Ian K.D. Pierce

Large Paleoearthquake Timing and Displacement near Damak in Eastern Nepal on the Himalayan Frontal Thrust

An excavation across the Himalayan Frontal Thrust near Damak in eastern Nepal shows displacement on a fault plane dipping ~22° has produced vertical separation across a scarp equal to 5.5 m. Stratigraphic, structural, geometrical, and radiocarbon observations are interpreted to indicate the displacement is the result of a single earthquake of 11.3 ± 3.5 m of dip-slip displacement that occurred 1146 – 1256 AD. Empirical scaling laws indicate that thrust earthquakes characterized by average displacements of this size may produce rupture lengths of 450 - > 800 km and moment-magnitudes Mw of 8.6 to > 9. Sufficient strain has accumulated along this portion of the Himalayan arc during the roughly 800 years since the 1146 – 1256 AD earthquake to produce another earthquake displacement of similar size.

On a flawed conclusion that the 1255 A.D. earthquake ruptured 800 km of the Himalayan Frontal Thrust east of Kathmandu

A reexamination of the observations and analysis recently reported to conclude that an 800 km section of the Himalayan Frontal Thrust ruptured in 1255 A.D. shows that the conclusion is flawed and without merit because of misinterpretations of trench logs and incorrect interpretation of radiocarbon statistics.

Characterizing the Quaternary expression of active faulting along the Olinghouse, Carson, and Wabuska lineaments of the Walker Lane

The northern Walker Lane (southwestern USA) accommodates ~5–7 mm/yr of right-lateral Pacific–North America relative plate motion. The northwest trend of major right-lateral faults in the Walker Lane is interrupted by the presence of northeast-striking left-lateral faults within the Carson and Excelsior domains. Previous studies in the Carson domain have suggested that left-lateral slip on the northeast-striking Olinghouse, Carson, and Wabuska lineaments accommodates Walker Lane transtensional dextral shear through the clockwise rotation of intervening crustal blocks. Our observations confirm and document the presence of late Pleistocene–Holocene faulting along each of these lineaments. Fault scarps along the Carson and Wabuska lineaments are discontinuous and sparse, and show evidence for left-lateral faulting, locally including linear fault traces, alternating scarp face directions, and lateral offsets of small gullies and ridges. The trends of scarps that define these lineaments link at their western ends with north-trending active normal faults. In this manner, it appears that the 5–7 mm/yr of right slip taking place across the northern Walker Lane is being accommodated by the combined processes of basin opening in the west and block rotation to the east. This mode of slip transfer differs from the Excelsior domain, where active left-slip faults and clockwise rotation of crustal blocks are confined to, and the result of, a distinct right step between right-lateral faults of the southern Walker Lane and central Walker Lane, respectively. The observation of these apparently diverse modes of development of left-slip faults and vertical axis rotations provides an example of the complexity that may be expected in the structural development of continental shear zones that have been characterized by transtension. INTRODUCTION The Walker Lane (southwestern USA) is a major intraplate shear zone defined by a northwest-trending zone of discontinuous active faults, basins, and ranges that is between the Sierra Nevada to the west and the north-northeast– striking faults and ranges of the Basin and Range to the east (Fig. 1). It is unique in its width and discontinuous character when compared to other major continental strike-slip faults observed around the globe, including well-known examples such as the San Andreas, which is located just to the west (Fig. 1), the Anatolian fault of Turkey (e.g., Şengör et al., 2005), the Altyn Tagh of Tibet (e.g., Yin et al., 2002), the Denali fault system of Alaska (e.g., Hickman et al., 1978), and the Alpine fault system of New Zealand (Zealandia) (e.g., Norris and Toy, 2014; Mortimer et al., 2017). Cumulative right-lateral slip taken up by the Walker Lane ranges from ~30 km in the north to >50 km in the south. Geodesy shows the Walker Lane corresponds to a well-defined zone of northwestdirected transtensional dextral shear ranging from ~12 mm/yr in the south to ~5–7 mm/yr in the north (Bennett et al., 2003; Hammond and Thatcher, 2007). In Wesnousky (2005b) it was conjectured that the discontinuous nature of faulting in the Walker Lane as compared to the San Andreas may be attributed to the greater cumulative offset having accrued along the San Andreas, and that the San Andreas is transpressional along most of its length, in contrast to the transtension that characterizes the Walker Lane. In this regard, efforts to characterize the varying modes of deformation observed in the Walker Lane may provide clues to understanding the early development of continental shear zones exhibiting larger displacement during periods of time in their development where observations show they were characterized by transtension. The Carson and Excelsior domains of the Walker Lane are each characterized by the presence of left-lateral faults that strike approximately transverse to the general northwest trend of the Walker Lane. In this paper we focus on presenting new observations bearing on the recency, sense, and style of slip of faults within the Carson domain. These observations are then compared to those of others collected within the Excelsior domain to show that the developments of the left-lateral systems differ significantly in the two regions. GEOSPHERE GEOSPHERE; v. 13, no. 6 doi:10.1130/GES01483.1