JOURNAL OF GEOPHYSICAL RESEARCH, VOL. 96, NO. B9, PAGES 14,481-14,492, AUGUST 10, 1991 Strain Accumulation Along the Denali Fault at the Nenana River and Delta River Crossings, Alaska J. C. SAVAGE AND M. LISOWSKI U.S. Geological Survey, Menlo Park, California Surveys of trilateration networks acrossthe Denall fault at the Nenana River in 1982, 1984, and 1988 and at the Delta River in 1975, 1979, 1982, and 1984indicatea minor (0.10 4-0.04/•strain/yr) northeastward uniaxial extension. The component of right-lateral shear-strain accumulation across the fault is not significant at the two-standard-deviation level. At the Delta River network the strain accumulation rate decreases rapidly with distance from the fault, but evidence for a similar decrease with distance from the fault is lacking at the Nenana River network. The strain accumulation rates inferred from trilateration are consistent with the very long baseline interferometry (VLBI) measurement reported by Ma et al. (1990) and supporttheir contention that significantright-lateral shear is not accumulating along the Denall fault at the present time. Savage et al. (1981) had earlier concluded erroneously that preliminarygeodetic measurements at the Delta River network demonstrated right-lateral shear strain accumulation. The absenceof significant right-lateral deformation across the Denall fault in the 1975-1988 interval is in marked contrast with the abundant geomorphicevidencefor Holocene right-lateral secular slip at the rate of 10-20 mm/yr on the Denali fault in this sector. INTRODUCTION The Denali fault is a topographically prominent feature that crosses central Alaska in a great east-west arc concave to the south (Figure 1). Althoughthe section of the fault west of the 150th meridian may not have been active in Quaternary time, offsetsin topographic features trending across the fault between 143øW and 150øW suggest a secular right-lateral slip rate of 10-20 mm/yr plus minor vertical movement during the Holocene [Plafker et al., 1977]. Evidence for Holocene slip on the Denali fault east of 143øW is absent. The Totschunda fault (Figure 1) apparently replaces the Denali fault as the active element in the fault systemeast of the junction of the two faults. The secular right-lateral sliprate on the Totschunda fault is estimated to be 10-20mm/yr during the Holocene [Plaficer et al., 1977]. Thus, the Denali and Totschunda faults form a singlefault system which has slipped at a secular right-lateral rate of 10-20 mm/yr duringthe Holocene at least asfar westas the 150th meridian. This motion is apparently accommodated at the western end by unnamed southwesterly trending thrustfaults[Lahr and Plafker, 1980,p. 485]. The role of the Denali-Totschunda fault system in the schemeof plate tectonics is not at present understood (see,however, Stout and Chase [1980] and oeahr andPlafker [1980]). We are concernedhere with the McKinley fault, the active Nenana River crossing of the Alaska Range and the other at the Delta River crossing. The canyons of these two rivers are the principal highway, rail, and small-aircraft routes through the Alaska Range. In aircraft weather advisories the Nenana River route is referred to as Windy Pass and the Delta River route as Isabel Pass. The Nenana River and Delta River geodetic networks are parts of the triangulation arcs acrossthe Alaska Range surveyed in 1941-1942 by the U.S. Coast and Geodetic Survey(USC&GS; now National Geodetic Survey). In 1970 the Delta River network was resurveyed by trilateration by the U.S. Geological Survey (USGS) and partially by triangulation by the USC&GS. A few lines in the Nenana River network were also resurveyed by trilateration by the USGS in 1970 [Page, 1972]. Unfortunately, little could be learned about strain accumulation along the Denali fault from the deformation of the Nenana River and Delta River geodetic networks in the 1941-1970 interval because the coseismic deformation associatedwith the great Alaska earthquake (March 28, 1964; Mw = 9.2) formed the major contribution to the strain accumulated in the interval [Savage, 1975]even at that distance (about 250 km) from the epicenter (star in Figure 1). The Delta River trilateration network was resurveyed by the USGS in 1975 and 1979 usingprecise trilateration (see Savageand Prescott [1973] for procedures employedand strand of the Denali fault west of145ø40'W. The other precision obtained). From these surveys, Savage et al. [1981] strand, the Hines Creek fault (Figure 1), has exhibited only found evidence for the accumulation of right-lateral shear localized dip-slip movement since 95 Ma [Csejtey etal., strain (0.14-½0.05 #strain/yr tensor shear)across the Denali 1982]. The Hines Creek fault is a terrane boundary, whereas fault and extension (0.32 -½ 0.07 #strain/yr) perpendicular the McKinley fault cuts across the interior of a terrane. The to it. The 1970 trilateration was not included in this solution geologic setting along the McKinley fault isdescribed by because the 1975 and 1979 surveys were appreciably more Csejtey et al. [1982] and Stanley et al. [1990]. precise. Since publication of that paper, the Delta River The U.S. Geological Survey has monitored two geodetic network has been resurveyed in 1982 and 1984. The strain networks (Figure1) along the McKinley strand of the Denali fault to determine the rate of strain accumulation. The two networks are located about 150 km apart, one at the This paper is not subject to U.S. copyright. Published in 1991 by the American Geophysical Union. Paper nmnber 91JB01285. rates inferred from the combined 1975, 1979, 1982, and 1984 surveysreported in this paper are only marginally consistent with the results of Savage et al. [1981].There appears to be a 20-30 mm error in measuring two distances in the 1975 survey that introduced an erroneous northwest-southeast contraction in the strain rates inferred from the 1975-1979 surveys. Thus, the accumulationof significant right-lateral 14,481