KECK GEOLOGY CONSORTIUM

PROCEEDINGS OF THE TWENTY-FOURTH ANNUAL KECK RESEARCH SYMPOSIUM IN GEOLOGY

April 2011 Union College, Schenectady, NY

Dr. Robert J. Varga, Editor Director, Keck Geology Consortium Pomona College

Dr. Holli Frey Symposium Convenor Union College

Carol Morgan Keck Geology Consortium Administrative Assistant

Diane Kadyk Symposium Proceedings Layout & Design Department of Earth & Environment Franklin & Marshall College

Keck Geology Consortium Geology Department, Pomona College 185 E. 6th St., Claremont, CA 91711 (909) 607-0651, [email protected], keckgeology.org

ISSN# 1528-7491

The Consortium Colleges The National Science Foundation ExxonMobil Corporation

KECK GEOLOGY CONSORTIUM PROCEEDINGS OF THE TWENTY-FOURTH ANNUAL KECK RESEARCH SYMPOSIUM IN GEOLOGY ISSN# 1528-7491

April 2011

Robert J. Varga Keck Geology Consortium Diane Kadyk Editor and Keck Director Pomona College Proceedings Layout & Design Pomona College 185 E 6th St., Claremont, CA Franklin & Marshall College 91711

Keck Geology Consortium Member Institutions: Amherst College, Beloit College, Carleton College, Colgate University, The College of Wooster, The Colorado College, Franklin & Marshall College, Macalester College, Mt Holyoke College, Oberlin College, Pomona College, Smith College, Trinity University, Union College, Washington & Lee University, Wesleyan University, Whitman College, Williams College

2010-2011 PROJECTS

FORMATION OF BASEMENT-INVOLVED FORELAND ARCHES: INTEGRATED STRUCTURAL AND SEISMOLOGICAL RESEARCH IN THE BIGHORN MOUNTAINS, WYOMING Faculty: CHRISTINE SIDDOWAY, MEGAN ANDERSON, Colorado College, ERIC ERSLEV, University of Wyoming Students: MOLLY CHAMBERLIN, Texas A&M University, ELIZABETH DALLEY, Oberlin College, JOHN SPENCE HORNBUCKLE III, Washington and Lee University, BRYAN MCATEE, Lafayette College, DAVID OAKLEY, Williams College, DREW C. THAYER, Colorado College, CHAD TREXLER, Whitman College, TRIANA N. UFRET, University of Puerto Rico, BRENNAN YOUNG, Utah State University.

EXPLORING THE PROTEROZOIC BIG SKY OROGENY IN SOUTHWEST MONTANA Faculty: TEKLA A. HARMS, JOHN T. CHENEY, Amherst College, JOHN BRADY, Smith College Students: JESSE DAVENPORT, College of Wooster, KRISTINA DOYLE, Amherst College, B. PARKER HAYNES, University of North Carolina - Chapel Hill, DANIELLE LERNER, Mount Holyoke College, CALEB O. LUCY, Williams College, ALIANORA WALKER, Smith College.

INTERDISCIPLINARY STUDIES IN THE CRITICAL ZONE, BOULDER CREEK CATCHMENT, FRONT RANGE, COLORADO Faculty: DAVID P. DETHIER, Williams College, WILL OUIMET. University of Connecticut Students: ERIN CAMP, Amherst College, EVAN N. DETHIER, Williams College, HAYLEY CORSON-RIKERT, Wesleyan University, KEITH M. KANTACK, Williams College, ELLEN M. MALEY, Smith College, JAMES A. MCCARTHY, Williams College, COREY SHIRCLIFF, Beloit College, KATHLEEN WARRELL, Georgia Tech University, CIANNA E. WYSHNYSZKY, Amherst College.

SEDIMENT DYNAMICS & ENVIRONMENTS IN THE LOWER CONNECTICUT RIVER Faculty: SUZANNE O’CONNELL, Wesleyan University Students: LYNN M. GEIGER, Wellesley College, KARA JACOBACCI, University of Massachusetts (Amherst), GABRIEL ROMERO, Pomona College.

