Contemporary seismicity, faulting, and the state of stress in the Colorado Plateau ~:~SGR~~ii;HREY } Woodward-Clyde Consultants, 500 12th Street, Suite JOO, Oakland, California 94607 ABSTRACT west- or northeast-striking planes at shallow with the installation of regional networks by the depths. The contemporary state of stress University of Utah Seismograph Stations The contemporary seismicity of the Colo­ within the plateau is characterized by approx­ (UUSS), the Los Alamos National Laboratory rado Plateau based on seismic monitoring in imate northeast-trending extension in con­ (LANL), and the USGS Albuquerque Seismo­ the past 30 yr can be characterized as being of trast to the previous belief that the plateau logical Laboratory (ASL) (Fig. 2). Despite the small to moderate magnitude, and contrary to was being subjected to east-west tectonic additional network coverage, however, detec­ earlier views, of a low to moderate rate of compression. One area of the plateau, the tion of earthquakes smaller than ML 2 within occurrence with earthquakes widely distrib­ eastern Wasatch Plateau and Book Cliffs, the plateau interior improved only slightly. This uted. Concentrations of earthquakes have may still be characterized by compressive paper presents the results of the first major pro­ been observed in a few areas of the plateau. stresses; however, the nature of these stresses gram of seismographic studies performed within The most seismically active area of the Colo­ is not well understood. the Colorado Plateau. This program, initiated by rado Plateau is the eastern Wasatch Plateau­ Woodward-Clyde Consultants (WCC) in 1979, Book Cliffs, where abundant small-magnitude INTRODUCTION included an extensive program of microearth­ seismicity is induced by coal mining. The quake monitoring. The results of these studies, largest earthquakes observed to date, of esti­ The Colorado Plateau in the western United combined with those of other recent studies to mated Richter magnitude (ML) 5-6, have States (Fig. 1) has attracted geologists since the be discussed, allow a comprehensive characteri­ generally occurred in northern Arizona. Al­ 1800s because the geologic past has been readily zation not heretofore possible of the contempor­ though very few earthquakes can be asso­ revealed by the excellent bedrock exposures, the ary seismicity, faulting, and state of stress of the ciated with known geologic structures or diversity of geologic structures, and the textbook Colorado Plateau. tectonic features in the Colorado Plateau, stratigraphy. Few seismologic and seismotec­ seismicity appears to be the result of the reac­ tonic investigations, however, have been con­ GEOLOGIC AND TECTONIC tivation of pre-existing faults lacking surficial ducted until recently, partly due to the belief that SETTING expression but favorably oriented to the tec­ the plateau was nearly seismically quiescent and tonic stress field. The small to moderate size that no large earthquakes had occurred within The Colorado Plateau has been characterized of the earthquakes and their widespread dis­ the plateau in modern times. These observations, as a relatively coherent uplifted crustal block tribution are consistent with a highly faulted however, were based upon a historical earth­ surrounded on three sides by the extensional Precambrian basement and upper crust, and quake record that was incomplete because of the block-faulted regime of the Basin and Range a moderate level of differential tectonic stress. sparse population, the relatively recent settle­ province and the Rio Grande rift (Thompson Earthquakes in the plateau generally occur in ment of the region, and inadequate seismograph­ and Zoback, 1979) (Fig. I). Geologic evidence the upper crust from the near-surface to a ic coverage prior to the 1960s. attests to a lack of major crustal deformation in depth of 15-20 km, although events have In the early 1960s, seismographic coverage of the plateau at least since the end of the Laramide been observed in both the lower crust and the Colorado Plateau as well as much of the orogeny 40 m.y. ago. Asymmetric basement up­ uppermost mantle in areas of low to normal western United States improved greatly, due lifts with long sinuous northwest-to-northeast heat flow. The latter suggests that tempera­ principally to the beginning of the Worldwide trends (Fig. 3) that are bounded on their eastern tures are sufficiently low at these depths that Standardized Seismograph Network. Several side by steeply dipping monoclines and asso­ brittle failure and hence earthquakes are still stations were installed within the intermountain ciated structural basins are the major structural possible. The predominant mode of tectonic United States, including the first seismographic features of the Colorado Plateau. Three episodes deformation within the plateau appears to be station within the plateau established by the U.S. in the structural evolution of the plateau have normal faulting on northwest- to north­ Bureau of Reclamation (USBR) at the proposed had significant impacts on the expression of re­ northwest-striking