The following are abstracts of papers presented orally at national meetings during 1995.
W.P. Clement, I.B. Morozov, J.B. Diebold, S.B. Smithson,
L.S. Hollister, and M.L. Crawford
Corresponding author, William P. Clement: Department of Geology, University of Wyoming, Laramie, WY 82071, phone:307-766-6227, fax:307-766-6679, firstname.lastname@example.org
W. P. Clement, I. B. Morozov, J. B. Diebold, S. B. Smithson, L. S. Hollister, and M. L. Crawford, 1995, ACCRETE -- a comprehensive seismic study of an accretionary continental margin. Abs, IUGG, Denver, July.
Southeast Alaska and northwest British Columbia consist of several terranes accreted to the continental margin. Further inland, the Coast Range Batholith of western British Columbia, the largest, continuous batholith in North America, represents a Paleogene island arc. Separating the accreted terranes from the Coast Range Batholith is the Coast Shear zone. Rocks exposed in the Coast Range Batholith were formed at depths of 20 km or more and are intruded by tabular mafic bodies. Problems concerning crustal formation and accretion can be addressed from detailed, multidisciplinary studies of this area. Specific problems include the dip of the Coast Shear zone at depth, the composition of the lower crust and its relation to batholith genesis, and accretionary processes and their effect on the Moho. To address these problems, a large-scale, controlled source seismic experiment was conducted in waterways along the Alaska - British Columbia border and in southeast Alaska. The experiment consisted of 60 onshore REFTEK three-component seismometers and a 3 km long marine streamer recording energy from a ship-towed air gun array. We present wide-angle and near-vertical seismic data to study the Coast Range Batholith and Coast Shear zone. Events are observed in this high quality data set to distances greater than 200 km. P- and S-wave Moho reflections are easily observed, as well as several intracrustal P- and S-wave events. Fan acquisition geometry enables us to determine the 3-D crustal structure. On the MCS data, a strong reflector between 8 - 10 seconds is potentially the Moho. A deeper event is observed between 16 - 18 seconds. Throughout the MCS data are dipping reflectors marking a possible crustal decollement. Our three-component data provide compressional and shear wave velocities for the crust and determine the presence of seismic anisotropy and our MCS data provide a detailed structural interpretation of this complex region of crustal accretion.
The Geological Framework of the Accrete Seismic Transect
M L Crawford (Geology Department, Bryn Mawr College, Bryn Mawr, PA 19010; 610-526-5111; e-mail: email@example.com) with other ACCRETE participants from University of Arizona, Beloit, University of British Columbia, Bryn Mawr, Columbia, Geological Survey of Canada, Princeton, Purdue, University of Sydney, University of Wyoming, and Virginia Tech
M. L. Crawford and the ACCRETE science team, 1995, The geological framework of the ACCRETE seismic transect. EOS, Transactions, 76, F594.
The ACCRETE transect which crosses the central Coast Mountains orogen follows the maritime border between southeastern Alaska and British Columbia. Relative plate motion for the ACCRETE study area changed from convergent to translational between the mid-Mesozoic and the present. Uplift and erosion along the transect have exposed mid-crustal rocks that record crustal growth by terrane and magmatic accretion. The absence of the upper 15-25 km of the crust and the observed plunge of rocks exposed at the surface to depth along the transect provides a unique opportunity to coordinate seismic and other geophysical studies with directly observed geological features and measured physical properties of deep crustal materials along the transect. A close relationship between deformation and pluton emplacement throughout the orogen recognized by geological studies in the ACCRETE study area has led to conclude that many of the large scale crustal features imaged by our seismic studies result from the fabric imposed on the rocks by high temperature ductile deformation combined with magmatic processes. In addition, this region has experienced igneous activity from the Jurassic to the Recent which provides samples of the deepest crust and upper mantle during orogen evolution. Two periods of voluminous igneous activity resulted in mid-Cretaceous (100-90 Ma) and Paleogene (65-45 Ma) batholiths. The Cretaceous plutons intruded during the convergent tectonic regime whereas the emplacement of the Paleogene plutons coincides with the inferred change from dominantly orthogonal to transpressive plate motion. The over 800 km long steep to vertical high temperature ductile Coast shear zone coincides with a thermal front between the western side of the high temperature rocks of the Paleogene pluton/ country rock gneiss complex and the previously cooled crust of the mid-Cretaceous orogen. Our seismic data imply a Moho offset of about 5 km (down to the east) across the same boundary. Pervasive mid-crustal ductile deformation and igneous activity have tectonically smeared the sutures between accreted terranes posing the challenge of how to recognize terrane boundaries below upper crustal levels.