GEOMORPHIC AND PALEOENVIRONMENTAL CHANGE IN GLACIER NATIONAL PARK, MONTANA, U.S.A. Faculty: KELLY MACGREGOR, Macalester College, CATHERINE RIIHIMAKI, Drew University, AMY MYRBO, LacCore Lab, University of Minnesota, KRISTINA BRADY, LacCore Lab, University of Minnesota Students: HANNAH BOURNE, Wesleyan University, JONATHAN GRIFFITH, Union College, JACQUELINE KUTVIRT, Macalester College, EMMA LOCATELLI, Macalester College, SARAH MATTESON, Bryn Mawr College, PERRY ODDO, Franklin and Marshall College, CLARK BRUNSON SIMCOE, Washington and Lee University.

GEOLOGIC, GEOMORPHIC, AND ENVIRONMENTAL CHANGE AT THE NORTHERN TERMINATION OF THE LAKE HÖVSGÖL RIFT, MONGOLIA Faculty: KARL W. WEGMANN, North Carolina State University, TSALMAN AMGAA, Mongolian University of Science and Technology, KURT L. FRANKEL, Georgia Institute of Technology, ANDREW P. deWET, Franklin & Marshall College, AMGALAN BAYASAGALN, Mongolian University of Science and Technology. Students: BRIANA BERKOWITZ, Beloit College, DAENA CHARLES, Union College, MELLISSA CROSS, Colgate University, JOHN MICHAELS, North Carolina State University, ERDENEBAYAR TSAGAANNARAN, Mongolian University of Science and Technology, BATTOGTOH DAMDINSUREN, Mongolian University of Science and Technology, DANIEL ROTHBERG, Colorado College, ESUGEI GANBOLD, ARANZAL ERDENE, Mongolian University of Science and Technology, AFSHAN SHAIKH, Georgia Institute of Technology, KRISTIN TADDEI, Franklin and Marshall College, GABRIELLE VANCE, Whitman College, ANDREW ZUZA, Cornell University.

LATE PLEISTOCENE EDIFICE FAILURE AND SECTOR COLLAPSE OF VOLCÁN BARÚ, PANAMA Faculty: THOMAS GARDNER, Trinity University, KRISTIN MORELL, Penn State University Students: SHANNON BRADY, Union College. LOGAN SCHUMACHER, Pomona College, HANNAH ZELLNER, Trinity University.

KECK SIERRA: MAGMA-WALLROCK INTERACTIONS IN THE SEQUOIA REGION Faculty: JADE STAR LACKEY, Pomona College, STACI L. LOEWY, California State University-Bakersfield Students: MARY BADAME, Oberlin College, MEGAN D’ERRICO, Trinity University, STANLEY HENSLEY, California State University, Bakersfield, JULIA HOLLAND, Trinity University, JESSLYN STARNES, Denison University, JULIANNE M. WALLAN, Colgate University.

EOCENE TECTONIC EVOLUTION OF THE TETONS-ABSAROKA RANGES, WYOMING Faculty: JOHN CRADDOCK, Macalester College, DAVE MALONE, Illinois State University Students: JESSE GEARY, Macalester College, KATHERINE KRAVITZ, Smith College, RAY MCGAUGHEY, Carleton College.

Funding Provided by: Keck Geology Consortium Member Institutions The National Science Foundation Grant NSF-REU 1005122 ExxonMobil Corporation

Keck Geology Consortium: Projects 2010-2011 Short Contributions— Teton Range, Wyoming

EOCENE TECTONIC EVOLUTION OF THE TETON RANGE, WYOMING JOHN CRADDOCK, Macalester College, DAVE MALONE, Illinois State University

FAULT-GENERATED CARBONATE INTRUSIONS FOUND AT WHITE MOUNTAIN, HEART MOUNTAIN DETACHMENT, WYOMING JESSE GEARY, Macalester College Research Advisor: John P. Craddock

INSIGHTS INTO THE ORIGIN OF THE SOUTH FORK DETACHMENT, WYOMING, USING CALCITE STRAIN ANALYSIS KATHERINE KRAVITZ, Smith College Research Advisor: Robert Burger

U-PB DETRITAL ZIRCON PEAK, WYOMING MOUNTAIN RAY MCGAUGHEY, Carleton College Research Advisor: Cameron Davidson

Keck Geology Consortium Pomona College 185 E. 6th St., Claremont, CA 91711 Keckgeology.org

