Fault-Related Folding in California’s Northern Channel Islands Documented by Rapid-Static GPS Positioning and other buried reverse faults depend has focused largely on using surface entirely on assumptions unique to the deformation measurement to test fault-bend fold geometry. We suggest subsurface models of fault-related Nicholas Pinter, Geology Department, that alternative models should be folding. Southern Illinois University, considered where geologically Precise positioning using the Global Carbondale, IL 62901-4324, USA, reasonable, and that surface uplift Positioning System (GPS) has [email protected] measurements across fault-related folds revolutionized investigations of can provide a crucial test of subsurface geodynamics and crustal deformation Bjorn Johns, UNAVCO, geometry. (University NAVSTAR Consortium, 1998). 3340 Mitchell Lane, Boulder, CO 80301, Data collection of ≥24–48 hours per site USA, [email protected] INTRODUCTION can yield precisions of a few millimeters Especially since the 1994 Northridge in measurements of intersite baseline Brandy Little, Geology Department, earthquake, buried reverse faults have distances. In contrast, this project used Southern Illinois University, been the target of intensive research. In the same geodetic-quality GPS Carbondale, IL 62901-4324, USA, southern California and elsewhere, the equipment as a high-precision surveying [email protected] locations of these structures, their tool, sacrificing roughly an order of geometry at depth, and rates and magnitude of precision for rapid- W. Dean Vestal, Geology Department, histories of slip are based primarily on acquisition capability and results Southern Illinois University, balanced cross sections utilizing the forthcoming in a single field season. Carbondale, IL 62901-4324, USA, fault-bend-fold and fault-propagation- Unlike geodynamic GPS studies, which [email protected] fold models (e.g., Shaw et al., 1994). directly measure tectonic motion over a Because so much crucial information is few years, we measured “fossil” strain model specific, there have been accumulated on uplifted coastal terraces ABSTRACT rumblings of doubt about the over periods of ~105 yr. Rapid-static Global Positioning System widespread application of these models positioning was used to measure late without independent corroboration Setting Quaternary deformation of uplifted (e.g., Ramsay, 1992; Stone, 1996). Our The Northern Channel Islands (Fig. 1) coastal terraces on Santa Rosa and San work in the Northern Channel Islands form the southern margin of the Santa Miguel Islands, which have accumulated regional warping and local faulting over 100+ k.y. Late Quaternary deformation on Santa Rosa is sharply partitioned, with several hundred meters of surface- rupturing left slip, but 15+ m of vertical motion taken up by folding, resulting in anticlinal growth of at least 0.12 m/k.y. Deformation on San Miguel Island to the west is consistent with warping on the north limb of the regional anticlinal structure. Deformation on both islands matches activity to the east, suggesting that the Santa Rosa Island fault represents an en echelon segment of the larger Transverse Ranges Boundary fault system and the Northern Channel Islands antiform. The Northern Channel Islands have been the focus of a debate over differing geometrical models of fault-related folding, and the Figure 1. The Northern Channel Islands form the southern margin of the Santa Barbara deformation measurements presented Channel. Faults are from Jennings (1994). DF—Dume fault; FF—Ferrilo fault; MCT— here suggest that uplift of the islands is Mid-Channel trend; ORF—Oak Ridge fault; SCFS—San Clemente fault system; SCIF—Santa occurring over a smoothly curved thrust Cruz Island fault; SRIF—Santa Rosa Island fault. Digital topography is from U.S. Geological ramp rather than a fault-bend fold. This Survey digital elevation models, and bathymetry is from National Oceanic and Atmospheric reinterpretation is important, as current Administration data, compiled by L. Mertes, Department of Geography, University of earthquake-hazard assessments on this California, Santa Barbara. 