Faculty Abstracts G-P

Departments:

• GEOLOGY
• MATHEMATICS
• PHYSICS
• PSYCHOLOGY

GEOLOGY

Chronology of Late Pleistocene Landslide-Dammed Lakes along the Rio Grande, White Rock Canyon, Northern New Mexico

David P. Dethier, et al., Professor of Geology

GSA Abstracts with Programs , 26 (6), 60 (1994)

The Rio Grande within White Rock Canyon, northern New Mexico, has incised 200-350 m through Miocene and Pliocene sedimentary rocks, Pliocene basalt, and Quaternary tuff, and massive landslides border the Rio Grande almost continuously for 16 km. Lacustrine sediment deposited within the canyon provides evidence that the Rio Grande has been dammed repeatedly by landslides during the late Pleistocene, producing lakes possibly as deep as 65 m.

The deepest lake is recorded by a 30-m thick sequence of laminated silt, clay, and fine sand, overlying ancestral Rio Grande gravels, exposed in bluffs near Cafiada Ancha. A radiocarbon analysis on charcoal collected from the basal lake sediments indicates an age of >43 ka. The landslide responsible for this damming event is probably located 8 km down river near Water Canyon, although the presumed dam has been heavily modified by later landsliding. Based on the elevation of the lake deposits and associated river gravels, the maximum lake depth is estimated to have been at least 65 m, with a corresponding length of at least 25 km.

Evidence for more recent landslide dams and associated lakes is present near Water Canyon, at the site of at least two toe failures from a large, older landslide complex. Upriver of this landslide complex, abundant remnants of lake deposits have been located, whereas none have been found down river. Laminated sediment plastered on the upstream side of a landslide records one lake probably at least 30 m deep overlying a buried soil dated at <17.4. Rio Grande gravels, river-sculpted basalt boulders, and flood levees along a high outflow channel on this landslide indicate the upper limit for the lake. At lower elevations, horizontally laminated fine sands, silts, and clays, separated by buried soils and overlain by river gravels, indicate at least 3 episodes of lake level rise at <13.6 ka, <13.1 ka, and <12.4 ka. These lakes had estimated depths of at least 15-25 m deep, and correlative deposits are preserved for at least 6 km upriver.

The triggering mechanisms for these landslides are speculative, but the temporal coincidence of the youngest landslide dams with the latest Pleistocene deglaciation suggests a climatic influence, such as destabilization of the toes of older landslides by the Rio Grande during periods of higher glacial discharges.

Wave Refraction as a Tool in Coastal Management: An Example from Hurricane Elena in Pinellas County, Florida

Richard A. Davis, Jr., Univ. of South Florida, William T. Fox, Professor of Geology, Margaret Andronaco, Terra Environmental Services, Inc.

Proceedings of the 1993 National Conference on Beach Preservation Technology, 399-409 (1993)

Coastal zoning typically includes conditions of elevation and distance from the shoreline but makes no mention of wave energy distribution. Even subtle bathymetric irregularities cause changes in the direction of wave approach and therefore in wave energy distribution along the shoreline. Detailed assessment of storm damage along a section of the Florida coast was combined with hindcast wave data and wave refraction to demonstrate the local variation in wave energy that reaches the shoreline. Correlation of storm damage with wave refraction patterns provides good evidence of the importance of including this variable in future management plans. The development of an appropriate data base is straightforward and requires only a desktop computer.

Dichotomous Facies on a Late Cretaceous Rocky Island as Related to Wind and Wave Patterns (Baja California, Mexico)

Markes E. Johnson, Professor of Geology, Marshall L. Hayes '92

Palaios, 8, 385-395 (1993)

Local facies forming the basal Rosario Formation of the Upper Cretaceous in the Erendira area of northern Baja California (Mexico) were deposited in distinctive windward and leeward settings. They are related by a non-conformity to the flanks of an andesite ridge of the Lower Cretaceous Alisitos Formation. Through rapid transgression and erosion in late Cretaceous time, the ridge was reduced to a maximum width of a half-kilometer and its neck was breached by Rosario sediments to form a small near-shore island. One end of the Alisitos outlier is now being truncated by marine erosion so that the original length of the peninsula-island is difficult to determine. It was not less than 1.5 km long, however, based on local geomorphology.

The windward side of the island is dominated by a thick, homogeneous wedge of andesite clasts reduced to small cobbles and pebbles in a limestone matrix. Crustose coralline algae referable to the genus Sporolithon prolifically encrust virtually all the clasts. The rudist bivalve Coralliochama orcutti is the only other abundant organism and it is found in life position cemented to the andesite substrate. Some juvenile echinoids and a rare undetermined type of gastropod also are members of this rudist facies. A thinner but crudely graded wedge of andesite clasts ranging from boulder to pebble size is present on the leeward side of the island. A previously described biota, here referred to as the oyster facies, has a diversity of 16 species including rare Coralliochama. Both the oyster and rudist biofacies thrived on rocky shores.

Rocky-Shore Biotic Associations and Their Fossilization Potential: Isla Requeson (Baja California Sur, Mexico)

Marshall L. Hayes '92, Markes E. Johnson, Professor of Geology, William T. Fox, Professor of Geology

Journal of Coastal Research, 9 (4), 944-957 (1993)

What is the likelihood that biotic associations from a contemporary rocky-shore setting may find their way into the geologic record? Previous studies on fossilization potential have analyzed recent faunas on an inventory basis. This study considers the fate of intact biotic associations arrayed among 30 macro-species of invertebrates and algae on a small island in Concepción Bay, Baja California Sur, Mexico. Field data collected from a network of 15 transects and 66 census stations were subjected to R and Q-mode analyses in order to establish and map windward and leeward facies on the island.

