Description:

Journal of Structural Geology 32 (2010) 1996-2008 Contents lists available at ScienceDirect Journal of Structural Geology journal homepage: www.elsevier.com/locate/jsg Late Pleistocene-Holocene ruptures of the Lima Reservoir fault, SW Montana David J. Anastasio a,*, Christina N. Majerowicz a, Frank J. Pazzaglia a, Christine A. Regalla a Lehigh University, Department of Earth and Environmental Sciences, 1 West Packer Avenue, Bethlehem, PA 18015-3001, United States b Pennsylvania State University, Department of Geosciences, State College, PA 16802, United States Article info Article history: Received 27 April 2009 Received in revised form 19 August 2010 Accepted 24 August 2010 Available online 11 November 2010 Keywords: Normal faulting Scarp diffusion model Active tectonics Basin and Range Montana Abstract Active tectonics within the northern Basin and Range province provide a natural laboratory for the study of normal fault growth, linkage, and interaction. Here, we present new geologic mapping and morphologic fault-scarp modeling within the Centennial Valley, Montana to characterize Pleistocene-Holocene ruptures of the young and active Lima Reservoir fault. Geologic relationships and rupture ages indicate Middle Pleistocene activity on the Henry Gulch (>50 ka and 23-10 ka), Trail Creek (>43 ka and w13 ka), and reservoir (w23 ka) segments. Offset Quaternary deposits also record Holocene rupture of the reservoir segment (w8 ka), but unfaulted modern streams show that no segments of the Lima Reservoir fault have experienced a large earthquake in at least several millennia. The clustered pattern of rupture ages on the Lima Reservoir fault segments suggests a seismogenic linkage though segment length and spacing make a physical connection at depth unlikely. Trail Creek and reservoir segment slip rates were non-steady and appear to be increasing. The fault helps accommodate differential horizontal surface velocity measured by GPS geodesy across the boundary between the northern Basin and Range province and the Snake River Plain. ? 2010 Elsevier Ltd. All rights reserved. 1. Introduction Extension in the shallow crust is accommodated by the growth of normal fault segments (Dawers and Anders, 1995; McLeod et al., 2000; Walsh et al., 2003). The transfer of deformation within extensional systems can lead to seismogenic linkage (King et al., 1994; Stein, 1999; Di Bucci et al., 2006) or the propagation of ruptures to nearby faults due to coor post-seismic reloading and relaxation of fault tips (Chery et al., 2001a, b; Kenner and Simons, 2005). These processes can result in a temporal clustering of seismic activity, which has been observed along several faults within the western United States (Lynch et al., 2003). Clustering of large magnitude earthquakes indicates non-steady, long-term slip rates. Understanding faults with long recurrence intervals requires combined paleoseismological data and the modern seismic record (Marco et al., 1996; Stickney and Lageson, 2002). A chronology of paleoruptures, fault rupture lengths, and fault displacements can inform assessments of seismic hazards and landscape evolution in tectonically active areas (Wells and Coppersmith, 1994). The actively deforming northern Basin and Range province in the western United States provides an excellent locality to study such normal fault development (Fig. 1). * Corresponding author. Tel.: +1 610 758 5117; fax: +1 610 758 3677. E-mail addresses: dja2@lehigh.edu (D.J. Anastasio), cmajerowicz@gmail.com (C.N. Majerowicz), fjp3@lehigh.edu (F.J. Pazzaglia), cregalla@geosc.psu.edu (C.A. Regalla). 0191-8141 $ e see front matter ? 2010 Elsevier Ltd. All rights reserved. doi:10.1016/j.jsg.2010.08.012 In this study, we use geologic mapping to constrain the rupture length and segmentation of the Lima Reservoir fault and fault scarp morphologic modeling to reconstruct the timing of ancient surface ruptures along fault segments. A refined Quaternary stratigraphy of Centennial Valley provides a temporal framework to understand Pleistocene and Holocene ruptures and to assess the fault’s development while accommodating deformation along the structural boundary between the Snake River Plain and the Basin and Range province. Models of fault scarp morphology estimate the time elapsed since surface rupture initiation and can provide a better estimate of fault slip rate than an estimate derived from dividing scarp offset by the age of the faulted horizon (e.g., Nash, 1980, 1986; Hanks et al., 1984; Mattson and Bruhn, 2001). 2. Geologic setting 2.1. Lima Reservoir fault In southwest Montana, the Centennial Tectonic Belt is characterized by young, underfilled grabens bounded by seismogenic normal faults of limited strike-length and throw (Stickney and Bartholomew, 1987). The Lima Reservoir fault is part of the Centennial Tectonic Belt, which extends westward from the Yellowstone eruptive center. (Fig. 1A). The Lima Reservoir fault outcrops as a series of discontinuous, south to southwest verging, extensional surface ruptures persisting for more than 24 km along the southern flank of the Laramide-style Snowcrest Range (Perry et al., 1988; Majerowicz, 2008; Fig. 1B). The fault includes the Henry Gulch, Trail Creek, and the reservoir segments (Fig. 2). In the adjacent Red Rock Valley, the Red Rock and Monument Hill faults are nearly coincident with the emergent Sevier thrust front (Harkins et al., 2005; Regalla et al., 2007) and in the Centennial Valley, the Centennial and Lima Reservoir faults border the Snowcrest Range suggesting that the locus and geometry of Quaternary-Holocene extensional faulting is inherited from earlier Sevier-Laramide contractional structures. The Centennial fault is a north-dipping normal fault, which together with the Lima Reservoir fault forms the Lima Reservoir graben defining the east-west trending Centennial Valley. The Centennial fault and the Lima Reservoir fault are listed as “Class A” faults by the U.S. Geologic Survey because of evidence of at least one rupture within the last 130 kyrs (Haller et al., 2000). Faults in northern Basin and Range province are less studied than to the south and may have displacement histories related both to regional extension and longwavelength epeirogenic deformation related to passage of the Yellowstone eruptive center (Wegmann et al., 2007; Bartholomew et al., 2009). 