| The Geology and Neo-Tectonics of the Teton Range, Wyoming Article |
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| The Geology and Neo-Tectonics of the Teton Range, Wyoming   | 
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Preview Since I am a geophysicist/geologist and since my primary area of work and research is the Teton range and its seismic potential, I thought I'd share some information about it here. This is based on a paper I wrote about the range and its earthquake potential.
I hope that it is not too technical for most to understand. I have bolded some of the more unique geologic terms that can be found in a glossary at the end of the article. I love this mountain range and have become quite familiar with it over the past few years. I feel that a deeper understanding of the forces at work that created these peaks and the surrounding area enhances the experience of being there.
Enjoy!
Introduction The Teton Range of north-western Wyoming is a 70 km, north – south trending normal fault-block mountain range. The mountain range is not large in comparison to other ranges of the western U.S. Cordillera; however, nearly 7,000’ of mountain rising straight up from a flat valley floor unimpeded by foothills has guaranteed national park status for this area of the state. The Teton Range occupies an area of the northern most Basin & Range province of the western U.S. Cordillera. Tectonically, it resides at the intersection of four major provinces that include: The Basin & Range, the Idaho-Wyoming Fold & Thrust Belt, the Rocky Mountain Foreland, and the Snake River Plain/Yellowstone Volcanic Plateau. A geologic history of the area stretches back all the way to the Archean. (Figure A)
 (Figure A) Geologic Time Scale
Several tectonic events define the physiography of the region ranging from crustal thickening and thrusting from late Mesozoic shallow back-arc subduction to more recent crustal thinning due to late Cenozoic normal back-arc spreading as well as the present day style of Basin & Range extension.
The latest Cenozoic (Quaternary) tectonics of the Northern Basin & Range has played the most important role in sculpting the mountain range as it is seen today. Since only during the past 2 Ma, nearly 3.5 km of throw has been established on the fault. Holocene age fault scarps ranging in height from 10 – 60 m near the central portion of the range reveal evidence of half a dozen or more large (Mw 7+) earthquakes. The most recent of which was ~4,000 years ago. With a recurrence interval of ~1,600 – 6,000 years, the Teton fault is most likely overdue for another large earthquake. Although the range is thought to be located in a region of seismic quiescence (for earthquakes of moment magnitude > 3.0), the fault is thought to be storing stress at normal rates. For the town of Jackson and the Jackson Lake dam, another earthquake of this magnitude could result in heavy damage mostly due to liquefaction of unconsolidated Quaternary river deposits within Jackson Hole.
Tectonic History (Evolution of the Northern Basin & Range; N.B.R.) The northern Basin & Range province is an actively deforming intracontinental plateau situated between the Sierra Nevada block and the Colorado Plateau (Thatcher et. al; 1999). Unlike other extended regions, the northern Basin & Range province is elevated on average 1.5 km above sea-level. Throughout the Paleozoic, the western U.S. Cordillera was subject to several episodes of terrane accretion, subduction accompanied by back-arc spreading, and contraction. There were also several orogenic events occurring throughout the Paleozoic. The first of which was the Antler orogeny.
 Mt. Moran on the northern end of the range.
In latest Devonian/Early Mississippian time, Antler assemblage rocks were thrust 200 km east onto the miogeoclinal rocks of the North American continental slope. The structure of the allocthon consists of numerous thrust slices that developed in an accretionary prism related to west dipping subduction. There are many hypotheses about the Antler assemblage: collision between the continent and an east-facing offshore arc, partial closure of a marginal basin, narrow upper plate thrust belt (accretionary prism) of oceanic rise and continental slope sedimentary rocks and a contemporaneous basin of sedimentation immediately to its west. From late Middle Devonian, arc volcanism waxed and waned throughout the rest of the late Paleozoic.
Eastward migration of deformation and magmatism continued from the Middle Jurassic into the early Cenozoic. This time heralds the beginning of back-arc compressional tectonics in the northern segment of the Cordillera. The Sevier orogeny, beginning in the Middle Jurassic to Early Cretaceous and ending sometime in the Eocene, was a major through going structural element of the eastern Cordillera from south-eastern California to northern Canada. It was an east vergent foreland fold & thrust belt also known as the Overthrust Belt. This belt was just west of and locally overlaps the Laramide Rockies.