faults with some localized Glen Canyon dam (GCA) in 1958 (Fig. 2). cent tectonic activity: ( 1) Precambrian tectonics, occurrences of strike-slip faulting on north- These stations provided the capability to locate (2) the Laramide orogeny, and (3) the Cenozoic all moderate-size earthquakes (Richter magni­ structural differentiation of the Basin and *Also at Arizona Earthquake Information Center, tude ML ;,, 4) occurring within the Colorado Range-Colorado Plateau province. Northern Arizona University, Flagstaff, Arizona Plateau. In the 1970s, seismographic coverage of Little is known of the Precambrian tectonics 86011. the plateau's margins improved significantly of this region. Prior to Cambrian time, however, Geological Society of America Bulletin, v. IOI, p. 1127-1146, 16 figs., I table, September 1989. 1127 ',tI _ 100° T ··-- .. .. ' ---- ··----··-·· Great Plains Northern Basin and Range 350- Rio Grande Rift I Great j Plains I \. \.. 0 300 600 km \ __, ··'"'-··- _________ Figure 1. Physiographic provinces in the western United States. The stress province boundaries of Zoback and Zoback (1980) (heavy solid and dashed lines) and the Colorado lineament as defined by Warner (1978) (shaded area) are also shown. The dotted line within the Colorado Plateau represents the low heat-flow thermal interior of Bodell and Chapman (1982). SEISMICITY, FAULTING, AND STATE OF STRESS, COLORADO PLATEAU 1129 112° 110° ,oso and laccoliths that were intruded between 48 to 44 Ma (Armstrong, 1969) (Fig. 3). IDAHO I Structural differentiation between the north­ 4 I ern Basin and Range and the Colorado Plateau -N~;~;-1-----:--i • :--1 provinces started sometime after 29 Ma and ap­ • 4 4 I WYOMING pears to have been underway by 26 Ma (Row­ ley and others, 1979), coinciding with possibly ~ ': UTAH :~ ! ... L-----,-------------- the first episode of uplift (Morgan and Swan­ ~ 4 4 .. 4 COLORADO 4 berg, 1985). By 24 Ma, the plateau was signifi­ N / 4 4 4 astern Wasatch I 4 UUSS Plateau- cantly lower than the adjacent Basin and Range. 40° 4 Book Cliffs 4 1 Crustal extension in the Basin and Range be­ j 4 1 Western ••• I • A 1Colorado ••• came widespread about 17 Ma (Stewart, 1978). l I 1-1•*.... This extension has been accommodated by both ... ••• listric and planar normal faults which produce ••MGC I Canyonlands Paradox large rotations of fault blocks and also by large ... ... I ..yalley displacements on nonrotational low-angle faults Capitol •• ~ 4 4 (Wernicke and Burchfiel, 1982). Active exten­ ... 0~ 4Reef ~T. ,.,., 4A COLORADO sion in an east-west to northwest-southeast di­ ... ;.,." •• ... ~SBR/USGS PLATEAU 4 A 0' wee BOUNDARY rection is now occurring in the northern Basin ,{' Glen and Range province and the High Plateau sub­ ;.,.<I::- Canyon I I .._ ___ .s--!- province of south-central Utah (Arabasz and Ju­ ------ lander, 1986) . GCA ---+--~-- • I Crustal extension in central and southern I San Juan 4 4 .. 4 Utah along the western margin of the plateau Basin / LANL ... Northern I• ... ........ also extends eastward well beyond the physio­ I USGS ...i •JI' ,. .. Arizona ...... graphic boundary. A transition zone on the NAU ... .. order of 50-100 km wide is characterized by ...... r.--:~ related occurrences of late Cenozoic normal ... faulting, late Tertiary and Quaternary basaltic ... volcanism, high levels of seismicity ( a part of the Intermountain seismic belt), high heat flow, and 1NMIMT ... low P n velocities relative to the plateau interior ARIZONA (Thompson and Zobaek, 1979). Thompson and Zoback (1979) proposed that Basin and Range extensional stresses in Arizona extend as much as 100-200 km into the Colo­ 0 100 200 300 km rado Plateau. On the basis of the present-day crustal extension of the transition zone and the Figure 2. Seismographic stations in the Colorado Plateau (designated by triangles) in the possible migration of volcanism toward the pla­ early 1980s from which data were used in this study and index map of areas discussed in text. teau interior, Best and Hamblin (1978) sug­ Appropriate network operators are also noted. Station GCA was the first permanent station gested that Basin and Range extensional stresses installed in the Colorado Plateau. may be actively encroaching into the plateau. (See Arabasz and Julander [1986) for an excel­ lent discussion of the transition zone.) the region must have undergone major deforma­ interior from aeromagnetic and gravity data and Smith and Sbar (1974) first stated that the tion and metamorphism to produce the complex from surficial geology (Hite, 1975). northern Colorado Plateau is being subjected to suite of metamorphic and igneous rocks exposed The Laramide orogeny took place in the tectonic compression between two zones of east­ in the Grand Canyon and the Uncompahgre up­ western United States from - 70 to 40 Ma. Most west- directed extension, the lntermountain lift (Fig. 3). Where the basement structure is deformation within the Colorado Plateau took seismic belt on the west and the Rio Grande rift exposed in the Grand Canyon, the east-dipping the form of reverse faulting on steeply dipping on the east.