Nearly all the species forming intertidal associations have hard parts which ensure a high probability of fossilization, but 37-63% of the species in the adjoining shallow-subtidal zone have no chance or poor chances of fossilization. Species of Sporolithon and allied red coralline algae in the intertidal and shallow-subtidal zones play a key role by encrusting large surfaces and binding in situ associated fauna such as barnacles and the bivalve Arca pacifica. Burial of leeward rocky shores is already being effected due to the in-filling of a lagoon; future burial of windward rocky shores is possible due to the faulted structure of Concepción Bay. Similar physical scenarios are known from the geologic record and they should afford a rich source of rocky-shore fossils for the study of community evolution.

Lateral Variation in the Geomorphology of a Pleistocene Rocky Coastline at Kalbarri, Western Australia

James H. Scott, Jr. '92, Markes E. Johnson, Professor of Geology

Journal of Coastal Research, 9 (4), 1013-1025 (1993)

The junction of the Indian Ocean with Kalbarri National Park and adjacent town site of Kalbarri in Western Australia exhibits two spectacular rocky coastlines. One is the modern setting with its broad rock platforms and towering sea cliffs eroded in the red Tumblagooda Sandstone (Silurian) capped by pale sandstones and shales (Triassic, Cretaceous). The other is an approximately 100,000 year-old Pleistocene rocky shoreline with a wide variety of deposits preserved unconformably against the Silurian sandstone. Over a short distance of 8 km along the water front, five very different geomorphological features may be observed in a continuum associated with the same ancient rocky coastline. They include: river-mouth bar, cobble pocket beach, intertidal abrasion platform with tidal pools, drowned paleovalley, and neptunian dikes in massive sea cliffs. A typical intertidal fauna with turbinid and patellid gastropods is fossilized. These deposits richly illustrate a range of conditions through which rocky shores are incorporated into the geologic record. They are described herein as a new rock unit named the Chinaman's Rock Member, attributed to the basal Tamala Limestone.

Neotectonica Del Area Loreto-Mulege

Jorge Ledesma Vazquez, Markes E. Johnson, Professor of Geology

Contribuciones a la Tectonica del Occidente de Mexico, 1, 115-122 (1993)

Rocky shoreline facies occur in the Upper Pliocene Infierno Formation from Loreto to Mulege along the western margin of Mexico's Gulf of California in Baja California Sur. Based on these definitive markers of former sea-level position, it is possible to compare the degree of neotectonic deformation and uplift for three different areas spread over a 130 km distance. The southern most area is at Arroyo Arce near Loreto; another is near Rancho Santa Rosaliita in the Bahia Concepción area; the northern most area is at Mulege. Maximum deformation and coastal uplift are shown at Arroyo Arce, where normal-listric faults oriented north by northwest are associated with antithetic faults. Grabens due to listric faults produced topographic gaps in this area that were subsequently filled by the development of antithetic faults. At 175 m above sea level, Pliocene coastal deposits denote a rate of uplift for the Loreto area at or well above the mean rate for the entire Baja peninsula. Farther north, Santa Rosaliita was unaffected by deformation but underwent moderate uplift and Mulege was the least affected area. This variation indicates that the regional effect is not that of a single tectonic block, but rather a group of thinly sliced blocks representing structural and topographic highs and lows. The neotectonic events of this region are associated with extension related to the continuing opening of the Gulf of California from Late Pliocene time onward.

Attracting Students to Science Through Field Exercises in Introductory Geology Courses

Paul Karabinos, Associate Professor of Geology, Heather Stoll '94, William T. Fox, Professor of Geology

Journal of Geological Education, 40, 302 (1992)

Field exercises in introductory geology courses allow students to be active and creative participants in their education. We bring small groups of students into the field and introduce them to the problem we will work on. Students work in groups of three or four, make observations, draw sketches, and take preliminary measurements. Then we regroup and discuss their observations, create a list of questions they would like to answer, and devise methods for answering their questions. They return to work. Then the students present their results in short papers that require them to synthesize what they discovered and to recognize the weaknesses in their data. Although this method is not efficient for transmitting information, it helps students: 1) understand the importance of separating observations from interpretations, 2) appreciate how difficult it is to ask good questions and gather data to answer them, and 3) be creative. The instructor provides context for the problem to be investigated, asks questions to help focus discussion, and offers advice on how to gather data. This approach can form the basis for an entire course or can be integrated with traditional courses.

Applications of Single-Grain Zircon Evaporation Analyses to Detrital Grain Studies and Age Discrimination in Igneous Suites

Paul Karabinos, Associate Professor of Geology, Peter Gromet, Brown University

Geochimica et Cosmochimica Acta, 57, 4257-4267 (1993)

207Pb/206Pb ages determined by direct evaporation of single zircons from metamorphosed and deformed rocks of the New England Appalachians permit an assessment of the applicability of this method to rocks that have been strongly disturbed by orogenic activity. Tests conducted on highly metamorphosed and deformed orthogneisses of known protolith age, whose conventional U-Pb zircon ages show profound post-crystallization discordance, demonstrate that the evaporation method is successful in recovering protolith ages. Uncertainties are ± 10 to 15 m.y. for rocks of Paleozoic age or older.

Preliminary results suggest that the method is particularly useful for provenance studies of sedimentary rocks, and in the discrimination among groups of rocks that are known to differ in age but are otherwise difficult to separate on the basis of field or petrographic characteristics. 207Pb/206Pb ages of detrital zircon grains from metasediments from trial sites in the western and eastern sides of the New England Appalachians reveal the distinct provenances of these rocks. The Missisquoi Formation in southeastern Vermont contains detritus derived from the 1.0 to 1.2 Ga Grenvillian province of Laurentia, whereas such ages are conspicuously absent from the Plainfield Formation in easternmost Connecticut. These results are consistent with the Missisquoi Formation having accumulated peripheral to the Laurentian craton during the early Paleozoic, whereas the Plainfield detritus was supplied by non-Laurentian, possibly West African, sources during the Late Proterozoic or Early Paleozoic. In southern Vermont, the evaporation method successfully distinguishes two groups of augen gneisses with ages of ~960 Ma and ~1120 Ma, respectively, which were first recognized through conventional zircon U-Pb methods.

