Dr. Robert Butler

Northward Transport of Terranes Debate

Although Dickinson, Gehrels, and I think the debate about post- mid-Cretaceous northward transport of Cordilleran terranes should be over and all should agree that we won the war, others seem unwilling to give up the Baja British Columbia and allied hypotheses. George Gehrels and I developed a theme session on post-mid-Cretaceous northward transport of Cordilleran terranes for the National GSA Meeting in Seattle. Our session eventually got incorporated with a similar one proposed by Paul Umhoefer and Ted Irving and developed into a lively half-day session at GSA.

We are continuing research on this topic through an NSF-funded project on the paleomagnetism and detrital zircons of Gravina Belt Rocks of southeast Alaska. This is a collaborative project with Dave Bazard of the University of Mississippi. During May, 1994, we spent two weeks doing field work via boats and helicopters in the Juneau region. Through George's thorough geologic knowledge of the area and no small measure of good luck with weather and logistics, we collected paleomagnetic samples from over 50 sites in Early Cretaceous Gravina Belt volcanics. Preliminary data indicate that we should be able to accurately determine the paleolatitude and directly address the magnitude of post-mid- Cretaceous northward transport of this outboard portion of the North American Cordillera. Preliminary indications are that the amount of northwards transport indicated is much less than called for in the Baja British Columbia hypothesis. We look forward to force feeding this data set to Ted Irving and the gang.

The Alexander Terrane, Southeast Alaska

This project is now in the wrap up stage of compiling and writing up results. We have two papers submitted for publication on the paleomagnetic and detrital zircon results. I plan to finish another large manuscript on the detailed paleomagnetic results before I leave for sabbatical this summer. Our main conclusions are:
1. Age populations of detrital zircons are inconsistent with placement of the Alexander terrane in proximity to the North American Cordillera during Early Devonian time. Possible paleopositions include the paleo-Pacific margin of Australia and the Scandinavian portion of Baltica. The paleomagnetic data from the Early Devonian Karheen Formation indicate a paleolatitude of 14¡±4¡ (north or south) which is also consistent with these Australian and the Baltica paleopositions. Based on the paleontological data, as interpreted by Connie Soja, we favor the Baltica paleoposition for the Alexander Terrane in mid-Paleozoic time.
2. Paleomagnetic data from many Paleozoic units (Ordovician Descon, Mississippian Peratrovich, Pennsylvanian Klawock) indicate chemical remagnetization which is remarkably similar to the widespread remagnetization of Paleozoic Appalachian carbonates. These formations do not retain a primary magnetization and cannot be used for determination of original paleolatitude. This is an important point which we must carefully document because Van der Voo et al. (1980) based major paleogeographic conclusions on paleomagnetic data from these rocks. I have followed up the paleomagnetic analyses with much rock-magnetic work at the Institute for Rock Magnetism at the University of Minnesota. This work documents that the bimodal magnetite grain-size distribution and unusual hysteresis parameters characteristic of remagnetized Paleozoic Appalachian carbonates are also present in the Alexander terrane carbonates.

The Bolivian Project

Our NSF-funded project in Bolivia involves paleomagnetic study of mid-Paleozoic strata, with particular emphasis on Silurian and Devonian red sediments. Most of the paleomagnetic work on the Silurian and Devonian of Gondwana has been done on Australian or African rocks with little work done on South American rocks. The Eastern Cordillera of Bolivia, and to a lesser extent the Subandean region, contains abundant outcrops of Silurian and Devonian rocks which are prime paleomagnetic targets. During July and August, 1994, Thierry Sempere, Larry Marshall, David Richards, and I spent five weeks sampling mid-Paleozoic units within the Eastern Cordillera and Subandean zones between Cochabamba and Bermejo. We returned with samples collected from >100 paleomagnetic sites. Processing of this large collection will guide us in our follow-up field season during 1995. During this and a preceding trip, we also collected samples from the Paleocene Santa Lucia Fm, which is widespread throughout the Eastern Cordillera. Previous paleomagnetic results indicate that this formation has recorded an interesting pattern of vertical-axis tectonic rotations which are related to the development of the Bolivian Orocline. During spring, 1995, David Richards and I wrote a proposal to the Petroleum Research Fund for funding of a post-doctoral position for David to undertake a thorough paleomagnetic study of the Santa Lucia Fm. This proposal has just recently been approved, and David will commence his appointment in January, 1995.

New Cryogenic Magnetometer

I received an equipment grant from NSF for acquisition of a new cryogenic magnetometer and funds for major upgrade of our thermal demagnetization equipment. Due to dumb luck in the scheduling of manufacturing and Gayle Zizzo's twisting of arms to force the university administrative offices to move faster than usual, we took delivery on our new magnetometer in September, 1994. The new system has a helium recirculation and refrigeration system which allows the instrument to operate for at least 400 days on 50 liters of liquid helium. Our old magnetometer required weekly fills with 30 liters of liquid helium for an annual cost of >$10,000. This operating expense is now a thing of the past. With much help from Steve Sorenson, Andrew Pappanikolas, Dave Steinke, and Ben McElhaney, the new instrument is now fully operational. The noise and drifts levels are extraordinarily low. Except for a few software bugs yet to be worked out, the new instrument is a dream to operate and is quickly making up for the substantial down time necessitated for its installation.