T Das, Dept. Geology, Princeton University, Princeton, NJ 08544,
J B Diebold, Lamont-Doherty Earth Observatory, Palisades, NY, 10964, firstname.lastname@example.org;
L S Hollister, Dept. Geology, Princeton University, Princeton, NJ 08544, email@example.com
T. Das, J. B. Diebold, and L. S. Hollister, 1995, Enhanced imaging of upper crustal structure, SE Alaska. ibid.
1700 km of 224 channel marine multichannel [MCS] data were shot by R/V EWING over accreted continental terranes in southeast Alaska during the ACCRETE project, 1994. While conventional processing of these data reveals deep crustal structures, including Moho, reflectors in the shallow [0 - 10 km] structure are poorly imaged due to the topographic complexity of the water-sediment and sediment-crust interfaces. When these surfaces undulate with wavelengths shorter than the size of the seismic hydrophone array, traveltimes of reflected arrivals are perturbed, reducing coherence along the simple hyperbolic paths assumed during the imaging procedure. As a result, stacked images are degraded. The magnitude of this degradation is frequency dependent, so that deeper reflections, richer in low frequencies, may be less affected than shallow ones. The shallow section also suffers the most from other deleterious effects, such as water column multiples and diffractions from the sediment-crust interface. Reflector coherence can be improved by applying a systematic set of time shifts, called "statics" corrections. In typical (ie in data shot and recorded on land) these corrections correspond to an earth model obtained from analysis of refracted arrivals. With the ACCRETE data set, the problematic surfaces are imaged directly, and a model constructed from a reflections section which has been migrated to show true surficial geometry. Since such a two dimensional model cannot properly represent a three dimensional surface, residual effects are to be expected. For the most part, however, two dimensional processing removes a large percentage of the incoherence.
I B Morozov, W P Clement, S B Smithson (Department
of Geology and Geophysics, University of Wyoming, P. O. Box 3006, Laramie
WY 82071-3006; 307-766-6227; e-mail: firstname.lastname@example.org)
L Hollister (Department of Geology, Princeton University, Princeton, NJ 08544; 609-258-4106; e-mail: linc@Princeton.EDU)
I. B. Morozov, W. P. Clement, S. B. Smithson, and L. S. Hollister, 1995, ACCRETE: first results of the wide-angle experiment. ibid.
The ACCRETE project is a collaborative endeavor among a group
of geologists and geophysicists to study terrain accretion in the central
segment of the Coast Mountains orogen, on both sides of the border between
SE Alaska and British Columbia. The seismic part of the project included
1) multi-channel near-vertical seismic reflection survey (MCS) and 2) a wide-angle refraction/reflection study using the same airgun shots recorded by nearly 60 3-component REFTEK instruments deployed at about 3-5 km intervals along the fjord of Portland Canal. We present preliminary results of this wide-angle experiment. The quality of data is very good, enabling observations of P- and S-wave Moho reflections on most record sections. Pn is observed at offsets exceeding 120-140 km. P/S converted phases (including those from the Moho) and intracrustal reflections are abundant in some records. As in other similar studies, the reflected signal has a ringy, mixed-phase, character, centered around 8 Hz in frequency domain. Deconvolution of the records helps to improve imaging of the intracrustal events. To remove the effects of the sediment cover of the bottom of the fjord, we apply static corrections obtained from the results of the MCS study. The analysis of common midpoint gathers from the Portland Canal line indicates a subsurface area which is highly refletive to S-waves. Ray tracing and preliminary travel-time tomography (carried out after the spatial resampling of the picked travel times) show the thickening of the crust in the landward direction, and features possibly related to accreted terrain boundaries. Records from the fan line along the coast (Clarence Strait) shows Pn refractions from the region of the Coast Shear. These results show that the acquisition of the ACCRETE "pilot study" was successful, and will substantially contribute to the understanding of tectonic accretion mechanisms, through the cooperation of the many geoscientists involved in this project.
