Northern Rockies Geology in Space and Time:
Reflections on the Big Questions of Northern Rockies Geology
and how can EarthScope play a role?
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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?
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.
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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.
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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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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.
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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.
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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.
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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?
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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.
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.
Ben A. Van der Pluijm, University of Michigan
Basil Tikoff, University of Wisconsin - Madison
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