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Project HOTSPOT: Scientific Drilling of the Snake River Plain Held Its Inaugural Workshop in Twin Falls, Idaho, on May 18-21, 2006

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Project HOTSPOT: Scientific Drilling of the Snake River Plain Held Its Inaugural Workshop in Twin Falls, Idaho, on May 18-21, 2006 Above For Official Use Only ICDP Proposal Cover Workshop PreliminarySheet Full New Revised Addendum Please tick or fill out information in all gray boxes Title: Proponent(s): Keywords: Location: (5 or less) Contact Information: Contact Person: Department: Organization: Address: Tel.: Fax: E-mail: Permission to post abstract on ICDP Web site: Yes No Abstract: (400 words or less) 1 / 2 Scientific Objectives: (250 words or less) Summary of Support Requested from ICDP Requested Estimated Total Project Budget ICDP funds: (ICDP funds plus other sources): (in US$) Planned Estimated Duration in Month Start: (On-site operations only): Requested Drill Engineering Operational (Please contact ICDPs Operational Support Support: Group if required) Downhole Logging (Please contact ICDPs OSG if required) Field Lab Equipment (Please contact ICDPs OSG if required) Training Course (Please contact ICDPs OSG if required) Details such as a Budget Plan, Management Plan, and Drilling Plan to be provided as attachment to the Proposal. OSG contact: U. Harms ([email protected]), Phone: +49 331 288 1085 2 / 2 HOTSPOT: The Snake River Scientific Drilling Project Tracking the Yellowstone Hotspot Through Space and Time Introduction Mantle plumes are thought to play a crucial role in the Earth’s thermal and tectonic evolution. They have long been implicated in the rifting and breakup of continents, and plume-derived melts play a significant role in the creation and modification of sub-continental mantle lithosphere. Much of our understanding of mantle plumes comes from plume tracks in oceanic lithosphere, but oceanic lithosphere is recycled back into the mantle by subduction, so if we are to understand plume-related volcanism prior to 200 Ma, we must learn how plume-derived magmas interact with continental lithosphere, and how this interaction effects the chemical and isotopic composition of lavas that erupt on the surface and of the lithosphere. Hotspot volcanism in oceanic lithosphere has been the subject of focused recent and ongoing studies by the Hawaii Drilling Project, the Reykjanes Drilling Project and IODP. These studies will provide base-line information about where mantle plumes originate, how they behave under oceans, and the volcanic products of these processes (DePaolo & Manga 2003). However, hotspot volcanism within continental lithosphere has not been studied in such detail, and is potentially more complex (e.g., Burov et al, 2007). The Yellowstone-Snake River Plain (YSRP) volcanic province, which began ≈17 Ma under eastern Oregon and northern Nevada and is currently under the Yellowstone Plateau, is the world’s best modern example of a time-transgressive hotspot track beneath continental crust (Fig. 1). Recently, a 100 km wide thermal anomaly has been imaged by seismic tomography to depths of over 500 km beneath the Yellowstone Plateau (Fig 1c; Yuan & Dueker, 2005; Waite et al 2006). The Yellowstone Plateau volcanic field consists largely of rhyolite lavas and ignimbrites, with few mantle-derived basalts (Christiansen 2001). In contrast, the Snake River Plain (SRP), which represents the track of the Yellowstone hotspot, consists of rhyolite caldera complexes that herald the onset of plume-related volcanism and basalts that are compositionally similar to ocean island basalts like Hawaii (Pierce et al 2002). The SRP preserves a record of volcanic activity that spans over 16 Ma and is still active today, with basalts as young as 200 ka in the west and 2 ka in the east. Thus, the Snake River volcanic province represents the world-class example of active time- transgressive intra-continental plume volcanism. The SRP is unique because it is young and relatively undisturbed tectonically, and because it contains a complete record of volcanic activity associated with passage of the hotspot. This complete volcanic record can only be sampled by drilling. In addition to this complete record of hotspot volcanism, the western SRP rift basin preserves an unparalleled deep-water lacustrine archive of paleoclimate evolution in western North America during the late Neogene. Continental Volcanism and The Mantle Plume Paradigm Recent studies suggest that continental flood basalts and associated linear volcanic trends of basalt and rhyolite form from deep mantle plumes. The basalts have trace element characteristics similar to ocean island basalts, and are commonly more iron-rich than normal mid-ocean ridge basalts. Although plumes were originally thought to consist of thin vertical tails that feed a bulbous plume head (e.g. Olson 1990), recent numerical models suggest that deep mantle