Collectively, these results indicate that the dating of single zircon grains by the direct evaporation method can be widely exploited in the study of complexly deformed and strongly metamorphosed orogenic belts.

Constraints of the timing of Taconian and Acadian Deformation in Western Massachusetts

Paul Karabinos, Associate Professor of Geology, B. F. Williamson '93

Northeastern Geology, 16, 1-8 (1994)

The Hallockville Pond Gneiss intrudes the Moretown Formation of the Rowe-Hawley belt and contains deformation fabrics comparable in intensity and orientation to those in the surrounding rocks. It is tonalitic in composition and may have formed in a magmatic arc, possibly related to the proposed Shelburne Falls arc of similar age (Karabinos and Tucker, 1992). It intruded before or during the earliest stage of the Taconian orogeny along this segment of the Laurentian continental margin. 207Pb/206Pb single-grain zircon evaporation ratios for this pluton yield an age of 479±8 Ma (weighted average of 4 grains). This age places a lower limit on the timing of the Taconian orogeny and also constrains the undated Moretown Formation to be older than approximately 479 Ma.

The Middlefield Granite intrudes the Rowe and Moretown Formations at their contact. The pluton is foliated parallel to the dominant regional schistosity, but xenoliths contain an older deformation fabric predating their incorporation into the magma and of presumed Taconian age. The 207Pb/206Pb single-grain zircon evaporation age for this pluton is 447±3 Ma (weighted average of 4 grains). Field relations and the age of the Middlefield Granite indicate that if the Rowe-Moretown contact is a fault, as suggested by Stanley and Hatch (1988) and Ratcliffe et al. (1992), then Taconian thrusting in this area ended by 447 Ma.

The Williamsburg Granodiorite, of minimum-melt composition, intrudes Silurian and Devonian formations of the Connecticut Valley Synclinorium, east of the Rowe-Hawley belt, and contains no obvious deformation fabric. The 373 + 5 Ma 207Pb/206Pb zircon evaporation age of the unfoliated Williamsburg Granodiorite (weighted average of five grains), together with a U-Pb zircon age on a strongly deformed sill in the Granville dome of 376 + 4 Ma, tightly constrains the main phase of Acadian deformation in western Massachusetts.

T. Nelson Dale: Diminutive Giant in New England Geology

R. A. Wobus, Professor of Geology

Geological Society of America Abstracts with Programs, 25 (6), A-256 (1993)

Thomas Nelson Dale (1846-1937) was a five-foot-six "dynamo" whose professional career spanned seven decades, four of them with the U.S. Geological Survey. It included a concurrent 9-year teaching position in geology and botany at Williams College that began a century ago (preceded by shorter appointments at Vassar and Drury Colleges), yet he had virtually no formal education beyond preparatory school. Trained largely through field experience in Europe with mentors such as Prof. Karl Zittel of Munich, his university "credentials" consisted of attending, over a period of years, a total of only five geology lectures, plus a four-month residency in mathematics (the latter at Cambridge University).

In 1880 he was hired by Raphael Pumpelly of the USGS to assist in collecting mining statistics. Pumpelly later directed Dale's summer field studies of fossil localities in the metasedimentary rocks of western New England, a project which grew into the mapping of the Mt. Greylock massif, the Rensselaer Plateau, and substantial parts of the Taconic and Green Mountain ranges. This work led to his appointment as Assistant Geologist for the USGS in 1890 and his promotion to Geologist in 1892. His field studies of the complex structures of Western New England and eastern New York, involving over 12,000 miles of hiking and more of driving the primitive mountain roads, resulted in 28 published reports, many with his meticulous drawings of physiography and field relationships. After the turn of the century, he became an expert for the USGS on the sources of slate, granite, and marble in New England and the U.S. and was author of more than 20 reports on these industries. During his long retirement period he lectured frequently on geology and continued to pursue his life-long passion for religion and philosophy.

Geochemistry of Late Stage Alkaline Intrusions of the Pikes Peak Batholith, Colorado

R.A. Wobus, Professor of Geology, et al.

Geological Society of America Abstracts with Programs, 25 (6), A-261 (1993)

Late stage, alkaline intrusions emplaced within the coarse-grained granites of the Pikes Peak batholith (PPB) include two chemical series, a sodic trend (syenites, quartz syenites, fayalite and riebeckite granites) and a potassic trend (granodiorites and fine-grained biotite granites) (Wobus, 1976; Barker et al., 1975). New major and trace element data for 65 samples from these plutons clearly show A- type characteristics (e.g., high FeOT/MgO; Ga/Al+10000 = 2.8 to 6.2; Zr+Ce+Y+Nb = 410 to 2140 ppm). High SiO2 (>70 wt %) PPB granites of both the sodic and potassic trends are geochemically similar to continental epeirogenic uplift granites (after Maniar and Piccoli, 1989), whereas intermediate (60- 70 wt % SiO2) rocks of the sodic and potassic trends exhibit similarities to rift-related and continental epeirogenic uplift granites, respectively. The inability to assign these plutons to a single tectonic setting may be due to the fact that they were emplaced over a time span great enough for the tectonic environment to have shifted (e.g., from continental epeirogenic uplift to continental rifting). It may also be the result of involvement of more than one source composition in the origin of these rocks as suggested by ranges in Zr/Hf (23-52), Zr/Nb (1-24) and Hf/Ta (0.5-10.8). PPB granites and granodiorites have features indicating derivation from continental crust that was recycled via collision, whereas most of the syenites appear to be fractionation products from magmas derived from OIB- like, mantle sources (after Eby, 1992).

