Geologic mapping of 340 square miles (884 square kilometers) in the southern Beaverhead Mountains demonstrates that the area is a segment of the Mesozoic to early Tertiary Cordilleran thrust belt, and a northward continuation of the Idaho-Wyoming thrust salient. Five thrust plates are bounded by major west-dipping, low-angle folded thrusts that juxtapose older strata over younger, and are characterized by east-verging concentric folds, frontal ramp anticlines, and transverse tear faults. The five thrust plates, each with a distinctive stratigraphic sequence, are, in descending structural order: Hawley Creek, Fritz Creek, Cabin, Medicine Lodge, and Tendoy. The Cabin thrust plate locally includes Precambrian (Archean(?)) basement crystalline rocks along its leading edge. These crystalline rocks probably were part of an older Precambrian block uplift cut by the eastward directed Cabin thrust. Extension faults with youngest movements ranging in age from early Eocene to Holocene offset the five thrust plates. The Divide Creek extension fault zone, along which middle Paleozoic stratigraphic section is deleted, probably is of early Eocene age. Miocene to Holocene basin-range faults offset older structures, and loci of the younger faults have shifted westward through time. Thrusts and thrust plates identified in the study area also make up a large part of the central and northern Beaverhead Mountains. The Cabin thrust plate is the most extensive of these. IX The redefined Cabin thrust is more than 125 miles (200 kilometers) long, has two major lateral ramps, has Archean(?) crystalline rocks along a 48-mile (75-kilometer) segment of its leading edge in central parts of the mountains, and Middle Proterozoic Yellowjacket Formation and overlying Lemhi Group rocks in northern parts. The Cabin plate includes Archean(?) through Lower Triassic rocks in the central Beaverheads. In the northern Beaverheads, rocks of the Yellowjacket Formation and the Lemhi Group have been thrust over Belt Supergroup rocks creating a structural culmination of Proterozoic rocks in east-central Idaho. The foreland that was overridden by the easterly transported Cabin and Medicine Lodge thrust plates was deformed by northwest to southeast directed reverse faults that probably were formed and partly eroded before the thrust plates arrived in Late Cretaceous (Maestrichtian(?)) time.

The Central Electricity Generating Board are seeking licence and planning approval to build a Pressurized Water Reactor just to the west of the existing Advanced Gas-Cooled Reactor at Hinkley Point, Somerset. As part of a seismic hazard evaluation both regional and local geology have had to be studied in detail. The site is located in a complex structural setting at the eastern end of the North Somerset coastal fault belt. Detailed field mapping was carried out on the foreshore from just east of Hinkley Point to just west of Watchet with the purpose of identifying structural style, sequence of faulting and approximate age of the last movement. When assessing the seismotectonics of the region it was necessary to study the degree of middle and late stage Tertiary inversion prominent in southern England, less evident in the immediate vicinity of Hinkley Point. Of especial interest is the north-west-south-east trending Watchet-Cothelstone-Hatch Fault and its relationship to the east-west trending faults passing close to Hinkley Point. The Watchet Fault truncates the main east-west trending faults in the foreshore inducing a marked drag towards the fault plane, thus this phase of movement on the north-west trending fault is younger than the generally vertical displacements associated with the east-west trending faults. Examination of gypsum slickensides shows that the influence of the later movement of the north-west trending fault on reactivation of the east-west trending faults, did not extend beyond 350 m east from the main fault plane. The fault plane of the Watchet Fault is

Microtremor networks have been instituted by the CEGB around the potential sites of PWR nuclear reactors at Hinkley Point in Somerset, Dungeness in Kent and at two locations in north Wales, as part of a programme for the assessment of seismic hazard. Critical attention has been paid to the quality of the data obtained and advantage has been taken of recently developed software to explore the confidence which can be attached to the findings of the networks. During the periods of operation of the networks eight microearthquakes have been recorded within 60 km of Hinkley Point and 22 close to the north Wales sites, but none within 60 km of Dungeness. Magnitudes range from 2.9 M L down to -0.5M L . Well constrained hypocentre locations indicate that mid- and lower crustal depths of origin are most common in the first two areas. Focal mechanism solutions for these events suggest that ‘mixed mode faulting’ characterizes current tectonic behaviour in both areas. These findings are exemplified by results from the Hinkley Point network. The direct and indirect implications for hazard evaluation procedures are discussed.

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