The overall deformation in the Cordilleran thrust belt of western Wyoming, Utah and Idaho progressed from west to east through time (Brown, 1988). Deformation of the basement-involved features also began in the west. There is evidence of uplift of the Laramide Wind River Mountains in Maastrichtian time. In general, the North American plate has moved in a westerly direction since the Jurassic. Following closely on the heels of the Sevier orogeny, the Laramide orogeny began in Maastrichtian time and ended in the Eocene. This orogenic event stemmed from the convergence of the North American plate with the Farallon plate. The high speed of convergence resulted in a flattening of the subduction zone, shifted areas of high heat flow far inland to the east, and thermally weakened the crust which yielded along low-angle reverse faults (Brown, 1988).
The Cenozoic was a time of severe tectonic compartmentalization in the U.S. Cordillera. Basin & Range normal faults are widely recognized as younger, steeper faults commonly superimposed across earlier detachment fault complexes. Faulting in the Great Basin province was and continues to be generally associated with bimodal volcanism and a variable tectonic environment; from a back-arc setting prior to northward passage of the Mendocino triple-junction to a transform tectonic zone afterwards. A change from compressional to extensional intraplate stress in the Cenozoic western U.S. may have accompanied a change in configuration of the subducting Farallon plate from shallow (Laramide) to steep (post-Laramide) dips.
Extensional tectonism responsible for the modern Basin & Range province appears to represent a unique late-stage episode of a much longer period of extension initiated in an ‘intra-arc’ setting contemporaneously with calc-alkaline magmatism (Zoback et. al, 1981). The northern Basin & Range block faulting developed after 10 Ma and continues to the present in response to a stress field oriented 45º clockwise to an earlier stress field. During the Miocene, the Yellowstone “plume” emerged on the scene and has strongly influenced Basin & Range extension in the area. There is evidence of a Yellowstone Influenced Strain Field (YISF) extending like a bow-wave or the wake of a boat in arms oriented to the north-west and to the south around the Yellowstone Volcanic Plateau.
The presence of this strain field immediately to the west and south of the hotspot is distinct from a north-east to south-west oriented Bain & Range extensional strain field to the south-west. The Yellowstone strain-field produced the bulk of the slip across the east-southeast dipping Teton normal fault south of the hotspot and the north-dipping Centennial normal fault west of the hotspot (Figure 1).
Figure 1 From "2.0 Teton Fault-Source Characterization" U.S. Dept. of the Interior, Bureau of Reclamation |
These two faults are anomalous in their trend relative to the dominantly north-west to north striking normal faults in the Basin & Range province further to the south-west (Janecke,2002).
Geologic History of the Jackson Hole Region The Precambrian rocks of the Teton Range record a long and complex history. The layered gneisses and schist’s which make up the basement complex were almost certainly laid down as sedimentary and volcanic deposits on the ocean floor, possibly on the flanks of a chain of volcanic islands (Love et. al., 1973). These layers were folded and contorted and intruded by bodies of granite and gabbro that were themselves sheared and converted to gneisses. Radiometric dates on some of the older units show that the main episode of metamorphism and deformation occurred 2.8 Ga (Ga = Giga-annos, or Billion years). Long after emplacement of the granite, tensional forces opened up nearly vertical east-west-trending zones of weakness. Along these fissures, molten rock welled up to form dikes. After emplacement of the dikes, the Precambrian rocks were uplifted and eroded. Thousands of feet of sedimentary rocks were deposited between 1.6 Ga and 570 Ma (Ma = Mega-annos, or Million years) but have since been removed from the Teton region by erosion. Their remnants however remain farther to the west.
For the next 500 million years, seas occupied the region intermittently up until the Cretaceous. A mile thick unit of marine sediment was deposited during the Paleozoic and nearly 8,000’ was deposited during the Mesozoic. Deposition was interrupted by periods of erosion. This is most evident by the complete absence of Silurian rocks in the Teton region. After the deposition of the Nugget sandstone, northwestern Wyoming underwent regional southward tilting, and before the re-advance of the sea in the Middle Jurassic, erosion took place in the area that is now the northern half of Jackson Hole (Love et. al.). The Bacon Ridge sandstone is the youngest marine formation of the Teton region. All of the later Mesozoic deposits were apparently laid down in coastal swamps and flood plains (Love et. al.). Volcanoes in or northwest of Yellowstone National Park supplied ash and sand-size volcanic rock fragments that are now found in the Harebell Formation.
