EarthScope in the Northern Rockies Workshop > Program > Session III

Session III

Northern Rockies Geology in Space and Time:
Reflections on the Big Questions of Northern Rockies Geology and how can EarthScope play a role?


Processes that make a continent

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Gene Humphreys, University of Oregon
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The Northern Rockies provide outstanding opportunities to address basic processes of tectonic and magmatic growth, and segregation and structuring of continental lithosphere. In particular:

  • What is tectosphere and what is its tectonic influence? The Northern Rockies and adjoining Great Plains provide EarthScope its best target.
  • What does accretion make, and have accretion processes changed through time? The Northern Rocky area contains many sutures, from Archean to Cenozoic, and includes accreted continent, island arc and ocean lithosphere.
  • What did the Laramide do that led to uplift of the Rocky Mountains and Great Plains? Was density modification a Laramide occurrence, or did it make the lithosphere susceptible to later Cenozoic processes?
  • What forces caused Laramide crustal contraction so far inland?
  • What is the Yellowstone hotspot and how has it affected the lithosphere (short- and long-term changes in density, composition and structure)?
  • Are plate stresses more a product of basal tractions or loads acting at the plate margins?
I'll focus on the last two of these questions.

Yellowstone hotspot system—A propagating zone of continental reconstruction Wallowa Mountains and Columbia River flood basalts—A delamination event. The Wallowa Mountains are the high center to a 200-km diameter circular "bulls eye" pattern of oscillating uplifts and sags, and they are comprised of the largest of the few granitic plutons in the area. The bulls eye region is the source area for most Columbia River Basalt (CRB) eruptions. Elevation maps of CRB flow interfaces are used to quantify the magnitude, timing and distribution of surface uplift, and teleseismic data is used to tomographically image the structure of the underlying mantle. The bulls eye area is characterized by mild pre-eruptive subsidence, syn-eruptive uplift of several hundred meters, and a persistent km-scale uplift of the Wallowa Mountains, and it overlies an anomalous volume of mantle that is devoid of partial melt compared with adjacent areas. Within the context of current geological understanding, only delamination of compositionally dense plutonic roots can explain the uplift timing and magnitude and the imaged upper mantle structure. This suggests that CRB magmatism is a consequence of lithospheric processes rather than arrival of a mantle plume; it is possible that delamination was triggered by the arrival of hot mantle, and that the local intensity of CRB magmatism resulted from the combined effects of anomalously hot mantle and lithospheric delamination.

Yellowstone Swell—Mantle residuum flattening beneath the continent. The Yellowstone swell is underlain by high-velocity mantle, cored by low-velocity (partially molten) mantle beneath the hotspot track. Swell uplift seems to require a flattening mass of hot and basalt-depleted buoyant mantle, consistent with but not necessitated by plume models. Yellowstone. Bob Smith will talk about recent findings in the Yellowstone area.

North America Plate stress By modeling the basal, marginal and gravitational loads acting on the North American plate, we find that gravitational potential energy and marginal forces are most important, but that basal tractions do contribute significantly. These include both tractions excited by global mantle flow and cratonic root drag. In Canada, North Atlantic and Artic ridge push compresses the continent against the Pacific Plate; root drag shields western U.S. from ridge push and allows it to extend. The transition between these two states is across the U.S. Northern Rockies. Also important to the Northern Rockies is the additional PE supplied by Yellowstone, which has the effect of focusing Basin and Range extension narrowly across the Yellowstone Park area.

Click to enlarge
Figure I. Location map showing CRB source area, the Wallowa (W) and Blue (B) Mountains. Productive Grande Ronde dikes are shown as short lines. Major extensional (lines with balls) and contractional (double arrows) structures are indicated. Major plutons west of Precambrian North America are outlined with heavy dotted lines. (inset) Map of western U.S. showing locations of major magmatic centres in relation to the Yellowstone hotspot, including: McDermitt caldera (M), Steens Mountains (S) and the source of the CRB eruptions in NE Oregon (C). The arcuate Snake River Plain (SRP) is the Yellowstone hotspot track leading to the active Yellowstone caldera (Y). Note bull's eye pattern of uplift at NW end of SRP.

