Can flexural-slip faults related to evaporite dissolution generate hazardous earthquakes? The case of the Grand Hogback monocline of west-central Colorado Francisco Gutiérrez 1,† , Domingo Carbonel 1 , Robert M. Kirkham 2 , Jesús Guerrero 1 , Pedro Lucha 3 , and Vincent Matthews 4 1 Departamento de Ciencias de la Tierra, Universidad de Zaragoza, C/. Pedro Cerbuna 12, 50009 Zaragoza, Spain 2 GeoLogical Solutions, 5253 County Road 1 South, Alamosa, Colorado 81101, USA 3 Departamento de Didáctica de las Ciencias Experimentales, Universidad de Zaragoza, C/. San Juan Bosco 7, 50009 Zaragoza, Spain 4 Leadville Geology LLC, 519 West 7th Street, Leadville, Colorado 80461, USA ABSTRACT A paleoseismological investigation of flexural-slip faults related to interstratal evaporite dissolution suggests that such gravitational structures might have the po- tential to generate earthquakes with dam- aging magnitude. The Carbondale collapse center, in the southern Rocky Mountains of Colorado, is a morpho-structural depression of ~1200 km 2 where Miocene volcanic rocks are downdropped as much as 1200 m due to interstratal dissolution of halite-bearing evaporites. On the western margin of the col- lapse center, the debuttressing effect related to active evaporite dissolution drives un- folding of the steeply dipping late Laramide Grand Hogback monocline, accompanied by displacement on bedding-parallel faults. These flexural-slip faults rupture uncon- formable Miocene basalts and Quaternary mantled pediments, generating conspicuous half-graben depressions bounded by anti- slope fault scarps parallel to the underlying strata of the monocline. Two trenches dug across flexural-slip fault scarps developed in each stratigraphic marker (basalt cap, mantled pediment) revealed unexpected evi- dence of multiple late Quaternary faulting events (e.g., faulted colluvial wedge, sharp unconformities), with displacement-per- event values of ≥1 m. Three faulting events were inferred from the trench dug in the pediment (<32 ka, 32–28 ka, 5.6–1.5 ka), and four events from the trench sited in the basalts, all probably older than 20 ka. The probable length (~25 km) and downdip width (~7.5 km) of the flexural-slip faults associated with the Carbondale collapse center suggests that they might have the potential to generate damaging “unfolding earthquakes” with mo- ment magnitude (M w ) around 6. INTRODUCTION Trenching investigations of active tectonic faults have increased exponentially in number in the last few decades. They provide criti- cal parameters for seismic hazard assessment such as slip rate, recurrence, and timing of faulting events (McCalpin, 2009a). However, trench investigations of active faults related to or potentially attributable to gravitational processes (e.g., landsliding, evaporite dissolu- tion) have rarely been attempted. This approach may provide practical data on (1) the deforma- tion style, kinematics, and deformation history of hazardous surface-rupturing structures (e.g., slip rate, creeping versus episodic displacement, most recent event); (2) controlling factors; and (3) parameters that may help to differentiate between tectonic and nontectonic faults—a relevant issue, from the seismic-hazard perspec- tive, with a still poorly developed scientific basis (Hanson et al., 1999). Recent trenching studies on active normal faults related to interstratal dis- solution of salt-bearing evaporites conducted in the Iberian Chain and the Pyrenees, Spain (Gutiérrez et al., 2012a, 2012b; Carbonel et al., 2013), and in the Paradox Basin, Utah (Guerrero et al., 2012), indicate episodic displacement on these gravitational structures, and slip rate and displacement-per-event values significantly larger than those expectable for tectonic faults. Interstratal dissolution of evaporites may gen- erate a wide variety of deformation structures (Warren, 2006; Gutiérrez and Cooper, 2013), including active faults with surface expression. The cartographic scale of these structures may be of the order of tens of kilometers, and strain rates may reach values significantly higher than those reported for tectonic structures due to the high solubility of the evaporites. In the Delaware Basin (New Mexico and Texas), dissolution of Permian evaporites has generated depositional basins more than 150 km long filled with Upper Neogene continental sediments that reach around 500 m in thickness (Hill, 1996, and ref- erences therein). Monoclinal flexures in supra- evaporitic units atop downdip-migrating disso- lution fronts have been documented in numerous regions, including northern England (Cooper, 2002), Canada (Hopkins, 1987; Anderson et al., 1988), and the Paleozoic evaporite basins of the southwest and central sectors of the USA (De Mille et al., 1964; Walters, 1978; Ander- son et al., 1994; Neal and Colpitts, 1997). In the Interior homocline of central Saudi Arabia, the downdip migration of dissolution fronts in east-dipping Upper Jurassic evaporite units has generated a west-facing sinuous monoclinal escarpment more than 550 km long (Powers et al., 1966; Memesh et al., 2008). Subsidence due to interstratal dissolution of evaporites may also play a significant role in the evolution of major fluvial systems. According to Gustavson (1986), the development of the Canadian River valley in the Texas Panhandle, for 200 km of its length, is largely related to subsidence due to the downward and lateral migration of dissolution fronts in deep-seated salt units, rather than to erosional lowering. Deep-seated dissolution of evaporites may also result in the formation of faults passing through the supra-evaporitic units and the devel- opment of grabens, foundered blocks controlled by circular or ellipsoidal faults and collapse structures bounded by failure planes with highly For permission to copy, contact editing@geosociety.org © 2014 Geological Society of America 1481 GSA Bulletin; November/December 2014; v. 126; no. 11/12; p. 1481–1494; doi: 10.1130/B31054.1; 9 figures; 2 tables; published online 23 June 2014. † E-mail: fgutier@unizar.es