We combined Laramide minor faults with a GIS database of structural trends to determine the tectonic controls on Rocky Mountain basement-involved foreland structures. GIS anal- yses show that post-Eocene (RIO) structures are dominated by bimodal fault orientations consistent with dual-stage NE-SW and E-W extension during Browns Park and Rio Grande extension. Ancestral Rocky Mountain (ARM) structures generally paral- lel Laramide structures, supporting a collisional push from the southwest for the Ancestral Rocky Mountain orogeny (e.g., Ye et al., 1996; Poole et al., 2005).
Unlike the analogous Sierras Pampeanas of southern Argen- tina, where active Laramide-style structures parallel Paleozoic structures, Rocky Mountain Precambrian (PRE) ductile folds, contacts, shear zones, and foliations dominantly trend NE-SW, at high angles to most Phanerozoic structures. Thus, our data sug- gest that reactivation of pre-orogenic ductile fabrics has played only localized, nonessential roles in determining structural trends within basement-involved foreland orogens.
Preexisting major faults may or may not have been reacti- vated, perhaps depending on their position relative to basement- involved arches. Local control by preexisting faults occurs throughout the Rockies (e.g., Brown, 1993), particularly in the E-W Uinta arch. Hypotheses suggesting that Laramide faults parallel a widespread grid of Precambrian faults (Marshak et al., 2000; Timmons et al., 2001) are not supported by our data: the fault modes commonly occur independently and systematically change their orientation within the orogen. Thus, reactivation of regionally extensive fault sets was not the major control on Laramide deformation.
Impingement and/or collapse of the Colorado Plateau and the Cordilleran thrust belt also were not primary controls on Laramide structural trends. For the Colorado Plateau, fault and fold orientations sweep systematically across the eastern plateau boundary in western Colorado as if the boundary had minimal
mechanical signifi cance. For the thrust belt, Laramide deforma- tion only parallels thin-skinned thrust belt deformation immedi- ately adjacent to its boundary.
For syn-Laramide minor faults, average slip (N67E-01) and compression (N67E-02) directions indicate unimodal, ENE-WSW shortening and compression, in excellent agree- ment with shortening directions predicted by Laramide arches (N67E) and folds (N66E), assuming shortening perpendicular to fold axes. The lack of consistent multidirectional fold trends appears to negate hypotheses invoking major, punctuated rota- tions of compression directions due to changing plate boundaries (Gries, 1983; Bird, 1998, 2002). This result is consistent with syn-Laramide igneous activity near the U.S.-Canada border that suggests only one oceanic plate, the Farallon plate, underlay the Rocky Mountains during the Laramide orogeny (Haeussler et al., 2003; Madsen et al., 2006).
The eastward change from NW-SE to N-S fold trends in Wyoming and Colorado appears to be proportional to distance from the eastern limit of the Laramide orogen. This could be construed as evidence for the partitioning of NE-SW Laramide shortening into N-S dextral strike-slip and E-W thrust motion at the eastern boundary of Rocky Mountain deformation (Karl- strom and Daniel, 1993; Cather, 1999; Cather et al., 2006). However, continuous Precambrian ductile structures in southern Wyoming (Fankhauser, 2006; Sims et al., 2001) show no evi- dence of through-going, N-S dextral strike-slip faults. The lack of large strike-slip systems in the analogous Sierras Pampeanas (Allmendinger et al., 1990; Ramos et al., 2002) also indicates analogous horizontal contraction without orogen-scale strain partitioning. In addition, the nearly identical average Laramide slip and compression directions suggest no large-scale, regional strike-slip shear in the Rockies. The obliquity of en echelon Laramide arches in New Mexico (N-S trends) and northern Wyo- ming (more E-W trends) to province boundaries appears to result from transpression required by three-dimensional strain compat- ibility, where the southeastern orogen margin needs a component of dextral motion and the northern orogen margin needs a mirror- ing component of sinistral motion.
