EQUIPOS AUXILIARES, SISTEMAS DE REGULACIÓN Y CONTROL

We extended the kinematic framework established with minor fault data by using more extensive and uniformly distrib- uted map-scale structures. Phanerozoic fault and fold data were combined with Precambrian contact, shear zone, and foliation data from Wyoming, Colorado, New Mexico, and parts of east- ern Utah and Arizona in a GIS database (Bolay-Koenig, 2001; Bolay-Koenig and Erslev, 2003). The boundaries of the area, somewhat arbitrary due to time constraints and limited data sources, extended south from the Montana-Wyoming border to central New Mexico and east from eastern Utah and Arizona to the eastern edges of Colorado and New Mexico (Fig. 2), including most of the Colorado Plateau. Thin-skinned Sevier thrust belt structures were excluded in our attempt to focus on basement-involved deformation.

Sources for fault and fold orientations include 19 USGS 1° × 2° geologic quadrangle maps (listed in references separately) covering all but north-central New Mexico and southern Wyo- ming and 22 basin structure contour maps on Early Cretaceous and late Paleozoic formations provided by the Rocky Mountain Map Company. Areas not covered by these data sets were cov- ered using the Wyoming basement contour map (Blackstone, 1993), larger-scale USGS geologic maps (Colton, 1978; Houston and Karlstrom, 1992), Kelley’s (1955) tectonic map of the Colo- rado Plateau, and large-scale maps over the Sierra Nacimiento

(Woodward, 1987), southern Sangre de Cristo (Miller et al., 1963), and Sandia (Kelley and Northrop, 1975) mountains of northern New Mexico. The pattern of anastomosing Laramide arch trends (Erslev, 1993) was defi ned by examining regional structure contour maps. Most exposed Precambrian contacts and shear zone traces were acquired from USGS 1° × 2° geo- logic quadrangle maps. Subsurface Precambrian contacts were acquired from basement maps of Sims et al. (2001), who extrapo- lated surface contacts to the subsurface using core and aeromag- netic data. Colorado Precambrian foliation data were collected from an unpublished foliation map compiled by Ogden Tweto and loaned to us by John C. Reed Jr.

Acetate overlays were generated and digitized for each source map, and data were subdivided by structure type and the age of involved strata. Fault and fold data were subdivided into four age-based subsets (PRE, ARM, LAR, and RIO), which were determined by the age of the youngest strata involved in the structure. PRE structures only involve Precambrian rocks, ARM structures involve pre-Permian rocks, LAR structures involve Permian to Paleogene rocks, and RIO structures involve Neogene rocks.

These age-based subsets assured that no older structures were attributed to younger orogens. It should be noted, however, that younger structures can and were attributed to older orogens. For instance, the PRE fault subset contains Phanerozoic faults that happen to be limited to Precambrian exposures. In addition, the certainty of a structure’s age varies with the type of struc- ture. The LAR arch data set is more reliably Laramide in age than the LAR fault data set, which probably contains numerous post-Laramide faults. PRE contact (crystalline rock contacts), shear zone, and foliation data are more certain to be Precam- brian because they formed or were modifi ed by ductile processes that, for currently exposed rocks in the Rocky Mountains, were largely restricted to the Precambrian.

The raw data (Table 3) were digitized from the acetate overlays into ARCINFO™ and transferred into ARCVIEW™ software. In order to quantify and compare the relationships between structural fabrics on geologic maps, a subroutine (script) broke each arc into individual line segments. This was necessary because if an arc’s orientation is solely calcu- lated from its end points, there would be no difference between straight and highly curved arcs.

Individual structure data sets were divided into geographic subsets by outlining an area for analysis using ARCVIEW™’s selection capabilities or by using another script to create an evenly spaced array of subdomains. For each subdomain, subsets can be created to include all line segments having center points within a user-specifi ed distance from central grid points.

Once a subset was selected, vector mean and dispersion val- ues were calculated. Dispersion is a measure of clustering; a dis- persion of 0.0 indicates only one orientation within the data, and a dispersion of 1.0 indicates no single preferred orientation. The vector mean was calculated for the subset’s line segments using

1986), which essentially (1) doubles each line angle, (2) does a standard vector mean calculation of the doubled angles (here weighted for line length), and (3) divides the fi nal value by two to determine the mean line orientation.

Length-weighted rose diagrams of line segment orientations were created to provide a more direct visualization of line distri- butions. The total line segment length for every 1° increment was calculated and smoothed using a window of 10°, where the extent of an individual petal from the center is proportional to the sum of the lengths of lines within 5° of that petal’s orientations. Rose diagrams are scaled to effi ciently fi ll available space and are not scaled to the total amount of data.

SUMMARY OF ROCKY MOUNTAIN STRUCTURAL