24th Annual Keck Symposium: 2011 Union College, Schenectady, NY INSIGHTS INTO THE ORIGIN OF THE SOUTH FORK DETACHMENT, WYOMING, USING CALCITE STRAIN ANALYSIS

KATHERINE KRAVITZ, Smith College Research Advisor: Robert Burger

INTRODUCTION GEOLOGIC SETTING

The South Fork detachment (SFD) is exposed for 35 The temporal and spatial overlap of the Sevier and km in the South Fork Shoshone River valley south- Laramide orogenies in the Eocene has resulted in west of Cody. The rocks exposed along the SFD many enigmatic structural features in northwestern range from Jurassic to Eocene in age. Much of the Wyoming. The Sevier belt, characteristic of thin- fault trace is lost underneath Eocene Absaroka vol- skinned deformation of Paleozoic and Mesozoic strata canics and alluvium in the west and south, so the with east-vergent 45° slab dip, was active from the full extent of the fault is unknown (Clarey, 1990). late Jurassic to Eocene. The Sevier orogeny created The decollement surface is in the Jurassic Sundance long, parallel ranges that trend north-south and north- Formation, ramps up section to the Cretaceous Cody west-southeast with 50% shortening. The Laramide Shale and once again to the Eocene Willwood Forma- orogeny, active from the late Cretaceous to Eocene, tion. Transport of the upper plate was to the SE in the exhibits thick-skinned shortening, typically with Middle Eocene, and hanging wall structures show as eastward 5° slab dip. The Laramide ranges generally much as 10 km of displacement. trend north-south and northwest-southeast but exhibit more diverse orientations than the Sevier ranges. The The origin of the SFD has been in question for over South Fork detachment (SFD) trends northeast-south- 90 years. Before 1936, it was widely accepted that west, inconsistent with the orientations of the two the SFD was emplaced by a low-angle thrust sys- orogenic events. tem. Bucher (1936) was the first to propose the SFD emplacement was driven by gravity. The SFD has Absaroka Volcanic field been variously interpreted as the front of a gravity slide (Blackstone, 1985; Pierce, 1957, 1986), the The Absaroka volcanic field is a northwest trending easternmost expression of the Cordilleran overthrust belt of volcanic and plutonic rocks that was active 55- belt (Clarey, 1990), or the toe of the Heart Mountain 45 Ma. It is the largest volcanic field in the Northern detachment (Beutner and Hauge, 2009). Rocky Mountains that spans an area over 23,000 km2 (Feeley and Cosca, 2003). There is evidence of three Northwestern Wyoming in the Eocene exhibited many large debris avalanches from the Absaroka Volcanic unique geologic events that may have contributed to field. One of these debris avalanches, the Deer Creek the formation of the SFD. The late Paleocene saw the avalanche, is speculated to have occurred from Sun- end of the thin-skinned, east-vergent Sevier orogeny light Peak, a volcano that was proximal to the HMD as well as the development of the thick-skinned, west- and SFD and active around the same time. Evidence vergent Eocene Laramide orogeny. The Absaroka such as this indicates that volcanic activity may have volcanic field became active at 55 Ma and remained played a role in the emplacement of the two alloch- active for 10 m.y. The goal of this current research is thons (Malone, 1995). to better understand the emplacement mechanics and deformation history of the SFD using calcite strain Heart Mountain Detachment analysis. This may illuminate the cause, kinemat- ics and dynamics of the SFD to explain how it ties in The Heart Mountain Detachment (HMD) allochthon with contemporaneous geologic events. is greater than 3400 km2 and consists of dozens of 351 24th Annual Keck Symposium: 2011 Union College, Schenectady, NY mountain sized remnants, including Heart Mountain (Clarey, 2009). and White Mountain, that were transported south- east at least 50 km. The detachment surface is in the CALCITE STRAIN ANALYSIS lower Ordovician Bighorn Dolomite and had a dip of 2° at the time of emplacement. A breakaway bedding Calcite twins form at low differential