Tectonic Evolution of the Flysch of the Chugach Terrane on Baranof Island, Alaska

What: This project focuses on the tectonic evolution of the Chugach-Prince William terrane in Southeast Alaska, and it is a continuation of our 2011, and 2012 projects. This thick accretionary complex is dominated by Campanian-Paleocene (c. 75-55 Ma) trench fill turbidites likely derived from a volcano-plutonic complex. Near-trench plutons of the Sanak-Baranof belt imprinted a distinctive thermal event on these rocks and are a key indicator of plate position between 61-50 Ma. The primary study area for 2013 is the Sitka Graywacke in Sitka, Alaska, and the nearby and presumed metamorphosed equivalent, Baranof Schist in Whale Bay in the South Baranof Wilderness Area. Student projects will be focused on metamorphism and thermal evolution of these rocks, and sedimentary provenance including U/Pb dating of detrital zircon.

When: June 16-July 13

Where: Central and Southeast Alaska, transport through the Yukon. Schedule: [1] Gathering in University of Alaska (Anchorage) and field trips, [2] drive with gear east across Alaska to the Yukon, south to Haines [3] Travel by ferry to Sitka; [4] Whale Bay, Baranof Island; [5] return Anchorage, pack, and depart. Canada travel is part of this effort, so a passport is required.

Who: Four students and project Leaders: John I. Garver (Union) (tectonics, thermochronolgy, sedimentation and tectonics) and Cameron Davidson (Carleton) (metamorphic petrology, structural geology and tectonics).

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, ongoing; 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 Complex (CMBC) 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 2013 field season (2013-14 project) we plan to target two areas on Baranof Island near Sitka with a goal of understanding the relationship between the Sitka Graywacke and the Baranof Schist, and to address the timing of post-metamorphic cooling. The geology of Baranof Island SW of the Neva Straight Fault is our primary target and there are three basic units we need to consider: 1) Sitka Graywacke, 2) Crawfish Inlet Pluton (Eocene); and 3) Baranof Schist. Bedrock in and around Sitka consists of the Sitka Graywacke. Recent detrital zircon U/Pb work (Haeussler et al., 2005) on these prehnite-pumpyllite grade flyschoid rocks suggests that the flysch strata have two different ages. Those rocks to the SW have zircon as young as 72 to 74 Ma (Campanian-Maastrictian or younger), and more inboard rocks have older young ages of around 97-105 Ma (Albian-Cenomanian or younger).

To the south/southeast, the 51 Ma Crawfish Inlet Pluton intrudes these two units of the Sitka Graywacke, and the contact aureole is sharp and narrow (but not well studied). South, southeast of the pluton is the widely exposed Baranof Schist, which is inferred to be the metamorphic correlative of the Sitka Graywacke. The Baranof Schist is relatively well studied, and it provides some key insight into the metamorphic evolution of this area (Loney et al., 1975; Loney and Brew, 1987; Zumsteg et al., 2003).

The most detailed recent study of the Baranof Schist identifies two key metamorphic events that have affected these rocks (M4 and M3; see Zumsteg et al., 2004). The last metamorphic event (M4) is related to contact metamorphism and regional heating by what are inferred to be Eocene near trench plutons (here the Crawfish Inlet Pluton, 51 Ma). This metamorphism is well-dated because it is recognized as contact metamorphism adjacent to the pluton, and it resulted in high-temperature mineral assemblages of sillimanite and andalusite. This late high T metamorphism (M4) appears to overprint a regional burial metamorphism (M3) that resulted in Biotite and Garnet facies metamorphic assemblages that extend 12 and 25 km out from the pluton (and hence are inferred to predate plutonism). This is where the problems start. The biotite to garnet-grade metamorphism is inferred to be driven by regional burial in a subduction wedge, and elsewhere in this area M3 is inferred to be mid-Cretaceous (91-106 Ma) based on dated metamorphic actinolite and sericite. However, these dates are from rocks relatively far away and on the north side of the Nevi Fault and therefore probably do not apply to our proposed study area.

The relationship between the Sitka Graywacke NNW of the Crawfish pluton and the Baranof schist to the SSE of the pluton is not known, but it has long been thought that they are essentially the same, but for some reason different metamorphic levels are exposed (S. Karl, pers. com, 2012). If this is the case, then the Baranof Schist (SSE of pluton) is as young as Campanian-Maastrictian (Upper Cretaceous), and hence the early metamorphism (M3) cannot be mid-Cretaceous in age, but rather must to Late Cretaceous or younger.

Potential Student Projects

  1. 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 (prehnite-pumpyllite facies to sillimanite facies) 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).
  2. Provenance of the sandstones. The sandstone of the Valdez Group (Campanian-Maastrictian), the Shumagin Formation (Campanian-Maastrictian), the Kodiak Formation (Maastrictian) and the Sitka Graywacke (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, and our work in progress). We know that both the Sitaka Graywacke is replete with zircon (Hauessler et al., 2005; Garver, unpublished). Initial studies have suggested that the zircon ages are similar to what we’d expect from the Coast Mountains Batholithic Complex (or Coast Plutonic Complex) in BC (Haeussler et al., 2005). However we’d note that the nature of our field sites would allow for the most complete across and along-strike sampling program so that the temporal changes will be more obvious. We are particularly interested in detrital zircon, because we know that will be successful and will immediately break new ground.
  3. 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 on Baranof Island. 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 SE.

We will be using two 14 ft Zodiac inflatable boats with aluminum floors and 30 hp 4-stroke engines. Whale Bay in the South Baranof Wilderness area is remote and we will rent a Satellite phone for emergencies. A video from 2012 gives you a sense of how this fieldwork is done:

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 around Baranof Island in remote SE Alaska. Weather will be wet, rainy, and cold, so participants must be prepared with complete rain gear and rubber boots 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 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 field studies will be in part in Zodiac inflatable boats in cold marine conditions that require onsite safety training. A valid passport is required for this project because travel is in part in Canada.

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 (2013-14) 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. Student should 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 (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.