Click to enlarge
Figure II. (a) Western U.S. Geodynamics. Red and blue bars show modeled stress (red= tension, blue=compression). The light-colored area shows where tectonically important deformation is occurring, with Yellowstone (circled Y) at the NE apex. The colored line segments represent active faults (thick for major plate-boundary faults). Gold arrows show velocity relative to North America for the Pacific and Juan de Fuca (JdF) Plates, and for selected points within the deforming western U.S. (including the Sierra Nevada (SN) and Siletzia (S) blocks). Stress trajectories represent the observed stress field. (b) Gravitational potential energy and resulting modeled stresses. Contour level is 1 TN/m. Note extension within the regions of high GPE and compression in the regions of low GPE, with the compressive axis oriented with the GPE gradient. In the Basin and Range and California these stresses are responsible for most of the non-strike-slip deformation. (c) Boundary and basal loads (arrows) and resulting modeled stresses. Note the shear stress field established across California and western Nevada caused by transform interaction, the compression "ahead" (i.e., north) of California, and the tension common in the western U.S. interior.

Mesoproterozoic belt basin: Crustal-scale anomaly in the Northern Rockies

Title slide from the presentation: Mesoproterozoic belt basin: Crustal-scale anomaly in the Northern Rockies
J. W. Sears, University of Montana
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The Mesoproterozoic Belt basin dominates the Rocky Mountains of northwestern Montana and adjacent regions. Cross-sections based on surface data and limited industry seismic data and boreholes suggest that the wedge-shaped, NE-tapering basin was inverted into the Purcell anticlinorium upon overthrusting the flat surface of the neighboring North American craton in unusually large and thick thrust plates. Restored sections suggest that the Mesoproterozoic Belt Supergroup, which filled the basin, thickened from a zero edge along the NE margin of the basin to 20+ km along the basin axis to the west. The upper 15 km of section is exposed, the lower 5+ km is interpreted from industry seismic reflection profiles and is thought to largely comprise gabbro sheets. This unusually great thickness may represent the fill of an intracratonic rift system that nearly proceeded to sea-floor spreading. Metamorphic-grade diagenesis, driven by massive mafic intrusions in the floor of the basin dehydrated the quartz-rich Belt Supergroup, so that it formed extremely strong and coherent thrust sheets during Cordilleran orogenesis. In addition to the Mesoproterozoic section, there was 2 km of Paleozoic platform strata and 6 km of Mesozoic foreland basin strata and volcanics at the time of thrusting. The basin was truncated along its western edge by Late Precambrian-Early Cambrian continental rifting, so that the Cordilleran miogeocline overlapped the well-layered Belt Supergroup in this region. This created a crustal-scale structural anomaly during Cordilleran orogenesis. Structural analysis suggests that Belt Supergroup thrust sheets rotated clockwise, with intervening transpressive shear zones, such as along the Lewis and Clark line. Isostatic considerations suggest that these massive thrust sheets depressed large-scale thrust ramps as they were displaced out of the Belt basin, driving flexural bending of the foreland basin; rebound of the ramps may have driven uplift and extension of the thrust sheets after conclusion of the Cordilleran thrusting in early Paleogene. Because of the crustal scale and excellent layer anisotropy of the Belt Supergroup, Earthscope projects should be ideal for evaluation and refinement of structural models based on surface data.

Pre-Miocene magmatism in the Northern Rocky Mountains
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Reed S. Lewis, Idaho Geological Survey

Arthur A. Bookstrom, U.S. Geological Survey
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Major episodes of magmatism in the northern Rocky Mountains include (1) 1380 Ma A-type granites and associated mafic rocks, (2) Neoproterozoic dioritic and syenitic intrusions, (3) Cretaceous and Paleocene granitic rocks of the Idaho batholith, and (4) Eocene intrusive and extrusive rocks of the Challis episode. The A-type granites are present in Idaho in a northwest-trending belt extending from Salmon to north of Orofino; all are southwest of the Brushy Gulch-Green Mountain fault system. These granites are similar in age and composition to the better-studied granite-rhyolite province of the mid continent. The Neoproterozoic intrusions are also in a northwest-trending belt (about 50 km SW of the A-type granite belt) but differ in having quartz-poor compositions. The Taylor Ranch mylonite zone, characterized by subhorizontal lineations, is probably a remnant of a northwest-trending fault zone that localized the intrusions. Widespread Cretaceous and Paleocene granodiorite, granite, and tonalite of the Idaho batholith are predominantly sodic (Na2O typically >K2O) and thus similar to the "sodic series" of the Boulder batholith to the east in Montana (Tilling, 1973). Only the southeast part of the batholith is potassic, as are outliers in northern Idaho. Metamorphism in country rocks adjacent to the potassic suite is restricted to relatively narrow contact aureoles, indicating shallower levels of emplacement than the plutons of the sodic suite. Limited age data indicate that at least some of the plutons of the potassic suite are 10-15 Ma older than the sodic suite. The potassic suite originated from a relatively enriched but more mafic source, perhaps located at a different crust level (or more inboard?) than the source for the sodic suite. Two suites of Eocene plutonic rocks are present: (1) granite with some A-type characteristics, and (2) diorite, quartz monzodiorite, granodiorite, and granite. Shallow-level intrusions are common; deep-seated Eocene plutons that are mineralogically similar to the Cretaceous batholith have only recently been recognized and are poorly dated. Emplacement of the older Eocene plutons was in part controlled by east-west structures, whereas the younger intrusions formed along northeast-trending structures.