Field Relations, Petrography and Geochemistry of Sugarloaf Intrusive Center, Pikes Peak Batholith, Colorado

Rachel J. Beane '93, Reinhard A. Wobus, Professor of Geology

Transactions of the American Geophysical Union, EOS , 74 (43) , 634 (1993)

Sugarloaf is one of seven sodic intrusive centers that lie within or adjacent to the 1080 Ma Pikes Peak batholith (PPB) in central Colorado. Major element and modal analyses from the other six centers have been previously published by Wobus (1976) and Barker (1975). Sugarloaf is composed of fine, medium, coarse and pegmatitic syenite. The syenites lack quartz and are dominated by perthitic feldspar and ferro-richterite amphibole. The fine grained syenite intrudes both the medium and coarse grained syenites, all of which are younger than the coarse red Pikes Peak granite.

Sugarloaf syenites have geochemical trends that typify rocks in the A-type, anorogenic granitoid suite. The syenites have high total alkalis, high FeO(T), low CaO and low MgO concentrations. They are enriched in REE and HFSE. Tectonic discrimination diagrams, based on geochemical characteristics, indicate the syenites were emplaced in a non-collisional environment. The linear arrangement of several of the sodic centers parallel to major Precambrian fault trends, and the classification of the syenites as A1 type granitoids (Eby, 1992) suggest emplacement of Sugarloaf may be rift related.

Trace element variation among the Sugarloaf syenites is controlled by fractional crystallization (FC). FC trends show fine grained syenite is the most chemically evolved sample analyzed. Apatite saturation temperatures for the fine grained syenite are approximately 200o cooler than temperatures for the most primitive sample, a medium grained syenite (961oC). FC trends and apatite temperatures are consistent with field evidence showing fine grained syenite intruded later than medium and coarse grained syenites.

Barker's (1975) assimilation-fractional crystallization model for PPB suggests that the sodic rocks differentiated from a mantle source magma contaminated by reaction melting in the lower crust. Silica variation diagrams of the major oxides show that syenites from five of the PPB intrusive centers are petrogenetically related and represent different stages of chemical evolution. Positive ENd values for Sugarloaf samples document that the syenites were derived from a depleted mantle source. Thus, to date, Barker's model best explains observed major oxide trends and isotopic values for the syenites.

Origin of Interlayered Gabbroic, Dioritic and Granitic Rocks on Mount Desert Island, Maine

Robert A. Wiebe, Franklin and Marshall College, Reinhard A. Wobus, Professor of Geology, Samuel Kozak, Washington & Lee University

Abstracts Volume, Seventh Keck Research Symposium in Geology, 13-16 (1994)

The igneous rocks on Mount Desert Island, Maine, represent a long-studied, classical area of New England plutonic geology (Shaler, 1889; Chapman, 1970; Gilman et al., 1988). The wide variety of rock types, excellent coastal and inland exposures, and beautiful scenery (now largely in Acadia National Park) undoubtedly all contributed to early interest in studying these rocks. C. A. Chapman and many of his students have been largely responsible for the excellent geologic map and the most accessible descriptions of the rocks (Chapman, 1970), and this publication has been revised by Gilman et al. (1988). Unfortunately, because these recent works were largely intended to be guides for park visitors, they do not provide the kind of detail or documentation necessary to establish the petrogenetic relations of the various igneous units. Until now, very little was known about the possible sources, crystallization history, and emplacement of the several plutons, about the relationships between the granitic plutons and the closely associated gabbro-diorites, or about the possible relations between the plutonic and nearby volcanic rocks of roughly comparable age. In terms of the potential for petrogenetic studies, it is surprising that the igneous rocks of Mount Desert Island have not been subjected more recently to detailed studies that integrate field, petrographic, and geochemical data.

The Precambrian Basement of the Florissant Area, Central Colorado

Reinhard A. Wobus, Professor of Geology

Geological Society of America Abstracts with Programs, 26 (6) , 69 (1994)

The Oligocene lake deposits at Florissant lie in a paleovalley incised into the Pikes Peak Granite (PPG), ca. 1080 Ma, within a few km of the western edge of the 3000 km2 Pikes Peak batholith (PPB). A prominent NNW-trending ridge of highly resistant sillimanitic schists and gneisses (ca. 1750 Ma) bounds the PPB just west of the lake beds, and relatively more resistant granite of the 1400 Ma Cripple Creek batholith is in contact with the PPB southwest of the lake deposits. Smaller plutons of gneissic tonalite and granodiorite correlated with the Routt (Boulder Creek) plutonic suite, ca. 1670 Ma, also occur just west of the PPB.

The anorogenic PPB typically consists of reddish, coarse-grained, easily weathered biotite- and hornblende-biotite granite. A fine-grained, often porphyritic variant locally forms a chilled margin along the western edge of the batholith and many small stocks scattered throughout it. Texturally more resistant than the coarse-grained PPG, this rock forms many of the promontories that rise above the extensive Eocene erosion surface developed across most of the batholith. Late sodic plutons of the PPB, such as the syenite stock and fayalite granite ring dikes of the Lake George intrusive center immediately north of the lake beds, also have greater resistance to erosion than the PPG.

Recent geochemical studies by participants in the Keck Undergraduate Geology Consortium (Sturm and others, 1993) conclude that the PPB is an A-type granite that was emplaced during a time of continental epeirogenic uplift and that the late sodic plutons may represent a subsequent rifting event. Nd-Sm isotopic studies by Douglass and Smith (1993) confirm that a mantle component was involved in the formation of the granite, thus helping to corroborate the petrogenetic model proposed for the PPB by Barker and others (1975).