2.2. Fault scarps 2.2.1. Henry Gulch segment The Henry Gulch segment was recognized by Majerowicz et al. (2007), as a westward extension of the Lima Reservoir fault (e.g., Haller et al., 2000; Stickney et al., 2000) on the basis of faceted spurs, fault scarps, and offset Paleozoic units. The Lima Reservoir fault gradually changes orientation from east-west trending along the reservoir segment to northwest-southeast trending along the Henry Gulch segment to become strike-parallel with the Monument Hill fault further north (Fig. 1B). Where preserved, the Henry Gulch fault scarps are 2-5 m high. Along much of the trace, however, the fault scarps are covered by more recent mass flow deposits, which make them unsuitable for fault scarp morphological studies. D.J. Anastasio et al. Journal of Structural Geology 32 (2010) 1996-2008 Fig. 2. Surficial geologic map along the Lima Reservoir fault system. See Fig. 1 for map location. Tertiary and older units blank, see Majerowicz 2007, 2010 for bedrock geology. Quaternary alluvium and debris flow deposits sourced primarily from the Tertiary Sixmile Creek Formation (uvial and volcanoclastic deposits and basalt flows) form the valley walls of the Red Rock River near the site of the modern Lima Dam (Fig. 2) (Majerowicz et al., 2007, 2010). Mass movement of these materials into the Red Rock River Valley during the Pleistocene blocked the ancestral watershed creating a large lake in the Centennial Valley. The debris flow deposits head at faceted spurs along the Henry Gulch segment and the age of the littoral deposits (Hill et al., 2005) constrain a maximum age for a Middle Pleistocene rupture of the Henry Gulch segment of the Lima Reservoir fault. The extent of the Pleistocene pluvial lake is recorded by lacustrine and palaudal deposits that are incised by a paleo-shoreline, which provides a piercing point for Lima Reservoir fault displacement. The Henry Gulch segment offsets alluvial fan and older lacustrine deposits of Middle Pleistocene age but the fault does not cut Holocene alluvium (Fig. 2) (Majerowicz et al., 2007). Table 1 Quaternary Stratigraphy of Centennial Valley. All Quaternary deposits are unconsolidated. Conventional 14C ages from Hill et al. (2005) and this study calibrated to calendar years before present (B.P.) using software available at http://www.radiocarbon.ldeo.columbia.edu/research/radiocarbon.htm, after Fairbanks et al. (2005), with 1s uncertainties and 14C ages from Harkins et al. (2005), calibrated to calendar years B.P. with INTCAL98 following Stuiver et al. (1998), with 2s uncertainties. OSL ages underlined with 1s uncertainties. See text for further discussion. Ключевые слова: calendar year, science, quaternary stratigraphy, field photograph, vertical exaggeration, single-slip, basin, younger, trail, rate, journal, ka, physical connection, gulch, modeled, prole, slip rate, normal, background slope, fault scarp, holocene rupture, creek, alluvial fan, morphologic, evolution, fault rupture, modern, seismological society, red, older, earthquake, based, map, chery, modeled age, single-slip age, active, qlko, table, offset surface, constant-slip, horizontal position, landscape evolution, regalla, temporal clustering, modern reservoir, age, rock valley, structural, anastasio journal, montana, western united, nash, stuiver, merrell locality, creek segment, diffusion constant, journal structural, elevation, sample, society america, journal structural geology, proles, maximum age, average, fault segment, rupture age, hill, hanks, reservoir segment, geologic mapping, lima reservoir, centennial fault, geophysical, lima reservoir fault, kyr, scale, time step, basin range, creek reservoir, relay ramp, pleistocene, henry gulch, reservoir fault, fan, northern, avouac, trail creek, reservoir, eastern, throw, range province, centennial valley, ha, bulletin, constant-slip age, society, majerowicz, location, rock, eastern sector, diffusion, result, scarp prole, mkyr, elsevier, lacustrine unit, wallace, fault slip, eld relation, quaternary, topographic prole, king, constant, quaternary faults, lima, age range, anastasio, radiocarbon, geomorphology, data, reservoir scarp, gile, fan deposit, pro?le, stickney, estimate, pattern, red rock, slip, wa, prole site, structural geology, measured, constant-slip model, america, throw measured, payne, centennial, henry, fairbanks, material, fault, model, middle, soil, geology, single slip, birkeland, rupture, university, unit, eastern tip-line, segment, large earthquake, fault throw, geologic survey, perry, morphologic dating, deposit, journal geophysical, single-slip modeling, valley, pro?les, stein, measured proles, holocene, uncertainty, single, report, pazzaglia, time, modeling, discussion, bartholomew, nash hanks, diffusivity constant, offset, northern basin, rms, ? ka, rupture length, conventional age, river, scarp, geologic, surface, gulch segment, alluvial, dawers, central sector, dated, surface offset, constant slip, position, province, mka, dated deposit, range