What: This project focuses on the tectonic evolution of the Chugach-Prince William terrane in south central Alaska, and it is a continuation of our 2011-13 Keck projects. The Chugach-Prince William terrane is a thick accretionary complex dominated by Campanian-Paleocene (c. 75-55 Ma) trench fill turbidites that were likely derived from the uplift and exhumation of terranes to the south in BC and Washington and then arrived in Alaska in the Eocene after coast-parallel translation. Near-trench plutons of the Sanak-Baranof belt imprinted a distinctive thermal event on these rocks and is a key indicator of plate position between 61-50 Ma (Fig. 1). For this project we plan to revisit and build on the results from two of the key field areas we have previously studied with Keck students: [a] Resurrection Bay area near Seward; and [b] Prince William Sound, where the thickest section and most diverse rocks types of the CPW are well exposed. Student projects will be focused on the thermal evolution of these rocks, provenance including U/Pb dating of detrital zircon, sedimentology/stratigraphy of turbidites, and igneous petrology and U/Pb dating of near-trench plutons.Project Overview and Goals: This multi-year Keck project is focused on understanding the tectonic evolution of the Chugach-Prince William terrane in south central Alaska. The Chugach-Prince William (CPW) composite terrane is a Mesozoic-Tertiary accretionary complex that is well exposed for ~2200 km in southern Alaska and is inferred to be one of the thickest accretionary complexes in the world (Plafker et al., 1994; Cowan, 2003). The CPW terrane is bounded to the north by the Border Ranges fault, which shows clear evidence of Tertiary dextral strike slip faulting, and inboard terranes of the Wrangellia composite terrane (Peninsular, Wrangellia, Alexander) (Pavlis, 1982; Cowan, 2003; Roeske et al., 2003). In the west, the southern margin of the CPW terrane is defined by the offshore modern accretionary complex of the Alaskan subduction zone, but east of Prince William Sound the Yakutat block is colliding into the CPW and this young collision has significantly affected uplift and exhumation of inboard rocks (Enkelmann et al., 2010).
Most of the Chugach-Prince William terrane is comprised of imbricated trench-fill turbidites deposited over a relatively short interval of time (Campanian to Paleocene – c. 75-55 Ma) and by some estimates the volume of sediment is between 1-2 million km3 (i.e. Decker, 1980; Sample and Reid, 2003). The turbidites are imbricated with oceanic igneous rocks (pillow basalts and locally full ophiolitic suites of Resurrection Bay and Knight Island) that provide important clues about the nature and location of adjacent oceanic lithosphere. Very soon after imbrication and accretion to the continental margin, rocks of the CPW were intruded by near-trench plutons of the Sanak-Baranof belt (SBB) that has a distinct age progression starting in the west (61 Ma in the Sanak-Shumagin areas) and progressively younger to the east (50 Ma on Baranof Island) (Keck project of Alex Short; Bradley et al., 2003; Haeussler et al., 2003; Kuskey et al., 2003; Farris et al., 2006). Following structural burial and intrusion of the SBB plutons, the entire assemblage was progressively and diachronously exhumed (Enkelmann et al., 2009, 2010).
Paleomagnetic and geologic data indicate that the CPW has experienced significant coast-parallel transport in the Tertiary, although this conclusion is controversial (cf. Cowan, 2003 and Haeussler et al., 2003). The CPW has apparent equivalents to the south, and this geologic match suggests that in the Eocene, the southern part of the Chugach-Prince William terrane was contiguous with the nearly identical Leech River Schist exposed on the southern part of Vancouver Island (Cowan, 2003). The geological implication of this hypothesis is profound yet elegant in the context of the Cordilleran tectonic puzzle: the CPW is the Late Cretaceous to Early Tertiary accretionary complex to the Coast Mountains Batholith (CMB) that intrudes the Wrangellia composite terrane (WCT, or Insular superterrane) and North America. Thus, the CPW is inferred to have accumulated in a flanking trench to the west and then soon thereafter these rocks were accreted to the margin and translated north. This geologic match is elegant because it suggests that the CPW accumulated outboard the Coast Mountains Batholith complex (Gehrels et al., 2009) and that the CPW essentially is the erosional remnants of that orogenic belt. Although elegant, it may be wrong. Soon to be published paleomagnetic data from Kodiak will show paleolatitudes of Paleocene rocks as far south as California (Housen and Roeske, unpublished), hence we need to search far and wide for candidate source rocks. Thus the focus of our effort is on the very thick rocks of the CPW accretionary terrane that were intruded by near trench plutons and then translated some controversial distance along the North American margin in the early Tertiary.
For the 2014 field season (2014-15 project) we plan to target two areas in the CPW. The first near Seward contains rocks of the Maastrichtian Valdez group and Paleocene-Eocene Orca Group. Emplaced into these rocks is the Paleocence Resurrection Ophiolite that appears to be a tectonic slice of 57 Ma oceanic crust and it’s sedimentary cover (Pettettie, 2012 Keck project). In the 2013 field season, we collected samples along the western shore of Resurrection Bay to test how far west this cover sequence extends (Keck project of Brian Frett, ongoing). As part of this work we visited an extensive band of deformed igneous rocks, presumably part of the Sanak-Baranof belt of plutons; these were unusual because they have sub-solidus s-c fabrics suggesting a major structure might cut through this area. Our goal this field season will be to determine the extent of this presumed shear zone, as well as to sample the Valdez Group (?) flysch in Aialik Bay. The second target is eastern Prince William Sound near Cordova, which is mapped as Orca Group. This area is dominated by flysch with interbedded conglomerates and mafic volcanic rocks and intruded by plutons and small stocks of the Sanak-Baranof belt. Hilbert-Wolf and Carlson (2011 Keck Project) showed that there are distinct packages (belts) of the Orca flysch in western Prince William Sound that share similar depositional ages and thermal histories. One of the main goals for this year will be to work out how (or if) these belts project into eastern Prince William Sound.