The Thirtynine Mile Volcanic Field of Central Colorado: The Guffey Volcanic Center and Surrounding Areas

R. A. Wobus, Professor of Geology, G. Kroeger, Trinity University

Geological Society of America Abstracts with Programs, 26 (6), 54 (1994)

The Thirtynine Mile volcanic field is second only to the San Juan field in southwestern Colorado in volume of material extruded during the mid-Tertiary. The Thirtynine Mile field is dominated by lava flows, agglomerates, lahars and dikes. Using the rock nomenclature of LeBas and others (1986), the volcanics range from basalt and trachybasalt at the mafic end through shoshonite, latite, trachydacite- trachyte to rhyolite at the felsic end of the compositional spectrum.

The Guffey volcanic center is the largest one within the Thirtynine Mile volcanic field. It is characterized by a central core complex of shallow plutonic rock bodies and domes located near the town of Guffey. This region forms a topographic high. Surrounding this high area is an 8 x 15 km elliptical, topographically low feature which is surrounded by a series of hills which have 400 to 900 m of relief. These hills are composed of extrusive material that exhibit quaquaversal dips. This landform was interpreted by Epis and Chapin (1978) as a subsidence caldera. We conducted a detailed gravity survey covering the northeast quarter of this topographic low. Repeated gravity measurements were made at 158 stations surveyed using a digital total-station. The resulting Bourguer anomaly was dominated by a linear trend associated with high angle thrust faults on the west side of the Front Range. Removal of this first-order trend produced a residual anomaly that showed no correlation with the topographic low. Lacking any sign of the radial symmetry that one would expect to result from the collapse of a large stratovolcano, other factors must have led to the formation of the distinctive topography.

Whole rock K-Ar ages were determined for 23 samples from the Thirtynine Mile volcanic field. Thirteen samples from lava flows and domes outside the elliptical rampart range in age from 35.8 to 29.7 Ma with a 2% error of approximately 0.5 Ma for each date. Ten of the thirteen K-Ar dates cluster between 35.8 and 34.0 Ma indicating a large percentage of the volume of the entire volcanic field was extruded in this 2 Ma span of time. Two thick latite flows (domes)? from the central area are 39 to 38 Ma, ages clearly older than the Wall Mountain Tuff. Clearly, volcanism in the Thirtynine Mile volcanic field preceded as well as postdated the Wall Mountain Tuff event at 36.6 Ma, which has previously been assumed to be a datum for the MAXIMUM age of the Thirtynine Mile volcanics. Eight samples of texturally variable intrusive rocks from the central area near Guffey, each being the freshest available material, range in age from 50.3 to 31.8 Ma. IF these ages can be shown to represent ages of crystallization, free from any excess Ar derived from nearby Precambrian Pikes Peak granitic bodies (such a 40/39 Ar study is in progress), then igneous activity in the region must have spanned 20 to 25 Ma. Coupling the K-Ar and gravity data weakens the argument that a large Mt. Shasta-like stratovolcano existed in the Guffey region 35 Ma ago.

MATHEMATICS

Quasifuchsian Surfaces in Hyperbolic Knot Complements

Colin C. Adams, Professor of Mathematics, Alan Reid

J. Australian Math. Society, Series A, 55: 116-131 (1993)

Examples of hyperbolic knots in the three-sphere are given such that their complements contain quasi- Fuchsian non-Fuchsian surfaces. In particular, this proves that there are hyperbolic knots that are not toroidally alternating.

The Knot Book

Colin C. Adams, Professor of Mathematics

W.H. Freeman and Co., New York (1994)

Take a piece of string. Tie a knot in it and then glue the two ends together. The result is a knotted circle, called a knot. But how many different knots are there? How can we tell if a balled up tangle of string is actually unknotted and can be disentangled? What mathematical properties can be used to distinguish between knots? What are their applications to biochemistry and physics? This book is an introduction to the fascinating study of the mathematical theory of knots. No special mathematical background is necessary to delve into one of the hottest areas in mathematics today.

Toroidally Alternating Knots and Links

Colin C. Adams, Professor of Mathematics

Topology, Vol. 33, No. 2, 353-369 (1994)

We extend the class of alternating links in S3 to a class called toroidally alternating links. This class includes all alternating links, almost alternating links, augmented alternating links with one augmenting component and Montesinos links. The definition also makes sense in S2 xS1 and any lens space L(p,q). We prove that the complement of a toroidally alternating knot in S3 or L(p,q) for p odd contains no incompressible meridianally incompressible surfaces. Moreover, we prove that a nontrivial prime non-torus toroidally alternating knot in S3, S2xS1 or a lens space L(p,q) is always hyperbolic for L(p,q) not homeomorphic to L(2,1) or L(4k,2k-1). We also examine toroidally alternating knots and links in more general manifolds.

On Mahler's Compound Bodies

Edward B. Burger, Assistant Professor of Mathematics

Journal of the Australian Mathematical Society, Series (A), 55: 183-215 (1993)

Let 0M, in Euclidean L-space, where L is the binomial coefficient, N choose M. The compound body M is describable as the convex hull of a certain subset of the Grassmann manifold in Euclidean L-space determined by K and M. The sets K and M are related by number of well-known inequalities due to Mahler.

Here we generalize this theory to the geometry of numbers over the adele ring of a number field and prove theorems which compare an adelic set with its adelic compound body. In addition, we include a comparison of the adelic compound body with the adelic polar body and prove an adelic general transfer principle which has implications to Diophantine approximation over number field.

