Kent et al.'s (2005) article, which claims a 60 k.y. record of extension and slip rates across the western boundary of the Basin and Range province, fails to deliver on its promises because of conceptual errors, flawed procedures, omissions, and other problems.
The basic premise of the paper, that shoreline terraces 15 km apart on opposite sides of an extensional basin like Tahoe (CR and RB, Fig. 1) record normal fault slip and slip rates across the basin, is false. It is well known that displacement gradients occur both along and perpendicular to normal faults, with negligible vertical separation 15–20 km into the hanging wall (e.g., Barrientos et al., 1987; Kusznir et al., 1991). Using vertical separation to calculate horizontal components and extension ignores important issues such as oblique slip, transtension, and tilt in half-grabens like Tahoe (Lahren et al., 1999; Schweickert et al., 1999, 2000b). Also, the terrace features themselves are misinterpreted.
Kent et al. also fail to address the Tahoe-Sierra frontal fault zone, with its abundant evidence of Quaternary activity, even though this zone forms the true boundary between the Sierra Nevada and the Basin and Range province (Fig. 1; Schweickert et al., 2000a, 2000b, 2004; Howle, 2000). Furthermore, many faults with youthful scarps within the northern part of Lake Tahoe are ignored, and only one fault is assumed to slip in the south half of the lake, implying that faults in the north half of the lake end at their youngest rupture tiplines (Fig. 1).
In addition, Kent et al. used flawed procedures. Large errors may arise from assuming that 14C ages on charcoal fragments closely date times of lake sedimentation, because charcoal may be sequestered on land for thousands of years before redeposition in the deep lake. The authors also use uncalibrated 14C ages, whereas calibrated calendar ages may be several thousand years older than the 14C ages (Stuiver et al., 1998). Other errors arise in comparing heights of terraces using spot elevations, rather than the elevation of the shoreline angle. The authors ignore variations in sedimentation rates, hiatuses, megaturbidites, etc., and extrapolate sedimentation rates at sites CR and NT (Fig. 1) to intervals far beneath and/or far from dated horizons to estimate ages of terraces and the megalandslide. These errors are compounded, making ages too old and slip rates too low.
Kent et al. have also mislocated important data points. The piston core and seismic profile at NT, which are shown as coincident (Kent et al. Figures 1, 3, and DR1d), are ~1.3 km apart (we were part of the team that selected the site, recorded the coordinates, and collected and analyzed the core), negating the use of the core to interpret the profile. Also, Kent et al.'s Table DR1 reports elevations and location of vibracore data at site CR as above lake level near the China Sea (35°N, 120°E). Elsewhere, reported heights of caves are significantly in error, some above ground level.
Other problems include correlation of terraces at CR and RB, the claim that a terrace surface is overlain by deltaic sediments at site CR, and interpretations of two vibracores at the same site. SHOALS lidar data reveal that two submerged erosional terraces occur around Lake Tahoe, and that the authors miscorrelated the shallower one at RB with the deeper one at CR, yielding an erroneous vertical separation. In addition, although a terrace surface cannot occur shoreward of its incised terrace riser, Kent et al. claim the existence of one at site CR, proven by a 6.5 m vibracore (Kent et al., Figs. 2B, DR1a, DR1b). However, the supposed terrace surface in the east half of their profile is a lake-bottom multiple, and is not visible where reported in the vibracore, which penetrates west-tilted, pre-terrace lake sediments. In a 0.5 m vibracore, the terrace surface is mislocated and misdated because the authors ignore a visible unconformity and a hiatus defined by their 14C data (Kent et al., Figs. 2B, DR1a). Reasonable alternative interpretations to Kent et al.'s conclusions are: 1) the terraces do not record the full amount of normal fault slip across the lake; 2) the Cave Rock terrace is a Holocene lowstand surface, with a large hiatus along it; 3) the age estimate on the megalandslide is too old; 4) reported slip rates on the West Tahoe and North Tahoe faults are too low; and 5) no reliable slip rate estimates are possible from the data presented.
Kent et al. provide neither credible estimates on individual faults in the Tahoe basin nor a 60 k.y. record of extension across the western boundary of the Basin and Range province.