stress, indepen- plane with a younger-over-older age relationship, a dently of temperature and normal stress magnitude, transgressive ramp, and a former-land-surface phase and can occur along three glide planes (Turner and with an older over younger age relationship have been Weiss, 1963; Lacombe and Laurent, 1992; Teufel, identified (Pierce, 1960). Fission track uplift studies 1980), making it a very sensitive strain gage. Me- have determined that Rattlesnake Mountain is older chanical twins will develop parallel to an e{0112} than the HMD and created the transgressive ramp plane in a specific glide direction, the intersection of e present in the HMD (Omar et al., 1994). and r (cleavage). Calcite strain-hardens once twinned and further twinning can then occur along the other The characteristics of the HMD are enigmatic and two e{0112} planes (Teufel, 1980). raise many questions about the emplacement rate, emplacement mechanism, and kinematics of the Calcite twin sets can be used as an internal strain gage detachment. The HMD was first described as a thrust to determine the aggregate deformation of calcite. fault (Dake, 1916), but this hypothesis was disproved The shear strain of a twin set can be determined by by the presence of extensional structures (Bucher, the grain width perpendicular to the twins and the 1947) and a breakaway fault (Pierce, 1960). Many angle of rotation of the grain edge before and after have argued for rapid emplacement of a continuous deformation. These values are obtained by measuring allochthon (Malone, 1995; Craddock et al., 2000) due the orientation of the optic axis of a calcite crystal, the to contemporaneous volcanism (Beutner and Gerbi, pole to the twin plane, the number and thickness of 2005; Beutner and Hauge, 2009). Beutner and Gerbi the twins, and the thickness of the grain perpendicular (2005) proposed that high CO2 from a volcanic or to the twins. The three-dimensional, deviatoric strain phreatomagmatic eruption greatly decreased friction tensor in a given sample can be determined by at least along the fault, allowing for rapid emplacement of five twin sets. The strains of each twin set measured the allochthon. The HMD is also considered to be the in its own e-g coordinate system are all transformed largest known subaerial detachment system due to the into the same x, y, and z thin section coordinate sector collapse of a volcano (Malone, 1995; Beutner system. Each twin set gives an equation that can be and Hauge, 2009; Craddock et al., 2009). solved to calculate the principal strain orientation and magnitude in the aggregate using a least-squares The South Fork Detachment and Heart Mountain method (Groshong et al., 1972). Detachment Relationship METHODS The HMD has been the subject of much research over Field Methods the last century, whereas the SFD has received much less attention. Both the SFD and HMD occurred in A total of 17 oriented samples were collected along the middle Eocene (~50 Ma), but the relative tim- strike of the SFD. Oriented samples were also col- ing and relationship between the two allochthons lected from the SW footwall (3 locations) and the remains unknown. Many have recently hypothesized northeast footwall (2 locations) to help identify any that the two proximal events are related to one an- strain overprints (Fig. 1). Reconnaissance mapping other. Due to age discrepancies and poor exposure of was conducted to further understand the structure of cross-cutting relationships, the connection between the SFD and to compare structural observations to the two events remains unclear. The SFD has been those of Pierce (1966). We observed a variety folds defined as a gravity detachment older than the HMD including, anticlines, synclines, recumbent folds and (Blackstone, 1985; Pierce, 1986; Beutner and Hauge, overturned bedding along the strike of the fault sys- 2009) as well as a detachment younger than the HMD tem. Many of these observations were not present in 352 24th Annual Keck Symposium: 2011 Union College, Schenectady, NY the geologic map produced by Pierce (1966). RESULTS Lower Plate Strain