Mountain building during the Paleocene had not been more active in the region since the Precambrian. Several uplifts and synclinal and anticlinal folding occurred during this time and several thousand cubic miles of quartzite debris which constitutes the Pinyon Conglomerate was shed off the Targhee uplift. This debris overlapped onto Paleozoic rocks at the northern end of the ancestral Teton uplift (Love et. al.). Deposition of coarse conglomerates continued into the late Paleocene in great fans that extended southward.
During most of the Eocene, enormous piles of volcanic debris accumulated north and northeast of the Tetons. Volcanic mudflow breccias of Eocene age contain boulders of Paleozoic and Mesozoic rocks. These rocks are folded but the age of folding is not known, however, it was between Eocene deposition and the emplacement of ash-flow tuffs of the Yellowstone Group which is Pleistocene age (Love et. al.). Oligocene geology in Jackson Hole is not very well known. During Miocene time, down warping occurred and 7,000’ of coarse clastic debris of the Colter formation developed. Regional uplift of much of Wyoming during the Pliocene epoch rounded out the Neogene period with a cycle of erosion that reexcavated many of the intermontane basins.
In latest Pliocene and earliest Pleistocene time, sediments continued to accumulate in Jackson Hole in response to progressive deepening of the basin of deposition and to the westward tilting of the valley floor concomitant with continued movement along the Teton fault (Love et. al.). Lots of volcanism occurred during this time from the north in Yellowstone. Large voluminous ash-flow sheets blanketed the valley. The second of these, the Huckleberry Ridge tuff occurred 2 Ma and was deposited rapidly as a flat ‘sheet’ which helped in the determination of subsequent deformation of the area. The vertical offset of this tuff sheet along the fault system at the northern end of Jackson Lake is ~2,000 feet. Projected dips on the sheets suggest that in the central part of the range, the displacement on the fault may have been as much as 4,000 feet after the eruption of the 2 million year old ash flow (Love et. al.).
Creation of the Teton landscape: Neotectonics of the Teton Fault The surface trace of the Teton fault lies along the western shore of Jackson Lake, within about 12 km of Jackson Lake dam and dips to the east potentially extending beneath the dam site. Late Quaternary fault-scarps, the product of multiple Holocene surface faulting events, are present along about 60 km of the fault trace. However, the Teton fault has been mapped only in moderate detail and there is relatively little detailed information available on the paleoseismic history of the fault. Slip rate estimates have been produced based on the offsets of the 2 Ma Huckleberry Ridge tuff and late Quaternary deposits along the front of the range. One trench was dug at the mouth of Granite Canyon providing detail of Holocene displacement history of the fault.
The Quaternary chronology of faulted deposits, together with the amounts of faulting, is the data from which estimates of faulting rates are derived. Currently four known groups of Quaternary deposits are clearly faulted. They have been identified as the glacial deposits in Jackson Hole related to ice lobes from the north, glacial deposits related to local glaciers from the range, deposits along glaciated drainages that post-date the major deglaciation of the Teton glaciers and Yellowstone ice sheet, and colluvial deposits that cover the slopes of the range front between drainages. These glacial deposits span 20,000 – 40,000 years and are from the Munger through Pinedale glaciers.
The Tetons as seen from the west. |
Late Quaternary fault scarps are recognized along approximately 60 km of the Teton fault. Quaternary displacement on the Teton fault has clearly extended beyond the limits of the fault scarps mapped along the range front. This is evidenced by the range front and structural relief at the southern end of the range and by numerous fault traces and offset of 2 Ma Huckleberry Ridge tuff at the northern end of the fault. The dip of the fault is only poorly constrained by surface geology, limited subsurface data and structural models, and observational information from historical earthquakes. It has been posed that the Teton fault be considered to be made up of three major sections; northern, central and southern. However, it is not clear from existing data whether or not these segments behave independently as separate fault rupture segments and earthquake sources or whether the entire fault ruptures in a single event.