Magmatism of the Snake River Plain and Yellowstone Region: Implications for continental lithospheric evolution above a mantle plume

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William P. Leeman, National Science Foundation
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Changing plate boundary conditions led to diachronous extension that has propagated northwards across western North America since Oligocene time, forming the Basin and Range province (BRP). Superimposed on this pattern, major flood basalt volcanism emerged at ca. 16 Ma near the Oregon-Idaho-Nevada state boundaries (to produce the Columbia River and other coeval lavas). Differences in lithosphere-scale structures led to dramatically different magmato-tectonic responses in western accreted terranes vs. eastern continental cratonic terranes. Subsequently, voluminous rhyolite-basalt volcanism migrated northeastward from this region to the Yellowstone volcanic field (NW Wyoming), forming the Snake River Plain. Although commonly attributed to the influence of an upwelling mantle plume, detailed investigations indicate that this magmatic activity differs significantly from that associated with oceanic hot spot tracks, and is a response to positive feedbacks between lithospheric and sub-lithospheric processes. Key observations and questions warranting further consideration can be formulated in the context of work in the SRP-Y region.

SRP magmatism is dominated by distinctly bimodal products, comprising early high-T, high-silica rhyolite, generally later olivine tholeiitic basalt, and minor intermediate products—some of which are mixtures of rhyolite and basalt. Rhyolite compositions indicate that they are crustal melts, the formation of which implies massive inputs of basaltic magma into the crust as the primary heat source. The scarcity of basaltic lavas in the early eruptive sequences, and their prominence following cessation of silicic magmatism suggests that the crust acts as a low-density filter until it has been extensively modified. The scale of this process is sufficiently large that, in order to conserve crustal thickness near the observed 40 km, significant extension is required—most likely parallel to the axis of the hot-spot track (i.e., normal to orientations of regional BRP faults).

Early silicic magmatism in the west and central SRP is demonstrably coeval with N- to NW-trending faults, and in some cases high-angle faulting is contemporaneous with major eruptive episodes. Moreover, widely distributed outbreaks of genetically unrelated rhyolite eruptions are closely associated in time and space with such faulting. These observations suggest that silicic (and presumably also mafic) magma bodies were present contemporaneously across broad areas (perhaps as much as 400 km wide). This view differs from the earlier notion that magmatism was controlled simply by migration of N. America over a focused hot spot.

Finally, origin of the underpinning basaltic magmatism is problematic. Available geochemical data for SRP basalts suggest that they are melts of relatively shallow domains within an isotopically evolved mantle. These data cannot easily be explained by contamination of basaltic magmas produced from sub-lithospheric mantle similar to sources for oceanic island basalts. Rather, the sources are likely to be within an old subcontinental lithospheric mantle keel (i.e., part of the upper plate). If this view is correct, models are required whereby primitive basalt can be (1) produced from lithospheric mantle in high volumes and just prior to onset of silicic magmatism, and (2) sustained long after demise of the silicic magmatic phase. One such model is that of Harry & Leeman (1995), in which extension can induce melting of eutectoid-like lenses in the lowermost lithosphere. In any case, the inferred pattern of basaltic melt production differs from that predicted for a simple plume model.

These concepts can be evaluated and quantified through high-resolution seismic and other geophysical investigations aimed at defining the density distribution within the crust beneath the SRP. Likewise, detailed seismic attenuation investigations, such as those in the vicinity of Yellowstone, can help constrain the distribution of melt as a function of depth. Further investigations are needed to better understand the history of crustal deformation, and its relation to volcanic stratigraphy and magmato-tectonic evolution of the SRP. Detailed information concerning the temporal-spatial variations in volcanic production rates across the region could help constrain the distribution, scale, and magnitude of magmatism. Finally, multidisciplinary investigations are needed to understand how the lithospheric mantle and crust evolve in response to large-scale magmatism.