plumes may have complex geometries that are not continuous from top to bottom, and may be tilted by flow of the asthenosphere (Farnetani & Samuel 2005; King 2007). Seismic tomography confirms these predictions for the Yellowstone plume, which can be imaged as deep as 600 km and appears to dip some 65º WNW (Yuan & Dueker 2005; Waite et al 2006; Fig 1c). A range of non-plume models have been proposed for the Yellowstone-Snake River Plain volcanic system. These models include a propagating rift (Christiansen and McKee, 1978), edge-driven convection (King 1 HOTSPOT: The Snake River Scientific Drilling Project and Anderson 1995; King 2007) and a convective roll or hotline driven by self-sustaining convection (Humphreys et al, 2000). All of these models imply that the source of basaltic magmatism is shallow asthenosphere that underlies the lithosphere. Since shallow asthenosphere is the source of mid-ocean ridge basalts (MORB), which are typically depleted in incompatible trace elements, this source is inconsistent with the geochemistry of the observed basalts, which resemble ocean island basalts in their major and trace element geochemistry (e.g., Vetter and Shervais, 1992; Hughes et al 2002; Shervais et al 2006). Further, none of the non-plume models predicts the sudden outpouring of flood basalt in < 1 million years or the time transgressive progression of silicic eruptive complexes – which represent the influx of huge volumes of mafic magma into the crust, now represented by subcrustal and midcrustal sill complexes (e.g., Peng and Humphreys, 1998). The cratonic edge effect model attempts to address the CRBG flood basalt province but do not explain the time transgressive Snake River Plain. In contrast, the propogating rift and convective roll (“hotline”) models specifically address the ESRP, but do not explain the CRBG flood basalts. Recently, the concept of mantle plumes as thermally or compositionally distinct entities has been challenged (e.g. Anderson 2001; Christiansen et al 2002; Foulger & Natland 2003; Foulger and Jurdy 2007; Foulger et al 2004). The contention arises partly from a lack of seismic tomographs that clearly demonstrate deep sources of mantle plumes, equivocal isotopic evidence for mantle plumes, and numerical models which do not support the upwelling of deep mantle in narrow conduits. Proponents of the plume model cite new tomographic studies that clearly image mantle plumes, including Yellowstone (Montelli et al 2004, 2006) and numerical models that show complex geometries for mantle plumes (King, 2007; Farnetani & Samuel 2005). The large volumes of magma erupted over short time periods in large igneous provinces, high eruption rates in some plume tails (e.g., Hawaii), large geoid anomalies (+15 m under Yellowstone; Fig. 1a), and high 3He/4He ratios (which may reflect the outgassing of unfractionated primordial mantle) also support the plume model (e.g., DePaolo & Manga, 2003). Thus, the mantle plume paradigm is the best available, although the extent, thermal flux, and depth of origin of plumes may vary. The drilling proposed here will provide additional evidence with which to further test and refine the mantle plume hypothesis. Regional Setting: The Snake River Volcanic Province The Neogene Snake River volcanic province can be divided into three provinces: the older western province, comprising the Owyhee Plateau and western SRP, the transitional central SRP, and the younger NE-trending eastern SRP (which lies generally parallel to North America plate motion; Fig 1b). The ESRP, a topographic depression that cuts across Basin and Range structures, is characterized by a thin carapace of basalt (100m-1500m) that overlies rhyolite volcanics and tuffaceous sediments extending to depths >3000 m (Champion et al 2002; Geist et al 2002a,b; Hughes et al 2002). The eastern SRP is underlain by a 10 km thick mid-crustal sill complex that has been imaged seismically that represents layered magma chambers where the basalts fractionate (Peng & Humphries, 1998; Shervais et al 2006). In the older western province, the Owyhee Plateau is a highland underlain by rhyolite and basalt, whereas the western SRP is a NW-trending graben bounded by en-echelon normal faults exposing rhyolite eruptives, and filled with up to 4 km of basalt and sediment (Wood & Clemens 2002; Shervais et al 2002). Large epicontinental lakes deposited several km of Miocene-Pleistocene sediments, which are both overlain and underlain by basalt (Shervais et al 2002). The western province is underlain by a mid-crustal mafic sill similar to that imaged under the eastern plain (Hill and Pakiser 1967), even though it lies north of the projected hotspot track based on reconstructions of North American plate motion (Gripp and Gordon, 1990, 2002). The central SRP represents a critical transition from the broad western province to the well-defined eastern province, but it has received comparatively little
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