The northern fault segment consists predominantly of longer fault traces that strike between N – S and N10ºE, that alternate with short fault traces that strike about N30 - 65ºE. In most cases, these shorter traces act as right steps to the overall trace of the fault. Surprisingly there is no evidence of faulting in the sediments beneath Moran Bay on the west side of Jackson Lake. The central section of the fault displays a more regular sawtooth pattern, defined by somewhat longer traces that strike about N10ºE, which alternate with traces only slightly shorter that strike NW. Throughout this central part of the fault system, existing mapping indicates that the fault scarps have a high degree of continuity. Scarps along the southern section define two arcuate to wedge shaped traces, a northern group which is set back slightly and centered on Phelps Lake, and a southern, more curved wedge in the Teton Village to Phillips Canyon area. South of Taggart Lake, overall, the fault scarps appear to be more discontinuous than along other sections of the fault, particularly in the areas near Phelps Lake and the Jackson Hole ski area. Whether these discontinuities reflect incomplete surface rupture along this section of the fault or destruction of the fault scarps by erosion and deposition is unclear based on the existing mapping. South of Jenny Lake is the only area along the fault where mapping has shown clear evidence of left-lateral offsets. These offsets along the more northeasterly striking sections of the fault were consistent with nearly E – W oriented extension affecting the fault as a whole, and not necessarily indicative of fault segmentation.
The apparent termination of the southern end of the range and fault near Mesozoic to early Tertiary thrusts suggests that these preexisting structures may influence its lateral extent (Byrd et. al., 1994). The Teton Range contacts the Snake River Range at an almost 90º angle at Teton Pass. The fault is hinged at its intersection with the Cache Creek thrust, requiring that the thrust act as a non-conservative barrier to rupture propagation along the Teton fault. Alternatively, the implied fault intersection may act as a nucleation point for ground rupturing earthquakes on the Teton fault, suggesting the Cache Creek fault dips 70º N if the earthquakes nucleated at 15 – 20 km depths (Byrd et. al., 1994).
The dip of the Teton normal fault lies somewhere between 45º - 75º. Along the late Quaternary fault trace, the fault exposed in the trench at Granite Creek had a dip of 75º – 85º based on the 2 m high exposure in the trench. Near-surface steepening of fault planes in surficial alluvium has long been recognized and dips from the trench exposures are clearly over steepened. The surface fault trace is very irregular, consisting of fault planes with multiple orientations, and many step-overs. This complex geometry, combined with the relative lack of detail associated with the existing complications of fault scarps, has thus far frustrated any attempts to use the fault scarp data to generate a meaningful estimate of fault dip. Historical earthquake evidence indicates that a preferred dip of ~45º and a range from 30º - 60º should be considered as the likely range of dips associated with fault rupture on the late Quaternary trace of the Teton fault.
Results from trenching studies performed at the mouth of Granite Canyon indicate two surface faulting events, one about 4 Ka and one about 8 Ka. These data appear to suggest intervals of nearly 4,000 years between faulting events on the southernmost portion of the Teton fault. At the northern end of the fault at Jackson Lake, submerged shorelines appear to indicate eight to ten earthquakes since deglaciation about 14 Ka. This implies average intervals between earthquakes of less than 2,000 years for at least the central portion of the fault adjacent to Spaulding Bay on Jackson Lake. The Teton fault is thought to occupy a zone of quiescence in the intermountain seismic belt. However, the southern portion of the fault is believed to be storing stress at normal rates and is therefore overdue for a large groundbreaking earthquake. Creep rates of 0.45 – 1.6 mm/yr have been measured across the fault and are actually larger than rates for other Basin & Range normal faults. From these measurements as well as measuring the dip and age of the Huckleberry Ridge tuff which was deposited only 2 Ma, it has been determined that the entire 2.5 – 3.5 km of displacement on the Teton fault has occurred in only the last 2 million years.