SFD-10 SFD-12 SFD-14.1 SFD-14.2 SFD-14.3 NE-1 NE-2

ε ε1 ε2 ε 2 ε2 1 ε3 ε1 ε 3 ε2 All samples in the southwest lower plate (SW-1, ε ε ε2 ε 3 ε 3 ε1 1 3 ε2 ε ε1 3 ε3 ε ε2 1 SW-2 and SW-3) contain >90% PEVs. The samples in the northeast upper plate (NE-1 and NE-2) had a lower percentage of PEVs <80% (Table 1). Such South Fork Fault low percentages of NEVs do not indicate an ad- N WY ditional non-coaxial event and were not analyzed 0 10km separately. All the samples in the lower plate have a Lower Plate Upper Plate slight layer parallel shortening (LPS) trend with the exception of NE-1, which shows layer normal short-

ε ε2 ε 2 1 ε ening (LNS) (Fig. 1). The shortening axes from the ε 1 ε ε ε3 1 1 3 ε2 ε ε2 ε 3 ε3 ε ε 2 1 ε 2 ε1 3 ε2 ε ε 3 ε3 1 ε2 ε ε ε ε 1 ε3 3 lower plate in tilted bedding have a slight W/NW 2 1 SW-3 SW-1 SW-2 SFD-1 SFD-2 SFD-3 SFD-5.1 SFD-6 SFD-7 trend. SW-2 and SW-3 are oriented approximately Figure 1. Stereonet plots of each sample from the upper W with a 72°plunge. The shortening axes from SW- and lower plate of the SFF showing PEV ε1, ε2, ε3 in 1, NE-1 and NE-2 are oriented approximately NW relation to bedding. Map from Pierce (1980). SFF trace with a more shallow plunge of <30°. The extension adapted from Clarey (2009). axes are oriented ~E with plunges ranging from 12° to 64°(Fig. 2). When the beds are rotated back to horizontal, the shortening axes are generally oriented NW/SE with a plunge ≤40°. The extension axes in Thin Section Analysis untilted bedding are oriented approximately E to SE and have steeper plunges of 38° to 90° (Fig. 2). Thin sections of each sample were cut in a vertical orientation, parallel to the transport direction. Each sample was analyzed using the calcite strain gage %NE Strain Orientation Sample Bedding to determine the orientations of the strain ellipsoid V (Azimuth, Plunge) (Groshong et al., 1972). Each twinned calcite grain ALL PEV NEV e1 e2 e3 e1 e2 e3 e1 e2 e3 was measured in an area of the thin section until ~50 SW-1 N30E,40NW 8.8 333°,87° 181°,11° 087°,16° 226°,72° 356°,12° 14°,086° grains were measured in each oriented sample (Gro- SW-2 N35E, 28NW 2.3 230°,77° 358°,13° 094°,18° 274°,72° 013°,02° 18°,103° 084°,12° 167°,23° 327°,74° shong et al., 1984). SW-3 N30E, 12NW 7.5 311°,31° 216°,18° 098°,55° 313°,28° 203°,32° 46°,078° SFD-1 EW, 88N 28.3 091°,10° 189°,42° 351°,46° 115°,24° 310°,32° 48°,346° 348°,44° 171°,56° 081°,02° SFD-2 N20W,90 24.0 266°,21° 153°,45° 012°,39° 216°,50° 120°,06° 40°,024° 031°,45° 152°,34° 272°,25° The data was separated into two groups, positive SFD-3 N40E, 18S 19.1 162°,35° 268°,21° 021°,48° 155°,33° 350°,56° 08°,249° 036°,56° 264°,12° 165°,36° SFD-5.1 N30W,90 37.5 019°,00° 280°,48° 110°,44° 026°,08° 288°,47° 42°,142° 134°,34° 308°,58° 042°,02° expected values (PEV) and negative expected val- SFD-6 N53E, 50SE 8.0 043°,09° 296°,64° 143°24° 040°,22° 274°,57° 24°,143° ues (NEV). A PEV is a twin set with an orientation SFD-7 N45E, 45S 