ACCRETE Project Paleomagnetic Studies;
Report of 1996 Field Season

From May 25 to June 9, 1996, a paleomagnetic sampling crew from the University of Arizona left "smokin' holes" along Portland Canal which forms the international border between British Columbia and southeast Alaska. Bob Butler and George Gehrels were joined by Graduate Student Julie Libarkin and Undergraduate Students Brian Darby and Matt Spurlin aboard the Barbara K, a 52' twin-diesel motor launch chartered out of Kethikan for this and several past field seasons. The Barbara K was captained by Rick Matthews and provided our floating "base camp" for the paleomagnetic sampling of Cretaceous and Cenozoic granitic rocks of the Coast Mountains. In part thanks to unusually favorable weather and calm seas, we obtained a much larger than expected collection of samples; ³8 oriented paleomagnetic core samples were drilled and collected from each of 136 sites on a transect across the Coast Mountains. We managed this due in significant measure to Julie becoming a strong driller, aided by those sessions in the Bear Down Gym weight room. We hope that our paleomagnetic research on these samples will yield data critical to solution of the current "tilt versus translation" debate regarding the origin on the Coast Mountains and adjacent areas of British Columbia and southeast Alaska.

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One of the most contentious tectonic issues about the North America Cordillera is whether the Cretaceous plutons of the Coast Mountains Batholith travelled from mid-Cretaceous paleolatitudes ³3000 km to the south OR instead post-mid-Cretaceous northward motion was limited to several hundred kilometers of northward displacement (May 5, 1995 issue of Science). The far-travelled hypothesis is referred to as the "Baja B.C. Hypothesis" because the proposed mid-Cretaceous paleogeographic location of much of British Columbia is in the area of present-day Baja California. The major impetus for the Baja B.C. Hypothesis comes from paleomagnetic data obtained from granitic rocks. Previous paleomagnetic studies conducted near and south of Prince Rupert, B.C. revealed "discordant" paleomagnetic directions from Cretaceous plutons west of the Coast Shear Zone. These directions do not match those predicted for this area during Cretaceous time and the discordance indicates that a major tectonic disruption has affected these rocks of the North American continental margin. A major ambiguity encountered in interpretation of these paleomagnetic directions is that paleohorizontal at the time of magnetization cannot be easily determined in intrusive igneous rocks. Thus there are two competing interpretations. Either these Cretaceous granites have experienced (1) systematic NE-side-up tilting of Cretaceous plutons during their uplift history (the "tilt" interpretation) or; (2) ~3000 km northwards translation from lower mid-Cretaceous paleolatitudes accompanied by clockwise vertical-axis rotation (the Baja B.C. Hypothesis = "translation" interpretation). A constraint on interpretations of the paleomagnetism of the Cretaceous plutons is provided by paleomagnetic directions observed from Paleocene and Eocene plutons east of the Coast Shear Zone. These data suggest that the major tectonic disturbance responsible for paleomagnetic discordance in the older plutons (either NE-side-up tilting or latitudinal transport) occurred between mid-Cretaceous and Eocene time and is located on or near the Coast Shear Zone. The existing paleomagnetic data were collected more than 20 years ago when methods of paleomagnetic research were much less rigorous than they are now. One of the objectives of our research is to test the regional significance of this pattern of paleomagnetic observations in the Coast Mountains of southeast Alaska and western B.C..

Our initial target for the 1996 field season was the mid-Cretaceous Dundas Island Pluton which comprises numerous islands west of the entrance to Portland Canal. It was here that our good luck with weather was most important because these exposures are quite open to the Pacific and can be impossible to access in poor weather. The logistics of geological field work in southeast Alaska require the use of inflatable, Zodiak-style boats for access to shoreline exposures which are essentially the only exposures due to the thick, rain-forest vegetation. (Ketchikan has an average annual rainfall of about 150 inches!) Fortunately, George Gehrels has many years experience with this style of field work, having done numerous geological mapping and U/Pb geochronologic studies in the area starting with his dissertation research while at Cal Tech. We had a clear skies, no wind, and flat seas for the first five days. I have a tradition of taking a swim in the ocean (~40¡F) after we return to the Barbara K following a day of paleomag drilling and collecting. My evening swim around the boat gained added incentive when the crew began demanding that I swim an additional lap for each successive day of sunshine. I couldn't quite manage this after the fifth day after which we had some days of rain, but still very nice weather by southeast Alaska standards. Because Dundas Island is a valued fishing and hunting area for the coastal indians of this area, Wayne Ryan joined us as a native observer for the Lax Kw'alaams Band. After several days accompanying us during our field work and evenings on the Barbara K, Captain Rick asked Wayne: "So Wayne, are most of the white people you know as crazy as these Arizona geologists?" After pondering this question for at least a full minute, Wayne replied: "Yep, pretty much." Our only disappointment in this early phase of the field work was the lack of a catch in our crab pot and the disinterest which the salmon displayed for the herring on our hooks.

