Tectonic Evolution of the Chugach Terrane, Shumagin Islands, Alaska
What: The overall project is focused on the tectonic evolution of the Campanian-Eocene Chugach-Prince William (CPW) terrane in southern Alaska. This project has several distinct objectives that include: 1) understanding the regional depositional setting and source for of the CPW flysch; 2) understanding the intrusive and thermal history of this belt. The 2012 project has a major goal of collecting and analyzing some of the westernmost rocks in this belt in the Shumagin Islands.
When: June 15-July 12
Where: This project will be in Alaska for fieldwork and then in Minnesota for laboratory work. In the field, the project will include the following locations:  orientation at the University of Alaska (Anchorage) and local field trips;  field work on the Kenai Peninsula in and around Seward;  travel to Sand Point, 4) work on Nagi island in the Shumagin Islands  return Anchorage.
Our fieldwork will target two different areas in southern Alaska: [A] Seward (southern Kenai Peninsula). The first area (A on Fig. 1) will focus on the Campanian Maastrictian Valdez Group, and plutons of the Sanak-Baranof plutonic belt (62-50 Ma). [B] Nagi Island. The second area (B on Fig. 1) is located in the Shumagin Formation, which is also intruded by granitic rocks of the Sanak-Baranof plutonic belt. These sites have been chosen to dovetail will current ongoing work in western Prince William Sound and Kodiak Island.
Who: Four Students and Professors Cameron Davidson (Carleton College) and John Garver (Union College).
Project overview and goals
This four-student project is focused on the tectonic evolution of the Chugach-Prince William terrane in south central Alaska. This thick accretionary complex is dominated by Campanian-Paleocene trench fill turbidites that were likely derived from the uplift and exhumation of terranes to the south in the North American Cordillera and then arrived in Alaska after coast-parallel translation. Near-trench plutons of the Sanak-Baranof belt and a younger suite of plutons provide distinctive thermal events and could be key indicators of plate position between 61 and 34 Ma. Two study areas will be: [a] rocks of the Valdez Group in the Seward on the Kenai Peninsula; and [b] along-strike correlative rocks of the Shumagin Formation of Nagi Island (photo above). Student projects will focus on the thermal evolution of these rocks, provenance including U/Pb dating of detrital zircon, and the geochemistry and geochronology of near-trench plutons.
This project will allow students to work in units that are classic in Cordilleran tectonics, and the results will directly feed into ideas of terrane translation and development of the Cordilleran tectonic collage. The project has broader significance because the translation, intrusive history, and exhumation of the CPW is directly related to deposition in flanking basins, including the hydrocarbon rich Cook Inlet basin.
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 abundant 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). Throughout much of the 2200 km long belt of the CPW terrane, it is bounded by the offshore modern accretionary complex of the Alaskan margin, but east of Prince William Sound the Yakutat block (yellow-green on map below) is colliding into the CPW and this young collision has significantly affected uplift and exhumation of inboard rocks.
Map of southern Alaska showing the distribution of rocks in the Chugach Prince William terrane (green) and the Yakutak terrane (yellow-green), which is colliding with Alaska. The two primary study areas are indicated on Nagi Island (in the Shumagins – B) and around Seward on the Kenai Peninsula (A).
Most of the Chugach and Prince William terrane is comprised of deformed 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 ophiolites of Resurrection Bay and Knight Island) that provide important clues about the nature and location of adjacent oceanic lithosphere. In the study area the primary units of this thick flysch facies are the Campanian-Maastrictian Valdez Group (Seward), and its western equivalent the Shumagin Formation.
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 on Sanak Island) and getting progressively younger to the east (50 Ma on Baranof Island) (Bradley et al., 200; 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 with the youngest exhumation ages east of Prince William Sound in the St. Elias Range (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 remains 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, 1982; 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 Complex (CMBC) that intrudes the Wrangellia composite terrane (WCT) 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. 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.