20.8 245°,38° 041°,50° 054°,12° 285°,68° 044°,10° 18°,139° 139°,12° 034°,60° 235°,27° that is consistent with the calculated aggregate strain SFD-10 N40E, 55SE 20.0 206°,05° 110°,48° 300°,42° 299°,07° 100°,54° 36°,294° 262°,17° 030°,65° 165°,20° SFD-12 N30E, 55SE 0.0 122°,61° 013°,12° 279°,28° 116°,62° 342°,22° 18°,244° tensor. A NEV is a twin set that does not have a SFD-14.1 N50E, 55N 2.0 285°,35° 032°,24° 148°,44° 281°,29° 028°,28° 50°,156° favorable orientation in terms of the calculated strain SFD-14.2 N45E, 45S 4.0 140°,77° 025°,05° 308°,12° 136°,76° 038°,03° 12°,294° tensor. A high percentage of NEVs in a given sample SFD-14.3 N40E, 25S 0.0 157°,53° 029°,20° 296°,30° 154°,53° 009°,32° 18°,267° NE-1 N70E, 32S 24.5 235°,14° 241°,16° 101°,59° 344°,29° 245°,21° 64°,106° 250°,49° 074°,39° 342°,02° (>40%) represents an additional non-coaxial event NE-2 NS, 28W 23.5 310°,18° 043°,06° 149°,70° 306°,22° 210°,18° 62°,085° 145°,69° 272°,12° 004°,15° (Teufel, 1980) and can be analyzed separately. To Table 1. Table showing all analyzed samples, bedding reduce noise in the data, 20% of the grains with the orientation, % NEVs and orientations of the strain el- largest magnitude deviations were removed from the lipsoid for ALL, PEV and NEV results. Samples without separate data sets (Groshong et al., 1984). The strains NEV strain orientations contained <5 grains with negative calculated were plotted in spatial orientation as well expected values and could not be analyzed separately. All as rotated and plotted in relation to untilted, horizon- samples analyzed were from the Sundance Formation. tal bedding. 353 24th Annual Keck Symposium: 2011 Union College, Schenectady, NY Upper Plate Strain Upper Plate ε The upper plate samples all contain <40% NEVs. A) Tilted Bedding 1 N ε SFD-5.1 contains 37.5% NEVs, and SFD-1, SFD-2, 000 2 ε SFD-7 and SFD-10 have 20% NEVs. The majority of 3 the remaining samples contained <10% NEVs (Table 1). All the samples exhibit LPS, though the proximity of the shortening axis to bedding varies (Fig. 1). The W E orientations of the shortening axes in the hanging wall 270 090 are scattered. SFD-5, SFD-6 and SFD-7 are in the NE quadrant and have a shallow plunge of 8°-22°. SFD- 14.2, SFD-14.3, SFD-12 and SFD-3 are in the SE quadrant and have a plunge ~20°-50°. The southern portion of the SW quadrant contains SFD-1, SFD-2 180 S and SFD-10 plunging from 6°-50°. SFD-14.1 plunges 19° in the NW quadrant (Fig. 3). The extension axes B) Horizontal Bedding N are also dispersed throughout the upper plate with 000 a concentration to the SE, W and N. The extension axes have moderate to shallow plunges ranging from 8° to 50° (Fig. 3). When the beds are rotated back to horizontal, the compression axes are still scattered, but all have a shallow plunge ranging from 4° to 30°. W E The extension axes in horizontal bedding are mostly 270 090 scattered within the western hemisphere and plunge from 2° to 80° (Fig. 3).