Influence of structure and composition of basement on mineral deposits across Montana and Idaho

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Lund, Karen, Klein, T.L, O'Neill, J.M., Sims, P.K., U.S. Geological Survey
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Topical studies of basement architecture and of regional stratigraphy, structure, and mineral deposits by the U.S. Geological Survey Headwaters Province study of Montana and Idaho resulted in a new understanding that basement composition and structure were key to formation of mineral deposits.

The age and configuration of Precambrian crystalline basement rocks in the region are problematical because of sparse, widely dispersed exposures, a generally thick cover of Mesoproterozoic and younger sedimentary rocks, and the complexity of superposed tectonism and magmatism. Thus, knowledge of basement is strongly dependent on geochemical heritage gained from isotopic studies of younger igneous rocks and geophysical data. Based on these data, the basement is dominated by the northern margin of the Archean Wyoming province to the southeast and the Paleoproterozoic Wallace ocean-arc terrane and two attached Archean minincontinents(?), the Medicine Hat and Pend Oreille, to the northwest. These are joined by the multiply reactivated, northeast-trending Paleoproterozoic Great Falls tectonic zone, a suture complex. The tectonic zone includes a 1.9-1.8 Ga Paleoproterozoic magmatic arc and a parallel Paleoproterozoic fold-and-thrust belt affecting Paleoproterozoic marginal basin sediments as well as the edge of the Wyoming province. In the Mesoproterozoic, north- and northwest-trending strike-slip shear zones segmented the assembled crystalline crust, resulting in transtensional and transpressional fault-bounded blocks that controlled Mesoproterozoic sedimentation as well as subsequent tectonic features.

From the 1860's to the present, large quantities of Au, Ag, Cu, Mo, Pb, and Zn have been produced from ore deposits in Montana and Idaho. Major mineral-deposit trends include the northeast-trending Idaho-Montana porphyry belt, the northwest-trending Coeur d'Alene belt, a less well-defined northwest-trending zone of Au-Co-Cu deposits, and a north-trending belt in the Atlanta lobe of the Idaho batholith. Most of these deposits are epigenetic and lie in the Idaho-Montana porphyry belt, a 200-km wide, more than 1,000-km long, northeast-trending belt extending from southwestern Idaho to north-central Montana. The Idaho-Montana porphyry belt contains the world-class Cu-Mo deposits of the Butte district and more than 80 deposits designated as economically significant. The epigenetic deposits of the porphyry belt are associated with granitoid rocks of many different compositions and textures belonging to the Cretaceous Idaho and Boulder batholiths as well as the Eocene Challis volcanic-plutonic episode, providing no age/compositional pattern of mineral genesis. However, the porphyry belt coincides spatially with the northeast-trending Great Falls tectonic zone and probably relates to metal-endowed basement and (or) structural character of the suture complex. Where the major batholiths lie over the older basement terranes, they are typically barren except where disturbed by north- or northwest-trending basement structures. Northwest-trending Mesoproterozoic strike-slip zones probably localized both sedimentation of the Mesoproterozoic Belt basin and related sediment-hosted deposits such as those of the Coeur d'Alene belt. Parallel Au-Co-Cu sediment-hosted deposits of the Blackbird type may have had a similar origin. Additionally, the world-class hydrothermal deposits at Butte and the highly mineralized area around Helena in the northern Boulder batholith lie at a critical juncture between the porphyry belt and the northwest-trending basement structure system related to the Coeur d'Alene district.