- Exhumation of the Chugach terrane. The Chugach terrane has a distinct thermal/metamorphic history (i.e. Vrolijk et al., 1988; Dusel-Bacon et al., 1993; Weinberger and Sisson, 2003) A major scientific question that we are trying to address is the timing of metamorphism and subsequent exhumation of the terrane (i.e. Garver et al., 2010). Our preliminary data from reset fission tracks in radiation-damaged grains indicates a profound west to east progression in cooling that occurs between about 55 and 25 Ma. We are interested in furthering this data set with student projects aimed at documenting the time-temperature history of these rocks using fission-track, helium, or Ar/Ar dating. We are also interested in projects that address the temperature history of shale and/or sandstones using illite crystallinity (XRD) VR, fluid inclusions, or carbon/graphic thermometry (Raman).
- Provenance of the sandstones. Sandstone of the Valdez Group (Campanian-Maastrictian) and Orca Group (Paleocene-Eocene) are relatively well studied by traditional analysis (i.e. sandstone compositions as in Zuffa et al., 1980). Only recently have workers started to look at U/Pb and Hf isotope geochemistry of detrital zircon (Bradley et al., 2009; Amato and Pavlis, 2010, and our work in progress). Our work thus far from over 2000 km along strike of the CPW show that detrital zircon ages are similar to what we’d expect from the Coast Mountains Batholith Complex (or Coast Plutonic Complex) in BC (Gehrels et al., 2009), and there also appear to be some links to southern Laurentia. Our work this summer will fill in two important gaps in our growing database of U/Pb and Hf isotope data from the CPW terrane.
- Tectonic significance of the Sanak Baranof plutonic belt. The Sanak-Baranof belt (SBB) is a distinctive suite of time-transgressive near-trench granitic intrusions (Bradley et al., 2003); the oldest ages are to the west on Sanak and Shumagin islands (61 Ma) and they progressively young to Baranof Island (50 Ma) (Bradley et al., 2003; Kusky et al., 2003; Farris et al., 2006). The origin of these near-trench plutons has been well-studied and many workers conclude that this plutonic belt was generated by the interaction of a spreading ridge and subduction zone along the western North American margin (Hudson,1983; Sisson et al., 2003; Bradley et al., 2003; Kusky et al., 2003). We are interested in finding out more about these plutons that are well exposed and poorly known in Kenai Fjords National Park, and near Cordova in eastern Prince William Sound. We would like to know the overall chemistry and petrology, the depth of emplacement, and the relations of these plutons to other intrusives in this part of Alaska or farther to the south. Thus we could envision projects that involved geochemistry, petrology, U/Pb dating, and an analysis of potential correlative rocks to the W and SE.
Working Conditions: Fieldwork will be in remote and isolated areas in Alaska that have special logistical challenges – please read this section carefully. We will use boats for fieldwork in Kenai Fjords National Park and eastern Prince William Sound. Weather will be wet, rainy, and cold, so participants must be prepared with complete rain gear, rubber boots (Xtra tuffs) and gear for SE Alaska. You must be comfortable with camping in remote harsh conditions with no nearby facilities or communication with the outside world (no internet or cell coverage); we will have a satellite phone for emergency use only. Personal music devices (i.e. iPod or equivalent) are prohibited in the field while in bear country. There will be required hiking and work in steep terrain with significant elevation, we will be camping with no electricity/refrigeration, and there is a high probability of bear encounters. Certain dietary restrictions may be accommodated, but many essential meals will be based on fish. You might be required to travel in small fixed-wing aircraft, and most of our field sites will be accessed using 14 ft Zodiac inflatable boats with aluminum floors and 30 hp 4-stroke engines. A video from 2013 gives you a sense of how this fieldwork is done:
Recommended Courses/Prerequisites: We are looking for rising seniors with an interest in Tectonics and those with a high degree of comfort in rough outdoor settings. To be accepted on this program you must be a rising senior who will complete course credit for a senior thesis (or equivalent) in the following year (2014-15) as part of this research effort. Suggested, but not required are those core courses in the Geology major: Historical Geology, Structure/Tectonics, Stratigraphy, Mineralogy and Petrology. Students should also have completed key cognate courses in Chemistry. Experience at a field camp or in a field geology course is strongly recommended but not required. Work from this project must carry over into the senior year (2014-15) as a required senior thesis (or equivalent) and the supporting letter from the on campus advisor must indicate how a thesis requirement figures into the senior year course load. Helpful, but not required in the letter from the on campus advisor is an indication of how well the applicant will function considering the special field conditions outlined above.
When: June 18-July 12 (tentative)
Where: 1) Meet in Anchorage, stay at the University of Alaska, local field trips, 2) drive to Seward, stay in Seward and/or camp somewhere in Kenai Fjords National Park (Ailik Bay), 3) take ferry to Cordova, stay in Cordova and/or camp somewhere in eastern Prince William Sound, 4) return Anchorage, pack, and depart.
Who: Six students and two project leaders: John I. Garver (Union) (tectonics, thermochronolgy, sedimentation and tectonics) and Cameron Davidson (Carleton) (metamorphic petrology, structural geology and tectonics).