The paleomagnetic data on rocks of the Chugach-Prince William terrane suggest long transport (see Coe et al, 1985, Cowan, 2003, and Gallen 2008 for reviews). Recent reanalysis of the Ghost Rocks on Kodaik Island suggest that they formed at a latitude of ~41°N +8°/-7° and were subsequently translated northward by >1500 km since the Paleocene (Gallen, 2008). Thus the focus of this project is on the thermal history and provenance of the very thick rocks of the CPW accretionary terrane that were intruded by near trench plutons and then translated along the North American margin in the Tertiary. The source rocks may lie far to the south.
Potential Student Projects
- Exhumation of the CPW. A key to understanding the evolution of the CPW is the timing and nature of exhumation revealed through cooling ages of zircon and apatite. The two study sites are ideally situated for thermochronology because their thermal history is poorly known and far enough apart to allow us to see along-strike changes in the orogenic belt. The Chugach-Prince William terrane has a distinct thermal/metamorphic history and a major scientific question that we are trying to address is the timing of metamorphism (prehnite-pumpyllite facies) and subsequent exhumation of the CPW 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 adding to 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 (e.g. Rahl et al., 2005) .
- Provenance of the sandstones and conglomerates of the CPW. A major question is whether the sediments of the CPW flysch are the erosional remnants of the Coast Mountains Batholithic Complex in Southeast Alaska and British Columbia, or perhaps some other source far to the south. U-Pb dating of detrital zircon from the Valdez/Shumagin rocks will provide important insight into the source of these sediments. The sandstone of the Valdez Group (Campanian-Maastrictian) 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 of detrital zircon (Bradley et al., 2009; Amato and Pavlis, 2010). We know that both the Valdez Group and the Orca Group are replete with zircon. Initial studies have suggested that the zircon ages are similar to what we would expect from the Coast Mountains Batholithic Complex (or Coast Plutonic Complex) in BC (Haeussler et al., 2005). We are particularly interested in detrital zircon, because we know that will be successful and will immediately break new ground. However we are also interested in the application of other provenance techniques (Nd isotopes, other varietal studies such as quartz provenance) that would shed light on the nature and age of the dissected volcano-plutonic source region inferred to have shed debris into this basin.
- Tectonic significance of near trench plutons. The Sanak-Baranof plutonic belt is inferred to be related to near trench plutonism that affected these rocks soon after deposition and imbrication in the accretionay complex. The rocks on Nagi Island are not well studied because this is such a remote site. Geochemistry of the plutons, depth of emplacement, better age control, and perhaps paleomagnetism will hopefully reveal the tectonic significance of these rocks. Some have suggested these represent a paleo-triple junction (i.e. Hauessler et al., 2003). We would like to know the overall chemistry and petrology, the depth of emplacement, and the relations of these intrusive rocks to other intrusive rocks in this part of Alaska or to those farther south as they might represent a sort of piercing point along the Border Ranges Fault. Reasonable tectonic reconstructions would allow these rocks to restore to a position in SE Alaska, where rocks of this age are common. Thus we could envision projects that involved geochemistry, petrology, U/Pb dating, and an analysis of potential correlative rocks in SE Alaska.
Fieldwork will be in remote and isolated areas in Alaska that have special logistical challenges – please read this section carefully. Weather will be wet and rainy, so participants must be prepared with complete rain gear and rubber boots and gear for relatively cool temperatures. Our primary mode of transportation will be using Zodiac inflatable boats with 30 hp outboard motors, and most of our fieldwork is at or near sea level. You must be comfortable with camping in remote harsh conditions with no nearby facilities or easy communication with the outside world (no internet or cell coverage); we will have a satellite phone for emergency use only. If you apply to this project keep in mind what we will be doing: Work will be in remote and rough conditions in Alaska. There might be hiking and work in steep terrain with significant elevation, we will be in sites with no electricity/refrigeration, we will be camping and there is a high probability of bear encounters. You might be required to travel in small fixed-wing aircraft, and field studies will be in part in Zodiac inflatable boats in cold marine conditions that require onsite safety training.
We are looking for students with an interest in Tectonics and those with a high degree of comfort in rough outdoor settings. Suggested, but not required are those core courses in the Geology major: Historical Geology, Structure/Tectonics, Stratigraphy, Mineralogy and Petrology. 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 (2012-13) 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 conditions outlined above.
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