On Frequency Distributions of Partial Quotients of U-Numbers

Edward B. Burger, Assistant Professor of Mathematics, Thomas Struppeck

Mathematika, 40: 215-225 (1993)

In this paper we investigate the asymptotic densities of the partial quotients of U-numbers, in particular U2-numbers. We prove that given any frequency distribution on the natural numbers, there exist U2- numbers whose partial quotients exhibit that distribution. Thus it is impossible to determine if a real number is a U-number by a statistical test as described above. We also prove the existence of real numbers exhibiting unusual Diophantine approximation properties; including the existence of a badly approximable number whose square is a Liouville number.

Beyond Hyperbolicity: Expansion Properties of One-Dimensional Mappings

Stewart Johnson, Assistant Professor of Mathematics, John Guckenheimer

Proceedings of the Smalefest, Springer-Verlag, 227-236 (1993)

This paper explores growth properties for large sets of iterated unimodal maps. We survey recent work in the area and delineate the major conjectures of the field. Our earlier techniques are applied to transformations that are approaching a tangent bifurcation.

Sex Ratio and Population Stability

Stewart Johnson, Assistant Professor of Mathematics

OIKOS, 69: 1, 172-176 (1994)

The ratio of males to females within a population can have a strong effect on the per capita growth rate. This paper contains a brief construction and discussion of a mathematical population model embodying sex ratio dynamics. Implications of the model are explored.

Monotonicity Theorems for Two-Phase Solids

Frank Morgan, Professor of Mathematics

Archive Rat. Mech. Anal., John Sullivan and Francis C. Larche 124: 329-353 (1993)

This paper extends our previous mathematical theory of alloys such as gold and copper, showing how certain quantities vary in given parameters for energy minima. The tools include convex function theory and the calculus of variations.

(M,e,d,)-Minimal Curve Regularity

Frank Morgan, Professor of Mathematics

Proc. AMS, 120: 677-686 (1994)

(M,e,d)-minimal curves are proved to be embedded C1,a/2 curves meeting in threes at 120 degree angles.

Mathematicians, Including Undergraduates, Look at Soap Bubbles

Frank Morgan, Professor of Mathematics

Amer. Math. Monthly, 101: 343-351 (1994)

It is an open mathematical question whether the common double bubble succeeds in minimizing area or whether there is some as yet undiscovered configuration of less area enclosing and separating the same three volumes of air. The analogous planar problem recently has been solved by undergraduates.

On Quotients of Nonsingular Actions Whose Self-Joinings are Graphs

Cesar E. Silva, Associate Professor of Mathematics, Dave Witte, Assistant Professor of Mathematics

International Journal of Mathematics, Vol. 5, No. 2 , 219-237 (1994)

We study the notions of minimal self-joinings (MSJ) and graph self-joinings (GSJ) (analogous to simplicity in the finite measure preserving case) for nonsingular actions of locally compact groups. We show that for a nonsingular action of a group G with MSJ, every quotient comes from a closed subgroup of the center of G whose action is totally non-ergodic. Thus, totally ergodic nonsingular flows with MSJ are prime. We then show an analogous result to Veech's theorem, namely that for a nonsingular action of a group G with GSJ, every quotient comes from a locally compact subgroup of the centralizer whose action is totally non-ergodic.

PHYSICS

A Lower Bound for Accessible Information in Quantum Mechanics

Richard Jozsa, Daniel Robb '93, and William K. Wootters, Associate Professor of Physics

Physical Review A, 49, p. 668 (1994)

It has long been known that the von Neumann entropy S is an upper bound on the information one can extract from a quantum system in an unknown pure state. In this paper we define the "subentropy" Q, which we prove to be a lower bound on this information. Moreover, just as the von Neumann entropy is the best upper bound that depends only on the density matrix, we show that Q is the best lower bound that depends only on the density matrix. Other parallels between S and Q are also demonstrated.

A Complete Classification of Quantum Ensembles Having a Given Density Matrix

Lane P. Hughston, Richard Jozsa, and William K. Wootters, Associate Professor of Physics

Physics Letters A, 183, p.14 (1993)

A complete constructive classification is given for all discrete ensembles of pure quantum states having a given density matrix. As a special case this provides a classification of positive operator valued measures with finitely many components. We also show that any chosen ensemble consistent with a fixed density matrix can be created at space-like separation using an entangled state depending only on that density matrix.

Is Time Asymmetry Logically Prior to Quantum Mechanics?

William K. Wootters, Associate Professor of Physics

In Physical Origins of Time Asymmetry, ed. by J. Halliwell, J. Perez-Mercader, and W. Zurek, Cambridge Univ. Press, 1994

In trying to imagine how quantum mechanics might be derived from a more fundamental theory, the author is led to consider a framework in which time-asymmetric events, rather than reversible unitary transformations, are taken as basic. In such a scheme it seems likely that quantum mechanics and the second law of thermodynamics would emerge together, both being consequences of a deeper law.

High-power Passively Mode-locked Er-doped Fiber Laser With a Nonlinear Optical Loop Mirror

Jefferson Strait,,Associate Professor of Physics, et al.

Optics Letters 18, 1444 (1993)

We report the generation of pulses having a duration of 1 ps and a pulse energy of 280 pJ directly from a passively mode-locked Er-fiber laser. The laser uses a nonlinear optical loop mirror at high switching threshold in an all-optical fiber resonator. Stable pulse trains are produced at the fundamental repetition rate of the resonator, 3.8 MHz.

PSYCHOLOGY

Suspicion and Dispositional Inference

Steven Fein, Assistant Professor of Psychology, et al.

Personality and Social Psychology Bulletin, 19, 501-512 (1993)

In the present paper we examine the role of suspicion in the dispositional inference process. Perceivers who are led to become suspicious of the motives underlying a target's behavior appear to engage in more active and thoughtful attributional analyses than do nonsuspicious perceivers. Suspicious perceivers resist drawing inferences from a target's behavior that reflect the correspondence bias (or fundamental attribution error), and they consciously deliberate about questions of plausible causes and categorizations of the target's behavior. They are, however, quite willing to make strong correspondent inferences about the target if they learn additional contextual information that renders alternative explanations for the target's behavior less plausible. Implications that these findings have for current multiple-stage models of the dispositional inference process are discussed, and the need for these and other models to give more consideration to the social nature of social perception is asserted.