Laramide and Sevier-style tectonics

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John Oldow, University of Idaho
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Mesozoic to Early Tertiary transpressional tectonism in the central North American Cordillera formed in response to oblique convergence between the oceanic plates of the Pacific basin and the western continental margin. In the mid- to Late Mesozoic, several hundred kilometers of crustal shortening were accommodated by a thin-skinned, foreland thrust belt (Sevier-style) with displacement directed toward the continental interior. The strike-slip component of oblique convergence was taken up by large-magnitude transcurrent faults that formed within the hinterland of the orogen. The foreland thrust belt and transcurrent faults were kinematically linked by a through-going basal decollement system that connected the orogen with the plate boundary. In the Canadian and northwestern US Cordillera, exotic terranes were accreted along the western margin of North America, whereas in the central and southern US Cordillera significant terrane accretion was not accomplished and the Mesozoic arc complex remained in contact with the ocean basin. Mesozoic backarc contraction occurred along the entire plate boundary, and in the western Idaho hinterland of the Northern Rocky Mountains, the fringing arc and backarc basin were imbricated in the west-facing Salmon River belt. This was followed by accretion of the Wallowa terrane and intra-continental transpression within the Western Idaho Shear Zone. In the latest Cretaceous and Early Tertiary, progressive foreland deformation in the Southern Rocky Mountains shifted from thin- to thick-skinned, basement cored thrusts (Laramide-style). In contrast, thin-skinned thrusting continued in the Southern Canadian Rockies until the demise of contraction in the Eocene. The transition between thin- and thick-skinned foreland structures is preserved in southern Montana and northern Wyoming, where the frontal decollement system of the Canadian Rockies stepped down to lower-crustal depths beneath Laramide thrusts. Thick-skinned shortening of the Laramide belt was transferred west to the Shuswap crustal duplex in hinterland of the southern Canadian orogen via a northwest trending oblique-ramp system. The oblique ramp resulted in the uplift and exhumation of high-grade metamorphic and plutonic rocks of the Idaho batholith and is marked to the north and south by the Lewis and Clark Shear Zone and Orofino Shear Zone, respectively. The transition between thin- and thick-skinned deformation in the foreland and the northwest-trending oblique-ramp transferring thick-skinned deformation into the Canadian hinterland appear to be controlled, at least in part, by pre-existing crustal structure. The Northern Rocky Mountains expose the transition between the fundamentally different tectonic regimes expressed by the northern and southern segments of the North American Cordillera. Through the EarthScope flexible array together with active source seismic reflection profiling, the crustal and upper mantle structure of this tectonic transition within the continental margin can be imaged and provide an unparalleled link between deep tectonic processes and deformation kinematics and timing determined from surface investigations. Assessment of the crustal geometry linking thin- and thick-skinned foreland structures will help resolve the degree of vertical coupling operating within the continental lithosphere during plate-boundary tectonism. Characterization of the northwest-trending oblique-ramp system will allow formulation of the 3D architecture of the orogen as the deep expression of the foreland structures are traced into the hinterland. Finally, the deep expression of intensely deformed rocks along western flank of the Idaho batholith will yield insight into the geometry and depth of the late Mesozoic intracontinental suture zone developed during back-arc basin collapse and terrane accretion.

Opportunities for understanding extensional processes in the Northern Rocky Mountains

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Susanne U. Janecke, Utah State University
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Extensional processes have shaped the crust of the Northern Rocky Mountains for more than a billion years. Rifting produced the Belt basin shortly after 1.5 Ga, and initiated the Cordilleran passive margin. Mesozoic to early Cenozoic convergence produced the Cordilleran fold-and-thrust belt, the uplifts of the Rocky Mountain foreland (Laramide) and an unusually broad volcanic arc in the northern Rocky Mountains. The structural grain of these contractional belts and the rheology of the Idaho batholith influenced the locus, kinematics and geometry of the >55 m.y. of extension that followed. Sparse data suggest that extension may have begun in the Cretaceous during the "Sevier orogeny", partially collapsing large culminations of the thrust belt, but widespread extension began in latest Paleocene to middle Eocene time. The large amount of core complex-related extension, starting during or just prior to the massive Eocene Challis-Sanpoil-Absaroka volcanic flare up, is highlighted by the unusual presence of pairs of core complexes at the same latitude. Despite this long history of extension and the Paleogene phase of detachment faulting the crust is still ~ 39 km thick across much of the region. This is ~ 9 km above the global average for rifted continental crust and thicker than the crust in the younger Great Basin to the south. Was the crust unusually thick before extension or did plutonism inflate the crustal column?

The Lewis and Clark zone is a fundamental crustal flaw in the Northern Rocky Mountains and may be a continental transfer fault. Across the zone, the dip direction of major normal faults changes, the thickness of thrust sheets increases abruptly northward, and the axis of core complexes steps and bends left. Does this feature penetrate to lower crustal levels, or even into the mantle? What is its geometry at depth?

The Paleogene phase of extension is the first of many kinematically distinct episodes of extension. Some areas near the Eastern Snake River Plain (ESRP) preserve as many as 6 different episodes of normal faulting with extension directions ranging from NE to NW to N. The normal faults with the largest displacement typically parallel the curving contractional structures of the Late Mesozoic to early Tertiary Sevier belt and basement thrusts in the Rocky Mountain foreland. Focal mechanisms should reveal oblique slip if this parallelism is largely due to crustal inheritance and dip slip if gravitational collapse, continuing more than 45 m.y. after the end of shortening, is a major cause of the extension.