When we completed sampling of the Dundas Island pluton, we headed up Portland Canal sampling Paleocene and Eocene plutons on the east side of the Coast Shear Zone. This deformation zone extends some 800 km parallel to the Coast Mountains orogen and has the aspect ratio of a shoelace. The Coast Shear Zone is a high temperature ductile shear zone which has been interpreted as: (1) a major strike-slip fault system that contributed to the northward translation of the western terranes; (2) a suture zone formed by orthogonal collision between the Insular and Intermontane Superterranes; and (3) locus of post accretion tilting of crustal blocks to the west containing the mid-Cretaceous plutons with discordant paleomagnetic directions. During our days sampling near the Coast Shear Zone, the crew arranged a surprise early 50th birthday party for me (I'd like to try to convince you that this 50th birthday party was early by say 10 years, but nobody seems to buy that line). A good time was had by all and I'm pretty sure we set a southeast Alaska tag-team record for killing a bottle of Pusser's rum.

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In the last week of field work, we collected samples from younger Cenozoic plutons at the eastern extreme of Portland Canal near Stewart, B.C. and Hyder, Alaska. We also struck it rich on crabbing and had several dinners of fresh Dungeness crab. Matt proved to be the gormand, putting away two or three whole carbs at one sitting. While anchored at Maple Bay, we had a chance meeting with Carol Evenchick of the Canadian Geological Survey who was also staying in this bay where she had pitched her tent on a float. She was doing early season geologic mapping on the British Columbia side of Portland Canal with her field assistant in preparation for field work in the high country later in the summer. Carol joined us for a last dinner at Maple Bay after which we talked Coast Mountains and Intermontane geology with maps spread around the Barbara K salon.

When we completed our paleomagnetic sampling and headed back for Ketchikan, Brian and Matt stayed on in Knat Cove near the entrance to Portland Canal for three weeks of geological mapping. A fairly nasty storm with high, but trailing seas in Dixon Entrance chased us back to Ketchikan. I headed back to Oregon, where I was on sabbatical leave last semester, a couple of days later. Julie spent a couple of days backpacking outside Ketchikan before heading south. George spent the balance of the summer in various locations in southeast Alaska with his wife Jennifer and three children aboard their small boat, the Wildcat.

Returning to the scientific issues, the UA paleomagnetic study of the Coast Mountains is part of the ACCRETE Project. This multidisciplinary research project is funded by the Continental Dynamics Program of the National Science Foundation and is coordinated by Linc Hollister of Princeton University and Maria Crawford of Bryn Mawr College. Other investigators are from Beloit College, Purdue University, Lamont Doherty Earth Observatory, Virginia Polytechic Institute, and the University of Wyoming. During September, 1994, a reflection seismic experiment was performed across the Queen Charlotte fault system (present plate boundary between the Pacific and North American plates), past Dundas Island just north of Prince Rupert, B.C., then up the full length of Portland Canal to Stewart, B.C.. This seismic experiment was designed to image the crustal and upper mantle structure of the Coast Mountains Batholith located along the suture zone formed by the collision of the Insular and Intermontane Superterranes. Our paleomagnetic studies are part of the second phase of the ACCRETE Project which is "An Integrated Study of Continental Growth at a Covergent to Transpressive Plate Margin" focused on the Coast Mountains orogen of British Columbia and southeast Alaska. In conjunction with the ACCRETE seismic, petrologic, structural, and geochronologic studies of these plutonic rocks which will provide independent control on the emplacement, cooling and uplift history, the current paleomagnetic and rock-magnetic studies may yield observations critical to solution of the "tilt versus translation" debate.

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