Typicality Effects in Attitudes Toward Social Policies: A Concept-Mapping Approach

Steven Fein, Assistant Professor of Psychology, et al.

Journal of Personality and Social Psychology, 66, 658-673 (1994)

Social typicality effects occur when people apply their attitudes more consistently toward typical than atypical category members -- presumably because attitudes are directed toward the prototypic category member. Four studies tested whether individuals also apply social policy attitudes more consistently toward typical than toward atypical persons affected by the policy. Study 1 used attitude concept maps to assess how extensively individuals spontaneously reference the typical person affected by a social policy. Death penalty proponents did so more than did opponents; welfare proponents and opponents did so equally. In three other studies, death penalty proponents displayed greater typicality effects in sentencing "convicted murderers" than did opponents, welfare proponents and opponents displayed equal typicality effects in evaluating cases of suspected welfare fraud, and individuals whose attitude concept maps about welfare referred more to persons displayed greater typicality effects than did individuals whose maps referred more to principles.

Effects of Prenatal Alcohol Exposure on the Developmental Pattern of Temperature Preference in a Thermocline

Betty Zimmerberg, Associate Professor of Psychology, Thomas Tomlinson, '92, et al

Alcohol, 10, 403-408 (1993)

Prenatal alcohol exposure is associated with a variety of impairments in neonatal state regulatory systems. Since prenatal alcohol exposure causes thermoregulatory deficits in response to both heat and cold stress in rats, body temperature set-point might be altered in alcohol-exposed offspring. The effect of prenatal alcohol exposure on behavior in a thermocline was investigated in 10-, 15-, and 125- day-old male and female rats from three prenatal treatment conditions: alcohol liquid diet, pair-fed liquid diet control, or standard control. Subjects were placed in the thermocline in the cold, hot, or middle start positions and observed for 60 min. Subjects exposed to alcohol prenatally had a wider "preference zone" than control subjects at 10 and 15 days of age, but did not as adults. This widening of the temperature set-point in young subjects prenatally exposed to alcohol may represent a developmental lag in the development of body temperature set-point or a central compensatory process allowing the animal to adapt to alternating experiences of heat and cold stress.

Sex Differences in Anxiety Behavior in Rats: Role of Gonadal Hormones

Betty Zimmerberg, Associate Professor of Psychology, Megan J. Farley, '92

Physiology & Behavior, 54, 1119-1124 (1993)

These experiments examined the role of gonadal hormones at both the organizational and activational time periods on sex differences in plus-maze behavior. In the first experiment, adult female Long- Evans rats were found to spend more time on the open arms of the plus maze than adult males, indicating less anxious behavior. In the second experiment, male and female subjects received a neonatal treatment (chemical castration with flutamide or tamoxifen, vehicle injection, or no injection) and a prepubertal treatment (gonadectomy, sham surgery, or no surgery). Adult females receiving either neonatal tamoxifen or prepubertal ovariectomy spent less time on open arms than control females, but females who received both treatments were the most defeminized subjects. Males were not affected by the absence of gonadal hormones at either time period. These experiments indicate that female gonadal hormones play an important role both organizationally and activationally in plus- maze behavior. the role of the GABA receptor complex in mediating this effect is discussed. Knowledge of sex differences in plus-maze behavior may help to make this maze a more useful tool in investigating anxiety behavior in rats.

Reduction of Rat Pup Ultrasonic Vocalizations by the Neuroactive Steroid Allopregnanolone

Betty Zimmerberg, Associate Professor of Psychology, et al

Pharmacology Biochemistry & Behavior, 47, 735-738 (1994)

Several of the recently characterized neuroactive steroids have been proposed to have anxiolytic effects in behavior models when subjects were tested as adults. In this experiment, the effects on infant subjects were examined using the isolation distress model of anxiety. The production of ultrasonic vocalizations in week-old rat pups after maternal separation was assessed after ICV injections of vehicle or allopregnanolone or sham injections. Subjects were also observed for activity and behavioral responses and tested on three measures of sedation. Allopregnanolone caused a dose- dependent decrease in ultrasonic vocalizations, with increasing motor incoordination, ataxia, and turning at the higher doses. Sex differences were not observed for any measure. These results suggest the GABAA, receptor binding site for neuroactive steroids is behaviorally active in neonates as well as in adults, and that the anxiolytic effects of the neuroactive steroids at this site may be dissociable from their sedative effects at low doses.

Effects of the Dynamite Charge on the Deliberations of Deadlocked Mock Juries

Saul M. Kassin, Professor of Psychology, et al

Law and Human Behavior, 17, 625-643 (1993)

When juries report that they are deadlocked, judges often deliver the dynamite charge, a supplemental instruction that urges jurors to rethink their views in an effort to reach a unanimous verdict. The present study evaluated the impact of this procedure on 378 subjects who participated in 63 deadlocked mock juries. Results indicated that the dynamite charge caused jurors in the voting minority to feel coerced and change their votes, reduced the pressure felt by those in the majority, and hastened the deliberation process in juries that favored conviction. These findings raise serious questions concerning the use of this controversial charge.