The youngest, currently active system of normal faults have a regular spacing and consistent N to NW trends, and more west dips than east dips west dips within the former fold-and-thrust belt. Faults become more widely spaced westward in the strong and isotropic Idaho batholith. Normal faults east of the thrust belt have a range of strikes and reactivate Laramide structures with NE, NW and N and E trends. These normal faults display additional complexities near the Yellowstone hotspot.

The Rocky Mountain Basin-and-Range province narrows northward and terminates near the Canadian border. The spatial pattern and geometry of the faults suggest clockwise rotation about an axis near the north edge of the province. GPS data sets will provide critical tests of this hypothesis and show whether the Idaho batholith and Lewis and Clark zone are discontinuities in the modern strain field.

The Yellowstone hotspot, the perturbed and densified crust of the ESRP, and the parabola centered on Yellowstone clearly influenced extension in the Basin-and-Range province. Normal faults within the parabola are unusually active, whereas structures beneath the ESRP are unusually quiescent because normal faults no longer extend across the ESRP. This suggests that there may be a mismatch of extensional strains between the Basin-and-Range province and the ESRP. If so, why is there no geologic evidence for faults along the margins of the ESRP, and no deflection of thrusts and facies belts? The NW dip of the Yellowstone plume to 500 km (Yuan and Dueker, 2005) is consistent with the asymmetry in extensional belts NW and SE of the ESRP (Pierce and Morgan, 1992).

Earthquakes in the greater Yellowstone region show domains of anomalous N-S extensional strains north of the ESRP that are consistent with the anomalous north-dipping Centennial fault and other older normal faults farther west. Flexure toward the ESRP plus SW subsidence away from Yellowstone could produce a northeastward migrating domain of E-W striking normal faults NW of the ESRP and NNE-striking normal faults south of the ESRP. Lateral flow of lower crustal rocks from beneath the ESRP might also produce such a pattern.

Click to enlarge
Figure 1. Regional map showing the major active normal faults adjacent to the Eastern Snake River Plain (ESRP), the north-northwest-trending Cache-Pocatello culmination (CPC), and other culminations of the thrust belt. Note that many of the largest normal faults coincide with the culminations of the older fold-and-thrust belt. Active normal faults with unusual trends are shown in orange. Processes near the hot spot may have produced these "cross faults". Cambrian and older pre-Tertiary subcrop defines the extent of culminations in the Sevier, Wasatch, and Salmon areas. The extent of Devonian and older subcrop is shown for the Cache-Pocatello culmination. Subcrop was truncated north of 44°45'. GSL = Great Salt Lake, SC=Sevier culmination; SAC=Salmon area culminations; WF = Wasatch fault, YH = Yellowstone hotspot. Compiled from Armstrong (1968), Hintze (1980), Schirmer (1988), Kuntz et al. (1992), Rodgers and Janecke (1992), Yonkee (1997), Stewart et al. (1998), Janecke et al. (2000), Janecke et al. (2001), and Carney and Janecke (2005). Modified from Carney and Janecke (2005).

Earthscope from the Montana Regional Seismic Network perspective

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Michael C. Stickney, Earthquake Studies Office, Montana Bureau of Mines and Geology
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The Montana Regional Seismic Network (MRSN) has evolved since 1980 to its current configuration of 40 stations covering most of the northern Intermountain Seismic Belt (ISB) in western Montana with an average spacing of ~50-60 km. The majority of the MRSN stations are vertical-component short-period stations with distribution and spacing controlled partly by telemetry constraints. Five three-component broadband stations operated cooperatively with the USGS provide the first continuous, high-quality digital data from Montana beginning in 1999. MRSN data are used to locate over 1,000 earthquakes per year with magnitudes ranging from less than 1 to 5.6. The MRSN catalog contains over 24,000 earthquakes located since 1982. A review of Montana seismicity reveals that the MRSN has operated during the quietest period with respect to historic magnitude 5 and larger earthquakes. Very few small to moderate magnitude earthquakes occur along mapped Quaternary faults. Instead, background seismicity occurs near the ends of Quaternary faults or in their hanging or foot walls and diffusely throughout the ISB. Most seismicity in southwest Montana results from normal faulting but strike slip faulting is more common in west-central and northwestern Montana. The dominant stress field along the northern ISB is ENE-WSW extension with a more localized area of NNE-SSW extension in the Hebgen Lake-northwest Yellowstone area. Scant seismological evidence suggests NE-SW-directed compression, consistent with the mid-continent stress field to the east of the ISB.