Lower Plate

A) Tilted Bedding ε N 1 180 000 ε2 S ε 3 Figure 3. Plots showing ε1, ε2, ε3 of PEVs in the upper plate of the SFF. A demonstrates the strain orientations when the bedding is tilted. B shows the strain orientation W . E 270 090 when bedding is rotated to horizontal.

180 S DISCUSSION B) Horizontal Bedding N 000 Teufel (1980) found that two deformation events can be distinguished if the second deformation event has a stress orientation >45° from the original. These W . E 270 090 two events are differentiated in the dataset by the presence of >40% NEVs. Craddock et al. (2000) examined calcite twinning strain within the upper and lower plate of the HMD to better understand the 180 S deformation of the allochthon during emplacement. Figure 2. Plots showing ε1, ε2, ε3 of PEVs in the lower The HMD carbonate rocks in the upper plate and plate of the SFF. A demonstrates the strain orientations lower plate contain a pre-HMD calcite twinning strain when the bedding is tilted. B shows the strain orientation from the Laramide-Sevier orogeny that shows thrust when bedding is rotated to horizontal. 354 24th Annual Keck Symposium: 2011 Union College, Schenectady, NY transport-parallel (E-W) shortening strain. The low percentage of NEVs implies the upper plate did not An overprint found in the Sundance Formation would contain a HMD related strain overprint, which has likely be the result of Laramide-Sevier strain. How- significant implications for the emplacement rate of ever, none of the samples within the SFD contain the allochthon. Because twinning is a time-dependent ≥40% NEVs. Therefore, there is no evidence for a deformation mechanism, the absence of a detach- non-coaxial event. The absence of Sevier-Laramide- ment related overprint indicates rapid emplacement related strain in the SFD is striking because it is of the allochthon. Craddock et al. (2009) later used abundant within the HMD upper and lower plate geochemical data and thermodynamic calculations to (Craddock et al., 2000). Because the strata related to quantify the emplacement rate of the HMD and found the SFD are younger than those of the HMD, it is pos- that it traveled as much as 50 km in 3-4 minutes. sible that these beds were not buried deeply enough The PEV shortening axes in the southwest footwall to produce twinning strain related to the Sevier- have an approximate WNW trend (Table 1), exhibit Laramide orogenic events. LPS (Fig. 1) and have moderately steep plunges even when bedding is rotated back to horizontal (Fig. 2). CONCLUSIONS The relatively steep plunges and WNW trend are not consistent with Sevier-Laramide-related shortening, The twinning strain in both the upper and lower plate which is characteristic of shallow E-W or NE-SW of the SFD are the result of SFD related emplace- trends (Craddock, 1992). It is very likely that this ment. This indicates that the allochthon was emplaced strain is directly related to the emplacement of the at a slower rate than the HMD because mechanical SFD allochthon. The absence of any Sevier-Laramide twinning is time dependent. Even if the SFD was related strain in the lower plate and the low percent- emplaced at a slower rate, it still could have been age of NEVs makes it unlikely the PEV strains within a catastrophic event. There are not enough NEVs the upper plate demonstrate Sevier-Laramide strain. within the sample suite to demonstrate a noncoaxial The variety of percent NEVs (Table 1) as well as the strain overprint. Therefore, there is no sign of Sevier- scatter within the strain orientations (Fig. 3) could be Laramide-related strain in the SFD. This is unexpect- the result of continuous strain as the beds were tilted. ed because Sevier-Laramide-related strain is present If the calcite was twinned by a constant orientation within the HMD. The strata within the SFD may not of maximum compressive stress while the beds were have been buried deeply enough to produce a Sevier- simultaneously tilting, the finite strain ellipsoid calcu- Laramide strain overprint. lated from calcite twins may represent a combination of many incremental strain ellipsoids. The calcite REFERENCES strain-gage technique does not account for rotational strain (Groshong, 1972), which could explain the Beutner, E.C., and Gerbi, G.P., 2005, Catastrophic scatter among the strain orientations. emplacement of the Heart Mountain block slide, Wyoming and Montana, USA: GSA Bulletin, v. Since the SFD has twinning strain that is most likely 117, p. 724-735. directly related to the SFD emplacement, the alloch- thon must have been emplaced in enough time to Beutner, E.C., and Hauge, T.A., 2009, Heart Mountain develop mechanical twins. Because twins are present, and South Fork detachment systems: Architec- the SFD allochthon was not emplaced at such a rapid ture and evolution of the collapse of an Eocene rate as the HMD, but still could have been emplaced volcanic system, northwest Wyoming: Northwest catastrophically. Clarey (2009) defines the SFD as a Wyoming: Rocky Mountain Geology, v. 44, p. superfault, which is an allochthon emplaced >100m 147-164. at >0.1m/s. If the SFD traveled 10 times faster than the minimum rate of a superfault, it would still allow Blackstone, D.L., Jr., 1985, South Fork detachment time for mechanical twins to develop (Friedman and fault, Park County, Wyoming; geometry, extent, Heard, 1974). source: Contributions to Geology, v. 23, p. 47-62. 355 24th Annual Keck Symposium: 2011 Union College, Schenectady, NY Ferrill, D.A., 1998, Critical re-evaluation of differen- Bucher, W.H., 1947, Heart Mountain Problem, in Uni- tial stress estimates from calcite twins in coarse- versity of Wyoming, p. 189-197. grained limestone: Tectonophysics, v. 285, p. 77-86. Clarey, T.L., 2009, Timing relations between the South Fork and Heart Mountain fault systems with Friedman, M., and Heard, H.C., 1974, Principal stress implications for emplacement, Wyoming, USA: ratios in Cretaceous limestones from Texas Gulf Geological Society of America Abstracts with Coast: AAPG Bulletin, v. 68, p. 71-78. Programs, v. 41, no. 7, p. 570. 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