I B Morozov L Vejmelek S B Smithson (Department of Geology and
Geophysics, University of Wyoming, P. O. Box 3006, Laramie WY 82071-3006;
307-766-6227; e-mail: email@example.com)
L Hollister (Department of Geology, Princeton University, Princeton, NJ 08544; 609-258-4106; e-mail:linc@Princeton.EDU)
I. B. Morozov, L. Vejmelek, S. B. Smithson, and L. S. Hollister, 1995, Tomographic imaging of the earth crust along the ACCRETE corridor. ibid, F591.
We present first results of the refraction/reflection wide-angle seismic experiment carried out in SE Alaska in August-September, 1994 as a part of the ACCRETE project. The dataset is very large (air gun shots at 50-100 m intervals in two passes along the 140-km fjord, with 3-component REFTEK recorders deployed at 3-5-km spacing) and generally of a very high quality. Moho P- and S-wave reflections are observed on most record sections, Pn is seen at farther offsets (with Pn crossover at 120-140 km). Intracrustal reflections and P/S converted phases are abundant in some records. We developed a tomographic P-wave velocity model for the main ACCRETE line along the SW-NE trending Portland Canal fjord. For the starting velocity model we used the model obtained previously by the ray tracing of 5 selected shots. To comply with the limitations of the inversion program, picked travel times were spatially resampled at 1-2 km spacing. To remove the effects of the sediment cover of the bottom of the fjord, we use the velocity model of the sediment layer, developed by John Diebold from the results of the MCS study. At the first step of the inversion, we used refracted arrivals to obtain the velocities within the upper 20 km of the crust. After that, Moho reflections were employed to constrain the velocities of the lower crust and Moho depth. The resulting tomographic velocity model shows the thickening of the crust in the landward direction, velocity contrasts, and Moho relief possibly related to the accreted terrain boundaries. The velocity and spatial resolution was analyzed using the singular value decomposition technique. This analysis shows that a more detailed coverage of the top of the crust can be achieved through a more versatile velocity parametrization scheme and modified damping techniques. The velocity model obtained in our present inversion will serve as a basis for a more detailed tomographic imaging using P- and S-waves, and also for the prestack migration. The final P- and S-wave velocity models and stacked images would provide valuable contribution to the achievement of the principal goals of the ACCRETE project.
P T C Hammer, R M Clowes, R M Ellis Dept. of Geophysics and Astronomy, University of British Columbia, Vancouver, CANADA, V6T 1Z4 ph: 604-822-2267; e-mail: firstname.lastname@example.org
P.T.C. Hammer, R.M. Clowes, R.M. Ellis, 1995, ACCRETE Seismic Refraction Profile Across the Coast Orogen of NW British Columbia and SE Alaska: Coast Plutonic Complex to Stikinia. ibid.
The Coast orogen of British Columbia and southeast Alaska contains the suture resulting from the mid-Cretaceous collision between the exotic Insular superterrane (Alexander and Wrangellia terranes) and the previously accreted Intermontane superterrane (Stikine, Taku and Yukon terranes). This geotectonic environment has been targeted by ACCRETE to address fundamental questions concerning continental evolution and dynamics during terrane accretion. One focus of the project is the Coast Plutonic Complex and its eastern transition to the Skeena fold and thrust belt of the Stikine terrane. Understanding the development of the Skeena belt is one of the keys to unravelling the tectonic history of the region.
During September 1994, ACCRETE conducted an onshore/offshore seismic experiment across the Coast Belt. The seismic source was an airgun array (136 l) towed by the R/V Maurice Ewing along a fjord that traverses the Coast Belt. Arrivals were recorded by a land-based array of 60 IRIS/REFTEK 3-component seismographs. Seventeen of these instruments were deployed inline with the shiptrack, extending 105 km northeast from the head of the fjord through the Coast Mountains. The resulting wide-angle dataset provides a densely sampled, inline profile across the plutonic suture into Stikinia. This dataset complements seismic reflection and refraction data collected along the fjord itself, and provides a link between ACCRETE and the LITHOPROBE Slave-Northern Cordillera Lithospheric Evolution transect. Data quality is excellent; numerous phases (e.g., Pg, PmP, Pn, PmS, Sg and Sn) are identifiable and distinct P-arrivals are observed to ranges greater than 250 km. Significant lateral inhomogeneity is observed across the transition between the Coast Belt and Stikinia. Crustal thickness changes of 6.5 km and/or lateral velocity changes of 0.3 km/s are inferred. Velocity structure models and their interpretation are presented.