P-wave travel times from 1,432 earthquakes recorded by the MRSN were used to derive a new western Montana crustal velocity model (Zeiler et al., 2005). The model consists of three layers with velocities increasing from 5.7 km/sec near the surface to 6.5 km/sec in the lower crust, overlying a Mohorovicic discontinuity at 39.7 km depth with an upper mantel velocity of 8.0 km/sec. Available seismic data are inadequate for velocity model determination outside the ISB.

Accurate depth determinations are problematic with regional station spacing, but well-resolved focal depths typically range from 3 to 16 km below the surface, consistent with seismogenic depths elsewhere in the ISB. However, a region of west-central Montana lying east of the Mission Mountains and north of the Lewis and Clark zone frequently has earthquakes with focal depths ranging from 18 to 22 km below the surface. This observation implies that the seismogenic depths of the Mission and Swan faults may extend 25% deeper than assumed in some seismic hazard analyses.

Questions that USArray data may help answer:

  • Where is, and what is the nature of, the boundary between extension along the ISB in western Montana and mid-continent compression to the east?
  • What are the differences between crustal and upper mantle structure in tectonically active western Montana and geologically stable eastern Montana?
  • Can surface wave studies of regional seismic sources and other seismic methods shed light on the geometry and characteristics of first-order geologic features such as Laramide basement uplifts (e.g. Beartooth Range), sedimentary basins (Belt and Williston basins), and fundamental structural discontinuities (Lewis and Clark zone, Brockton-Froid fault zone)?
  • How does the anelastic attenuation structure vary throughout the region?
  • Can the range of site response characteristics be accurately characterized and correlated with mapped geology?

Integrating undergraduate education and research in the Northern Rockies

thumbnail image of a slide from the presentation: Integrating undergraduate education and research in the Northern Rockies
Robert C. Thomas, Sheila M. Roberts and R. Stephen Mock, Environmental Sciences Department, The University of Montana-Western
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Geoscience faculty members have always recognized that research is one of the best learning experiences for undergraduate students. These research projects provide focus, ownership and responsibility for pursuing real scientific problems. Unfortunately, undergraduate research is commonly only available to students through senior theses and independent study and the bulk of the undergraduate geoscience curriculum is centered on the teaching of disciplinary terminology in lecture-based courses.

In order to allow for undergraduate research opportunities in every course in the geoscience curriculum, the faculty members in the Environmental Sciences Department at The University of Montana-Western in Dillon, Montana have adopted a research-based curriculum that utilizes a one-class-at-a-time scheduling model pioneered by Colorado College over 30 years ago. Under this approach, students take four classes per semester that are taught one at a time for an average of three hours per day. Each class lasts for 18 instructional days and the students and faculty both get a four-day break between each course.

This scheduling system is ideal for research-based courses because there is ample time to gather and analyze data and the students have no other courses competing for their time and attention. Courses are structured like the professional work environment, requiring students to get the job done rather than just put in their time. The students have a need to know information, so reading papers and understanding the language of the geosciences has purpose and meaning other than the fear of examination. Data from the first two years of a FIPSE (Fund for the Improvement of Post-Secondary Education)-funded pilot program at The University of Montana-Western show dramatic improvements in student performance, retention and satisfaction.


Recent Changes in the Rocky Mountains: Neogene Drainage Changes and Biogeography

Paul Karl Link, Idaho State University

As the heterogeneous crust of the North American Plate moved over the Yellowstone-Snake River Plain hotspot, a north-east migrating wave of high topography has forced the Rocky Mountain Continental Divide eastward by several hundred km over the last 15 Ma (Pierce and Morgan, 1992). This has caused stream drainage capture, of formerly east and south flowing streams, by the west-draining Snake River system (Beranek, 2005; Beranek et al., submitted). These topographic effects have also acted as limits and controls on local Pleistocene glacial, pluvial, and arid climates.