The close spatial correlations between the present geometry of the low-angle segment of the Andean Benioff zone, the Juan Fernandez Ridge on the Nazca plate, and basement-involved foreland arches of the Sierras Pampeanas show a clear causal relationship (Jordan et al., 1983; Ramos et al., 2002; Alvarado et al., 2005). These observations support the hypothesis of Saleeby (2003), which states that Laramide deformation of the cratonic interior and the disruption of the southern Sierra Nevada batho- lith were both due to the subduction of an oceanic plateau on the Farallon plate. Certainly, the arcuate Laramide front, with its con- centric fold and radial slip directions, is consistent with a primary impact occurring in the vicinity of modern-day southern Califor- nia. Gradual west-to-east changes in Laramide fold-axis orienta- tions from NW-SE to N-S may refl ect rotation of Farallon–North American convergence directions. In Wyoming, where deforma- tion progressed from southwest to northeast (Brown, 1988; Perry
et al., 1992), this predicts a clockwise rotation of shortening and compression directions consistent with rotations of plate conver- gence vectors hypothesized by Saleeby (2003).
The actual connection between subduction processes and basement-involved deformation remains uncertain. End load- ing of the North American plate by continental collision during the Pennsylvanian (Ancestral Rocky Mountain orogeny) and by plateau subduction during the Cretaceous-Paleogene (Laramide orogeny) could explain both orogens. The unusually thick crust of the Rocky Mountain region (Keller et al., 2005) may have allowed the remarkable extent of these orogens by providing hot- ter, and thus weaker, lower crust. For the Laramide orogeny, a lithospheric thrust wedge defi ned by the subducting plate below and a crustal detachment above is consistent with current models of Andean deformation (Jordan and Allmendinger, 1986; Ramos et al., 2002, 2004). We anticipate that the deep seismic data from EarthScope investigations will test current Laramide models invoking thrust imbrication of the entire lithosphere (McQueen and Beaumont, 1989), lithospheric buckling (Tikoff and Max- son, 2001), lithospheric hydration (Humphreys et al., 2003), and crustal buckling and detachment (Fletcher, 1984; Erslev, 2005).
In conclusion, the consistency of Laramide minor fault, arch, and fold trends supports hypotheses invoking one stage of ENE- WSW shortening and compression due to subduction-driven Laramide contraction. Local complications due to reactivation of preexisting weaknesses and external impingement of the Cordil- leran thrust belt were superimposed on this primary ENE-WSW contractional pattern. In general, our view of Rocky Mountain structural trends is analogous to looking through a thoroughly fractured window. Local complexities inherited from a long his- tory of prior events have blurred and distorted Laramide patterns, requiring careful integration to reveal their essential systematics. The minor fault and GIS methods presented here can help inte- grate complex deformation patterns into coherent, information- rich images.
ACKNOWLEDGMENTS
This research was made possible through generous contri- butions of geologic and geographic information systems (GIS) expertise by John C. Reed Jr., Paul Sims, Carol Finn, Victoria Rystrom, Melinda Laituri, Dave Theobald, Tim Wawrzyniec, John Geissman, Karl Kellogg, David Lageson, Steve Cather, Karl Karlstrom, and many others. Colorado State University (CSU) fault data were mostly collected by CSU students Bjorn Selvig, Phillip Molzer, David Hager, Joe Gregson, Stephanie O’Meara, Branislav Jurista, Tim Ehrlich, Jason Ruf, Mandy Fisher, Steve Holdaway, Melissa Copfer, Seth Fankhauser, Thomas Neely, Cyrus Gillett, John Detring, and Scott Larson. Rocky Mountain Map Company, a division of Barlow and Haun, donated structure contour maps of the Rocky Mountain basins. Funding came from the Petroleum Research Fund of the American Chemical Soci- ety, the Continental Dynamics Program of the National Science Foundation, Colorado Geological Survey, and Edward Warner of
Expedition Oil Company. We gratefully acknowledge a review by David Lageson of this paper.
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