1998 Karsten/Klein
Northern Chile Ridge/Valdivia Fracture Zone Program R/V Melville Cruise Goals: This cruise represents two field programs with distinct goals that have been combined into a single expedition: Northern Chile Ridge Study (Karsten/Martinez). This part of the program will map and sample the northern Chile Ridge, between the Chile F.Z. (~36¡S) and the western end of the Valdivia F.Z. (~41¡S). The primary objective of this study is to document the thermal effects of large, long-lived transform offsets on the tectonic and magmatic processes operating at a "transform dominated", intermediate rate spreading center. Theoretical models of ridge axis thermal structure near transform offsets, where the ridge axis is juxtaposed against older, colder lithosphere, predict that systematic relationships should be observed between transform offset age and parameters such as axial depth, crustal thickness, and lava composition. Previous examination of these predictions, using global comparisons of a few well-characterized ridge-transform intersections, have demonstrated a qualitative relationship between axial depth and offset age. Variations in crustal thickness and magma composition associated with transform fault effects have been less systematic and more ambiguous. Part of the problem with these earlier studies is that it has been difficult to separate out the effects of transform faults from the influence of other tectonic variables (e.g., spreading rate, regional depth anomaly, mantle temperature gradients) changing in the system. We propose to re-examine the predicted relationship between axial depth and morphology and transform offset age by mapping ~550 km of the northern Chile Ridge, where the only tectonic variable changing is the segment length-transform offset geometry. In addition, we propose to extend the inquiry by obtaining shipboard gravity and magnetics data and dredged rock samples, which will allow us to investigate variations in axial depth, inferred crustal thickness and structure, tectonic fabric and magma chemistry as a function of offset age and ridge-transform geometry. The results of this study will ultimately contribute to the refinement of theoretical models relating axial morphology and crustal accretion variables, as well as contribute to on-going characterization of extensive unmapped regions of the mid-ocean ridge system. We will conduct a SeaBeam 2000 bathymetry/side-scan, gravity and magnetics survey of the ridge axis out to the Brunhes-Matuyama boundary, followed by rock dredging/wax coring with 5-10 km spacing along axis. Sample spacing will be densest within 30 km of ridge-transform intersections, where the greatest thermal effect is expected. The northern Chile Ridge is an ideal site for investigating relationships between ridge axis structure and thermal effects of large transform offsets for the following reasons: 1) There are at least six first-order ridge segments in this poorly mapped region, bounded by long-lived transform offsets with ages ranging between ~2 Ma and ~6 Ma (plus one ~15 Ma offset). First-order ridge segment lengths vary between ~35 and ~150 km, providing an opportunity to investigate whether ridge axis behavior varies systematically as a function of offset/length ratio. 2) All other parameters which might affect the ridge axis thermal structure (e.g., variable spreading rate, regional thermal anomalies or gradients associated with mantle hot/cold spots) do not change over the study area, thereby allowing a very controlled investigation of the transform fault effect to be undertaken. 3) Theoretical models indicate that ridge morphology at intermediate spreading rate ridges, such as the Chile Ridge, is very sensitive to small changes in ridge axis thermal structure, so there is a high probability of observable changes. 4) Complications of mantle outcrops found at slow spreading ridges or "over-shooting" intrusions in the transform domain found at faster spreading ridges should be minimized at intermediate spreading rates. 5) The moderate latitudes (i.e., calmer seas) of the proposed study area will allow us to make the high quality shipboard gravity measurements necessary to resolve small gradients along short ridge segments. Valdivia Fracture Zone Study (Klein/Karsten). This study will map and sample the small, deep, stable, intra-transform spreading centers of the Valdivia Fracture Zone (VFZ). This proposed work is an outgrowth of our work on the Southern Chile Ridge near its intersection with the Chile Trench, south of the target. Our previous results showed that the southern Chile Ridge is erupting an unusually diverse and in many respects geochemically unique suite of lavas. One type of enrichment displays geochemical characteristics that are unlike MORB sampled previously, extending toward compositions more commonly associated with arc or back-arc lavas. It is tantalizing to consider that there is a relationship between this highly unusual lava chemistry and the peculiar tectonic setting of ridge subduction; that is, that the contaminant for this endmember may derive from the adjacent subduction zone. While previous studies have identified a recycled component in the sub-oceanic mantle, such contaminants are presumed to represent ancient subduction and deep recycling. In contrast, the Chile Ridge may represent an example of more recent and shallow contamination from the adjacent subduction zone. Thus, one of our goals is to determine whether the unique geochemical signature that we observed along two of the four ridge segments of the southern Chile Ridge is a spatially extensive phenomenon that persists to the central Chile Ridge or whether it occurs in close proximity to the site of ridge subduction. To explore this, we will focus on the short (~25 km), stable intra-transform spreading centers of the VFZ system because this setting is most likely to erupt a diversity of magma compositions representative of the diversity of mantle compositions. A second goal of our project is to determine the spatial extent of the Indian Ocean/Dupal isotopic signature we identified at several sites on the northern end of the Southern Chile Ridge. The Dupal anomaly has been described as a globe-encircling belt of anomalous ocean island and ocean ridge basalt compositions, centered at ~30-40¡S. It has long been recognized, however, that available data left a huge gap in that belt in the eastern Pacific, occupying several thousand kilometers between Dupal highs. Thus, the central Chile Ridge data will provide a valuable gap-closing data set. A third goal of this program is to extend the depth range of previously sampled MORB from the Pacific. Previous studies have shown marked distinctions in major element compositions between Atlantic and Pacific MORB; due to the limited depth range of most Pacific ridges, it is not known if this difference is independent of extents and pressures of melting, reflecting fundamental difference in major element source composition. Our sampling program will extend the Pacific ridge depth range to ~5000 m.