Summary We have learned that the Teton landscape was shaped by a long and complex series of events. Tracing its history from the Archean all the way up through the Quaternary, for nearly 2.8 billion years of time, gives us some idea of the vast scope of geologic processes that it took to shape the landscape to its present grandeur. If it weren’t for the right mix of plate tectonics, sedimentation, erosion and volcanism we might never have been witness to the amazing geologic laboratory that is the Teton Range. In one small corner of Wyoming, the disciplines of metamorphic and igneous petrology, geomorphology, paleoseismology, Quaternary geology, glaciology, volcanology, sedimentary geology, earthquake seismology, structural geology, geodynamics, hydrology, mineralogy, tectonics, landslide hazards and even paleontology could learn something new for many years to come. But it is in the realm of neotectonics that we stand to learn the most from this dynamic region. As man continues to populate Jackson Hole, a better understanding of the hazards associated with large normal fault earthquakes needs to be achieved.
BibliographyBrown, W.G. Deformational style of Laramide uplifts in the Wyoming foreland. Geological Society of America, Memoir 171, 1988
Byrd, O.D., Smith, R. B., and Geissman, J.W. The Teton fault, Wyoming: Topographic signature, neotectonics and mechanisms of deformation. Journal of Geophysical Research, Vol. 99, No. B10, Pages 20,095 – 20,122, October 10, 1994.
Janecke, S.U. Applications of the USArray and PBO facilities to tectonic problems in the northern Basin-and-Range province and the eastern Snake River Plain.
Love, J.D., Reed, J.C., Christiansen, R.L., and Stacey, J.R. Geologic Block Diagram and Tectonic History of the Teton Region. Grand Teton Natural History Association in cooperation with the U.S. Geological Survey 1973
Thatcher, W., Foulger, G.R., Julian, B.R., Svarc, J., Quilty, E., Bawden, G.W. Present-Day Deformation Across the Basin and Range Province, Western United States. Science. Vol. 283, Mar. 12, 1999
Zoback, M.L., Anderson, R.E., and Thompson, G.A. Cainozoic Evolution of the State of Stress and Style of Tectonism of the Basin and Range Province of the Western United States. Philosophical Transactions of the Royal Society of London. Series A, Mathematical and Physical Sciences published by the Royal Society, Vol. 300, No. 1454, Extensional Tectonics Associated with Convergent Plate Boundaries. Mar. 26, 1981, pp. 407 – 434.
Glossary of Geological TerminologyHere are the definitions of some of the more unique geologic terms. I hope this will help in the understanding of the article!
Allochthon - Rocks that have been moved a long distance from their original place of deposition by some tectonic process, generally related to overthrusting or folding or gravity sliding.
Anticline - A fold that is convex upwards.
Autochthon - A succesion of beds that have been moved comparatively little from their original site of formation, although they may be intensely faulted or folded.
Basement - A term that refers to the igneous and metamorphic rocks of Precambrian age that underly most of the Paleozoic rocks of the craton.
Bimodal Volcanism - Volcanism usually associated with continental rifting (opening) in which two forms of volcanic rocks are deposited, basalts and rhyolites.
Contemporaneous (Basin) - The basin formed at about the same time as continental thrusting and deformation.
Cordillera - A mountain range or system in some cases the main mountain axis of a continent; specifically the great mountain region of N. America between the central lowland and the Pacific Ocean.
Craton - A relatively immobile part of the Earth, generally of large size, i.e. - the relatively undeformed zone of the U.S. continent between the Appalachian mountains to the Rocky Mountains.
Detachment Fault - A basal layer/horizon where most, if not all, regional faults coalesce onto.
Fault scarp - The trace of a tectonic fault that breaks the surface of the Earth and can be traced and/or mapped in the field.
Geosyncline - A large and generally linear trough that subsided deeply throughout a long period of time in which a thick succession of stratified sediments and possibly extrusive volcanic rocks accumulated. The strata of many geosynclines have been folded into mountains.
Liquefaction - The transforming of unconsolidated moist sediments into a slurry of mud and water during intense seismic shaking.
Miogeosyncline - A geosyncline in which volcanism is not associated with sedimentation.
Normal Fault - A fault in which the hanging wall has been depressed relative to the footwall.
Orogeny - The process of building mountains, particularly by folding and overthrusting.
Reverse (Thrust) Fault - A fault in which the Hanging Wall rises up relative to the Footwall. They are called thrust faults generally when the angle of thrust is less than 30 degrees.
Strike - The course or bearing of a structure, bed, fault trace etc. on the surface of the earth.
Syncline - A fold that is concave upwards.
Terrane - A geologic formation or group of formations in an area over which a particular rock or group of rocks is prevalent. Images
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