Deja Vu All Over Again: Elliott's Critique of Eyewitness Experts

Saul M. Kassin, Professor of Psychology, et al

Law and Human Behavior, 18, 203-210 (1994)

Echoing McCloskey and Egeth (1983), and motivated by Kassin, Ellsworth, and Smith's (1989) survey of 63 eyewitness experts, Elliott (1993) recently attacked the use of psychological experts on eyewitness testimony. There are two principal shortcomings of this critique. First, it misrepresents the eyewitness literature and the experts who use it. Second, it merely parrots complaints of the past. The same old arguments are made about the lack of sufficient research evidence, the standards by which experts should conduct their affairs, and the impact of it all on the jury. Perhaps the field needs periodic prodding and consciousness-raising on this issue, but there is very little in this critique that is imaginative or new compared to those that preceded it. And what is new is based on an irresponsible review of the literature.

Young Children's Comprehension of Pretend Episodes: The Integration of Successive Actions

Robert D. Kavanaugh, Professor of Psychology, et al

Child Development, 65, 16-30 (1994)

In 3 experiments, children's comprehension of successive pretend actions was examined. In Experiment 1, children (25-38 months) watched 2 linked actions (e.g., a puppet poured pretend cereal or powder into a bowl, and then pretended to feed the contents of the bowl to a toy animal). Children realized that the pretend substance was incorporated into the second action. In Experiment 2, children (24-39 months) again watched 2 linked actions (e.g., a puppet poured pretend milk or powder into a container, and then pretended to tip the contents of the container over a toy animal). They realized that the animal would become "milky" or "powdery." In Experiment 3, children (25-36 months) drew similar conclusions regarding a substitute rather than an imaginary entity. The results are discussed with reference to children's causal understanding, their capacity for talking about objects and events in terms of their make-believe and real status, and the processes underlying pretense comprehension.

Probabilities and Utilities of Fictional Outcomes in Wason's Four-Card Selection Task

Kris N. Kirby, Assistant Professor of Psychology

Cognition, 51(1): 1­28 (1994)

Four experiments are presented that show that selections in Wason's (1966) four-card selection task can be increased by increasing the probability that cards will yield important outcomes. Experiment 4 also shows that selections can be influenced by varying the utilities of possible outcomes. These results suggest that some important content effects in this task may not arise from inferential processes, and examples are given of how probabilities and utilities have been confounded with other manipulations in previous research. The recent trend towards content-dependent theories of reasoning may have arisen largely because theorists are attempting to construct models of inferential reasoning that can account for what are, in part, non-inferential phenomena. Content-independent inferential processes and content-independent choice processes may operate together to account for content effects in the selection task.

A 30-Week Randomized Controlled Trial of High-Dose Tacrine in Patients with Alzheimer's Disease

Paul R. Solomon, Professor of Psychology, et al

Journal of the American Medical Association, 271(13), 985-991 (1994)

An early study of 17 patients with AD reported beneficial effects of tacrine dosage up to 160 mg/d (tacrine base). These findings prompted the initiation of large multicenter studies to further define the treatment effects; however, the maximum daily dose of tacrine was limited to 80 mg. Statistically significant improvements were reported on the Alzheimer's Disease Assessment Scale-Cognitive subscale (ADAS-Cog), the Mini-Mental State Examination (MMSE), and activities of daily living, but the magnitude and duration of the treatment effect remained unclear. The study reported herein evaluates the benefits of tacrine dosage up to 160 mg/d over 30 weeks, substantially higher doses and over a longer treatment period that previously studied in a large multicenter trial.

Efficacy and Safety of High-Dose Tacrine: A 30-Week Evaluation

Paul R. Solomon, Professor of Psychology, et al

Alzheimer Disease & Associated Disorders, 8(2), S22-S31 (1994)

Tacrine, a reversible, centrally active cholinesterase inhibitor, was evaluated for treatment of Alzheimer disease (AD) in doses up to 160 mg over a 30-week treatment period in a double-blind, placebo-controlled, parallel-group study. Men and women aged 50 years or older in otherwise good health with probable AD of mild-to-moderate severity were randomized to one of four treatment groups. Group I received placebo. Group II began tacrine at 40 mg/day for 6 weeks, then was given 80 mg/day for 24 weeks. Groups III and IV began tacrine at 40 mg/day for 6 weeks, then increased to 80 mg/day for 6 weeks and 120 mg/day for 6 weeks; Group III remained on tacrine 120 mg/day for a total of 18 weeks, and Group IV was escalated to 160 mg/day for the final 12 weeks. The primary outcome measures were the Clinician Interview-Based Impression (CIBI), the Alzheimer's Disease Assessment Scale Cognitive sub-scale (ADAS-Cog), and the Final Comprehensive Consensus Assessment (FCCA). Of 663 patients who entered the study, 279 completed the entire 30 weeks. Dose-response trends were statistically significant for all three primary outcome measures (p ¾ 0.001). Pairwise comparisons of tacrine 160 mg/day vs. placebo significantly favored tacrine on CIBI (p = 0.002), ADAS-Cog (p < 0.001), and FCCA (p < 0.001). Tacrine plasma levels were highly correlated with dose and response. Primary reasons for withdrawal in tacrine-treated patients were asymptomatic alanine aminotransferase (ALT) elevations (28%) and gastrointestinal complaints (16%). ALT elevations were reversible on treatment discontinuation, and 87% of discontinued patients were successfully rechallenged and maintained on long-term tacrine therapy. After 30 weeks of treatment, high doses of tacrine produced significant dose-related improvements on objective, performance- based tests (ADAS-Cog, Mini-Mental State Examination), which were clearly evident to the clinician as determined by the CIBI and FCCA. Family members reported significant improvements in quality-of- life measures.

A Model Systems Approach to the Study of Disorders of Learning and Memory

Paul R. Solomon, Professor of Psychology, and William W. Pendelbury, M. D.

>Neurobiology of Aging, 15, 283-286 (1994)

Alzheimer's Disease: Therapeutic Strategies for the 1990s

Paul R. Solomon, Professor of Psychology, and William W. Pendelbury, M.D.

Neurobiology of Aging, 15, 287-289 (1994)