated synthetic fabric for different propagation rates. The fabric is best fit for propagation rates between 400 and 900 mm/yr. Slower ( 900 mm/yr) propagation rates yield slightly poorer fits. The fabric results are thus in accord with the interpretation, based on magnetics alone, that the rates of rift propagation significantly exceeded the spreading rates. An interesting feature of the model is that the complex tectonic fabric is reasonably well fit on this scale ( ~ 1 km) by simple rift propagation models which do not invoke shear zone behavior and thus deviate from rigid plate tectonics.

Scientific Rationale for Establishing Long-Term Ocean Bottom Observatory/Laboratory Systems

The oceanographic community is in a position scientifically and technologically to initiate programs leading to the installation of one or more permanently instrumented observatory/laboratory complexes on submarine spreading centers. The dynamic nature of these systems is well established. Yet, there has been no long term, inter-disciplinary effort focused on specific sites to document rates of change in system components, nor the interactions linking the physical, chemical, and biological processes involved. The ultimate goal of this natural laboratory approach would be to establish, then model, the temporal, and the spatial, co-variation among the active processes involved in generation and aging of 60 percent of the planetary surface. The technological and intellectual stimulation involved in successful implementation of natural seafloor laboratories will provide a new generation of dynamically-based, quantitatively testable models of ocean lithosphere genesis and of the biological and chemical consequences of its formation.