Neogene drainage connections are demonstrated by detrital zircon and sandstone provenance studies (Link et al., 2002; accepted), and, totally independently, by studies of Recent and fossil fluvial vertebrates (fish, muskrats, beavers) and invertebrates (snails) (Repenning et al., 1995, Van Kirk et al., 2003; Herschler and Liu, 2004; R. Herschler, pers. comm. 2003-2005). For example, late Miocene and Pliocene fish of Lake Idaho have affinities with the Sacramento River drainage; late Miocene fossil muskrats at Hagerman Fossil Beds have affinities with the Humboldt drainage, as do late Miocene to Recent snails; there are striking differences in mitochondrial RNA in various fish species between the Bonneville basin and the Snake River basin (M. Campbell, IDFG, pers. comm., 2005; K. Mock, USU, pers. comm., 2005); etc.

By extending our database of detrital zircons in Neogene and Holocene stream and lake deposits of the Snake River basin into the headwaters of the Missouri River and the Clark Fork in Montana, we can produce a graphic illustration of drainage changes and capture over the last 100 Ma in southwest Montana. Huge Late Cretaceous boulder-choked streams drained eastward into Jackson Hole (Lindsey, 1973; Janecke et al., 2000). Those quartzite clasts and the detrital zircons they contain, derived from the Belt Supergroup in east-central Idaho, are now washing down the Snake River system. Eocene to Miocene extension along the Muddy-Grasshopper detachment fault in SW Montana controlled complex deposition of the Renova Formation and coeval units, which display radically different detrital zircon signatures in various facies and locations (Link et al., 2004).

In addition to the obvious importance for studies of biogeography, such research is eminently attractive graphically, and can be used to make Web- or Public TV-based programs to demonstrate topographic change in the northern Rockies. The concept of the migrating hotspot and even the 15 Ma time scale is comprehendible by the average American. The lessons taught about not-so terra firma will be graphic. I have a Geoscience Education NSF grant called Digital Geology of Idaho, which could help produce such visualizations.

This type of detrital zircon study, paired with Earth Scope deep-crustal geophysics, can locate specific crustal boundaries that have persisted or been reactivated through Neogene topographic change. This in turn allows predictions as to how crustal structures respond to stress from thermal expansion and contraction, loading, and Basin and Range extension, and has obvious implications for geologic hazard studies.

Click to enlarge
Click to enlarge
Figure: Detrital-zircon spectra from the upper Snake River and Portneuf River systems. Plots are both single 50 to 60-grain samples and lumped multiple samples. Grains over 3 Ga are not shown. Plots are shown for 0 to 150 Ma grains and for 150 to 3000 Ma grains. from Link et al., (accepted, Sed. Geol.)

A GeoTraverse in the Northern Rockies

thumbnail image from a slide in the presentation: A GeoTraverse in the Northern Rockies
David W. Mogk, Montana State University
Ben A. Van der Pluijm, University of Michigan
Basil Tikoff, University of Wisconsin - Madison
download PowerPoint presentation (PowerPoint 1.8MB Oct24 05)
GeoTraverse is a strategy to optimize the research capabilities of the EarthScope Project (e.g. USArray and PBO instrumentation) by deploying a densified network of flex array seismometers in a coordinated manner across the continent. Careful planning of the location of a GeoTraverse can help ensure that all EarthScope science and education goals are fully addressed. GeoTraverse is a cross-continent, transect-based research framework that supports integrated, multidisciplinary (geologic and geophysical) study of the three-dimensional structure and temporal evolution of the crust and uppermost mantle of the United States. The GeoTraverse concept will promote integrated geoscience research, drawing from the wealth of existing geoscience mapping and data (geological, geochemical, geochronological, and geophysical) and taking full advantage of the unique opportunities provided by EarthScope to provide a synoptic four-dimensional view of the North American continent. GeoTraverse may be designed as a continuous transect across the North American continent, with the advantage of spanning the transition zones between adjacent geological provinces. Or, GeoTraverse may also be developed in segments that address particular crustal segment of interest (e.g. Yellowstone-Snake River Plain, the Belt Basin) or regions where a particular geological process or phenomena merits special attention (e.g. Laramide structures, hydrothermal-magmatic processes, the roots of the Montana Alkalic Province, the transition from the Wyoming Province to the accreted terranes to the west). The GeoTraverse concept has the possibility of fully integrating the geological component into EarthScope and thereby broadening how the geological community does research. It also provides for the possibility of significantly enhancing the education and outreach components of EarthScope, potentially with collaboration of the National Parks. For geoscience researchers and educators in the northern Rocky Mountain region: Is there community support for the GeoTraverse concept, and if so, what are the key research targets for GeoTraverse? Where would you construct the GeoTraverse and why?


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