4 MAY 2001, GSA TODAY Barbara Channel and the southernmost Island, ~2000 m of Eocene to Miocene km, dual-frequency GPS receivers range of the western Transverse Ranges. sedimentary and volcanic rocks are record both the L1 (1227.6 MHz) and L2 This area is a natural laboratory for exposed (Dibblee and Ehrenspeck, (1575.42 MHz) signals in order to remove examining neotectonic processes that 1998). The Tertiary sequence is overlain ionospheric errors. include: (1) complex and partitioned by diverse late Quaternary deposits, To achieve the cm-level precision strain; (2) fault-related fold growth; and especially dune sand, which reaches required for most survey applications, (3) fault activity that threatens the local thicknesses of ≥50 m (Woolley, 1998). GPS carrier-phase data must be used. area, including Los Angeles. Two active Coastal terraces fringe Santa Rosa and Receivers that utilize phase data record left-lateral faults cut the islands—the form a broad bench across much of the not only the code from each satellite but Santa Rosa Island fault (Colson, 1996) northern half of the island (Orr, 1960). A also the phase of each incoming signal. and the Santa Cruz Island fault (Pinter uranium-series date of 120 ± 3 k.y., In postprocessing GPS phase data, the and Sorlien, 1991). These faults appear equivalent to oxygen-isotope substage software must find a unique or fixed- to be continuous with similar faults 5e, has been obtained from solitary integer solution for the number of along the northern margin of the Los corals on the second lowest terrace carrier-wave cycles for each satellite Angeles Basin, together forming a (Thorup, 1994), and this age has been signal. The resulting level of precision semicontinuous zone which has been reconfirmed by independent U-Th depends largely on the duration of called the Transverse Ranges Boundary analysis (J. Lundberg, 2000, pers. comm.). continuous measurements at a site. fault zone (Pinter et al., 1998). These The most prominent tectonic feature Rapid-static GPS positioning (Blewitt, surface-rupturing faults seem to coexist of Santa Rosa Island is the Santa Rosa 1990) refers to the mode of operation in with low-angle thrust faults at depth Island fault. Displacement of Miocene which one collects the minimum data (e.g., Shaw and Suppe, 1994; Rivero et rocks across the Santa Rosa Island fault necessary—as little as ~10 min of al., 2000). Regional thrust structures suggests at least 16 km of left slip on measurement—to obtain a fixed-integer have been inferred beneath the Santa this fault (Weaver, 1969), and more carrier-phase solution. For the Monica Mountains (Davis and Namson, recent motion is indicated by drainages equipment used here, vertical precision 1994; Dolan et al., 1994) and beneath deflected several hundred meters to the is estimated at 1.0 cm plus one part per the Northern Channel Islands (the left (Colson, 1996). There has been million times baseline length (Trimble Channel Islands thrust; Shaw and some disagreement about the degree of Navigation, 1998), suggesting that all Suppe, 1994). It has been suggested that latest Quaternary activity on the Santa elevations measured on Santa Rosa and uplift of the Northern Channel Islands Rosa Island fault. Suggestions of San Miguel were precise to 2.8 cm or chain is the result of south-vergent slip relatively rapid (~1 mm/yr) and recent better relative to the local base station. across a north-dipping ramp in the slip (≥2 ruptures in the past 20 k.y.) and Adding this uncertainty to the underlying thrust fault (Shaw and large earthquake hazard (up to M 7.7; uncertainty in the local geoid variations, Suppe, 1994; Seeber and Sorlien, 2000). Colson, 1996) are difficult to reconcile all elevation measurements in the study The Northern Channel Islands antiform with other observations, including the area should be accurate to better than 5 itself has been interpreted either as a eastern sea cliff exposure of the fault, cm, and all horizontal coordinates classic fault-bend fold (Shaw and Suppe, which shows little or no vertical accurate to better than 2.5 cm. 1994) or as the expression of horizontal- separation across the stage 5e terrace Although the rapid-static GPS axis rotation above a listric-shaped platform (e.g., Sorlien, 1994). measurement times were ~150–300 thrust fault (Seeber and Sorlien, 2000). times shorter than for campaign GPS, Importantly, each fold model predicts a GPS POSITIONING the positions were only about 5–10 different magnitude or rate of slip on the Several different GPS approaches are times less precise. This work extended causative fault and hence a different available that vary in precision, research from the previous 2–3 years on earthquake hazard. application, and equipment cost over Santa Cruz and Anacapa Islands using several orders of magnitude. Since the ground-based laser surveying. In a Study Area cessation of military signal degradation single field season, the new GPS-based This research was conducted on San in May 2000, handheld GPS receivers field work more than doubled the Miguel and Santa Rosa Islands. San yield position accuracies within ±20 m previous data coverage, with precisions Miguel is the westernmost island in the in the horizontal and ±40 m in the equal to or better than the ground-based chain. More than 3000 m of Cretaceous vertical. Sources of error include measurements. The principal advantage