GSA 1995 Annual Meeting, New Orleans. POSTER
HARRY, Dennis L., Dept. of Geology, The University of Alabama, Box 870338, Tuscaloosa, AL 35487-0338; OLDOW, J.S., and SAWYER, D.S., Department of Geology and iGeophysics, Rice University, P.O. Box 1892, Houston, Texas 77251
A series of finite element models illustrate the role which pre-existing heterogeneities within the lithosphere play during the evolution of orogenic belts. The models indicate that orogen evolution >>is strongly influenced by pre-existing crustal architecture during the first 150 to 200 km of shortening. Deep seated weaknesses (modeled as a region with thickened crust) produce intense strain at lower crustal levels. Crustal thickening at deep levels results in isostatic uplift of the upper crust, producing a topographically high orogen lacking a foreland basin. Shallow weaknesses (modeled by replacing a portion of the crystalline upper crust with weaker sedimentary rock) produce intense deformation near the surface. Crustal thickening at shallow levels creates a topographic load which causes isostatic subsidence and development of a pronounced foreland basin. Models in which crustal provinces of unequal strength are juxtaposed produce doubly vergent deformation zones bounded by oppositely facing retro- and fore-wedges. Many orogens (e.g., the Appalachian and Ouachita orogens) develop on previously rifted margins which involve each of the types of pre-existing heterogeneity described above. These include a thick sedimentary basin and a seaward decrease in crustal thickness near the margin and a lateral transition from dioritic to gabbroic crust at the ocean/continent boundary. Modeling of contraction on such a margin indicates that major decollements develop at midcrustal and lower crustal levels, partitioning deformation into upper crustal, lower crustal, and mantle domains. The magnitude and spatial distribution of strain within each domain are nearly uniform, but differ significantly >>between domains. The loci of maximum strain within the upper and lower crustal domains are laterally offset. As a result, the thickest crust lies continentward of the highest elevations and the site of most intense surface deformation. This pattern of crustal thickness variation is a primary feature of orogenesis, and is not related to post-orogenic collapse. The midcrustal decollement terminates near the leading edge of the thrust front but the deep crustal decollement extends more than 500 km landward, producing 5-10% strain well in front of the orogenic belt. After 150-200 km shortening, sufficient topography develops to cause the balance of compressional stress and gravitational forces to dominate the style of deformation. The orogen thereafter evolves in a manner analogous to a critically tapered wedge, and pre-existing mechanical heterogeneities exert little further influence on orogen growth.
Bringing Geological Research to the General Public: A Case Study
Lincoln S. Hollister, Geology, Princeton University, Princeton, NJ 08544, phone: 609-258-4106, fax:609-258-1274, email@example.com.
Hollister, L. S., 1995, Bringing science education to the general public: a case study. Geological Society of America, Abstracts with Programs, 27, A-92.
In order to obtain permits to do a seismic field experiment in
the fjords of northern British Columbia and southeast Alaska, I had to
convince the local people that shooting an array of airguns every 20 seconds
for 10 days would not harm marine life. The "local people" included Alaska
and British Columbia; the federal governments of the US and Canada; the
towns of Prince Rupert, Ketchikan, and Stewart; a wilderness area; and
three aboriginal tribes. My successful approach consisted of taking the
time to communicate directly with the people; it built upon their curiosity
about their immediate physical surroundings, it addressed their concerns
about natural hazards, and it used their everyday vocabulary to illustrate
how our techniques worked. We succeeded in getting the 30,000 inhabitants
so in favor of our project (called ACCRETE) that the inevitable half dozen
who disapproved of it did not find a constituency.
An important factor contributing to our permitting success was the "discovery" that virtually everyone is curious about their immediate surroundings. One example is the science class I spoke to at the North Coast Tribal Council Education Centre: forty First Nations students kept me two hours with their questions. The teacher used ACCRETE as a vehicle for teaching science during the 1994-1995 school year. When we held a workshop in April, 1995, the teacher brought two of her best students to New York to observe the process of doing science. After their return to British Columbia, she and her students went to the students' home villages to report on the results of the project. To get permission from the First Nations tribal councils, I promised we would return to report on the results. The process of keeping this promise is underway.
Based on my experiences, I strongly recommend that field project leaders go to the local high school and talk to a few science classes about geological research in the area, and return at a later date to describe the results. This will help us realize our goal of bringing science understanding to the general public.
address: Department of Geology
Princeton, New Jersey 08544
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