Mantle plumes are thought to be responsible for creating ocean archipelagos including Hawaii, Easter, and Galápagos. Similarly, mid-ocean ridges are an essential driving force in plate tectonics. The Galápagos Archipelago is one of the few places on the planet where mantle plumes and mid-ocean ridges interact, providing a rare opportunity to improve our understanding of Earth's important dynamic processes. This research develops a multi-disciplinary perspective on plume-ridge magmatic and tectonic interaction using geophysical data and geochemical analysis of rocks collected from the seafloor. Work includes carrying out a detailed bathymetric and gravity survey of the North Galapagos seafloor. Deep-sea camera traverses and rock dredging also provide essential data to resolve seafloor structural fabric and information about the crust and upper mantle. Geochemical analyses of collected lavas provide essential information for determining mechanisms by which plume material is transported between the archipelago and the mid-ocean ridge. This project is a fully integrated research and teaching collaboration between undergraduate (Colgate University) and research institutions (Woods Hole Oceanographic Institution, University of Idaho). Undergraduates participate fully in the project including the cruise and subsequent extended research projects during which they develop cutting edge analytical skills and present their work at international conferences. Broader impacts also include post-doctoral fellow support and training of students from Ecuador and marine biologists from the Charles Darwin Research Station, the primary institution responsible for conservation and research in the archipelago. Impacts of the work will assist in ongoing conservation efforts in the Galapagos.
The field work associated with this grant was conducted in Spring 2010 using the SIO ship R/V Melville (MV1007 cruise May-June 2010). All geophysical and multibeam data and metadata have been deposited in the UNOLS MGDS database (http://rvdata.us/catalog/MV1007 and www.marine-geo.org/tools/search/entry.php?id=MV1007). Over the past 2 years all the geophysical data that formed the core of the WHOI contribution to this research effort have been processed and reported on, primarily in 2 journal articles. One was published in 2012 and the other has been accepted for publication in a AGU Monograph to be published in late 2012 or early 2013. These data have been used to resolve the regional fabric of the seafloor in the northern Galapagos and deduce crustal and upper mantle structure. S. Adam Soule and Eric Mittelstaedt, a post-doc at WHOI funded through this grant, have been primarily responsible for processing and analysis of the geological and geophysical data sets including multibeam, sidescan sonar, sea-surface magnetics and BGM-3 continuous gravity. The maps and data summary figures have been published in Mittlestaedt et al. (2012) and (in press).The multidisciplinary approach we have undertaken with our collaborators, K. Harpp and D. Geist (at Colgate U. and U. Idaho) have enabled us to assess geodynamic models and significantly advance our understanding of plume-ridge interactions. Multibeam and sidescan data have been processed and integrated within a GIS environment to permit cross-correlation of seamount and prominent structural seafloor features throughout the surveyed area. Figures 1& 2 show the primary results of that integrated mapping effort as published in Mittelstaedt et al. (2012). Figure 3 shows the compiled data resulting from analysis of the multibeam and sidescan sonar to derive structural and volcanic lineament statistics for the surveyed area. Raw gravity measurements collected during the MV1007 cruise were reduced to a free air anomaly (FAA) by applying a 360 s Butterworth filter, removing instrument drift (< 0.1 mGal) and the 1984 reference ellipsoid, correcting for ship motion (Eötvos correction), and, finally, decimating the data to one measurement every 60 s. The final compilation is gridded at a 2 km resolution using the Generic Mapping Tools’ greenspline function with a minimum curvature spline at a tension of 0.2 [Wessel and Smith, 1991] (Figure 4). Additional details regarding gravity data are presented in Mittelstaedt et al., in press). Bathymetry and side scan sonar data reveal significant variations in faulting and volcanism across the study area. Fault populations were placed into three categories: (1) ridge-parallel; (2) transform-parallel; and (3) transform oblique. Categories (1) and (2) are consistent with a standard model of mid-ocean ridge spreading. However, the third set of faults, located in a ~60 km wide swath around the Galápagos Transform Fault, strikes 30° oblique to the trace of the transform and is not commonly observed at other transform faults. This orientation is likely associated with trans-tension across the GTF that is itself oblique to the ridge spreading direction. Volcanism in the study area is distinctly different between the Nazca and Cocos Plates and may indicate a sharp change in melting regime across the GSC. On the Cocos Plate to the north of the GSC, the observed number (219) and volume (70 km3) of seamounts are small. In contrast, on the Nazca Plate to the south of the GSC, there are more than 500 identified seamounts with a total estimated volume of 1178 km3. Seamount volcanism on the Nazca Plate is concentrated within 3 volcanic lineaments, the largest of which is the WDL. Within these lineaments, large, individual seamounts are generally elongate in a direction approximately parallel to the transform-oblique faulting, but the degree of elongation decreases toward the ridge axis. This pattern suggests that the lithospheric stress field plays a role in the pattern of Galápagos lineaments. The tectonic history of the NGVP and the presence of the nearby Galápagos Plume are linked to the creation of the Galápagos lineaments. We propose that creation of the GTF was caused by southward ridge jumps associated with weakening of the near-ridge lithosphere by the plume. Formation of the GTF established a stress field favorable to magma penetration of the inside corner region of the Nazca Plate and melting of the Galápagos Plume supplied magma to construct the lineament volcanoes.
