Abstract The objectives of this study were to assess possible differential movement across an inferred fault beneath Byrd Glacier, and to measure the timing of unroofing in this portion of the Transantarctic Mountains. Apatites separated from rock samples collected from known elevations at various locations north and south of Byrd Glacier were dated using single crystal (U–Th)/He analysis. Results indicate a denudation rate of c. 0.04 mm a −1 in the time range c. 140–40 Ma. Distinct age v. elevation plots from north and south of Byrd Glacier indicate an offset of c. 1 km across the glacier with south side up. A Landsat image of the Byrd Glacier area was overlain on an Aster Global Digital Elevation Model and spot elevations of the Kukri erosion surface to the north and south of Byrd Glacier were mapped. The difference in elevation of the erosion surface across Byrd Glacier also shows an offset of c. 1 km with south side up. Results support a model of relatively uniform cooling and unroofing of the region with later, post-40 Ma fault displacement that uplifted the south side of Byrd Glacier relative to the north. Supplementary material: Sample and apatite (U–Th)/He data are listed at http://www.geolsoc.org.uk/SUP18671

Abstract Surface and subsurface geological features of the Himalayan–Tibetan orogenic system may be explained by three sets of processes: those related to plate convergence, those related to the gravitaţional spreading of a fluid middle crust beneath the Tibetan Plateau, and those related to aggressive erosion along the southern margin of the plateau. In this paper, the possible relationships among the last two of these—and their tectonic manifestations—are presented in the form of a ‘Channel Flow–Extrusion’ hypothesis. This hypothesis, deriving from a series of ideas advanced by many geologists and geophysicists over the past two decades, suggests the definition of three phases in the Early Miocene–Recent history of the orogenic system. During Phase I (Early Miocene), the crust of southern Tibet was sufficiently hot and thick to enable lateral flow of a weak middle crust. To the north and east, this flow resulted in the expansion of the Tibetan Plateau. To the south, erosion at the Himalayan front permitted the mid-crustal channel to breach the surface; this process is recorded in the deformational history of the Himalayan metamorphic core and the Main Central and South Tibetan fault systems that bound it. While the lateral expansion of the plateau by mid-crustal flow has continued throughout Neogene time, some evidence suggests that extrusion across the Himalayan front waned substantially during the Middle Miocene–Early Pliocene interval (Phase II). In Middle Miocene time, large magnitude extension of the decoupled upper crust of southern Tibet led to the development of a subsidiary channel; its extrusion explains the existence of the North Himalayan gneiss domes. Phase III (Late Pliocene–Recent) has involved renewed southward extrusion of the main channel due to climatically induced increases in the erosion rate at the Himalayan range front.

Abstract Although K-Ar geochronology is one of the most time- honored methods of measuring the age of geologic materials, its practical application is limited by a fundamental difference in the physical states of potassium (a solid) and argon (a gas). This difference means that K-Ar dating of a sample requires splitting it into two fractions, one destined for K analysis (usually by flame photometry) and the other for Ar analysis (by isotope dilution and rare gas mass spectrometry). This procedure is simple enough for large, homogeneous samples, but it effectively precludes any form of K-Ar microanalysis. In the mid-1960s, the recognition that bombardment of K-bearing minerals and glasses with fast neutrons in a nuclear reactor could convert naturally occurring 39 K to 39 Ar led to the development of a derivative of the K-Ar method: 40 Ar/ 3 9Ar geochronology (Merrihue and Turner, 1966). It became possible to determine 40 Ar/ 39 Ar ages simply by measuring Ar isotope ratios on a rare gas mass spectrometer, dramatically extending the range of geologic problems that could be addressed through the K → Ar decay scheme. Most importantly, 40 Ar/ 39 Ar geochronology does not require knowledge of the mass of the analyzed material, permitting the use of microanalytical procedures that extract Ar from samples so small that measurement of their weight is impractical. One such procedure involves the use of a laser beam to melt, ablate, or gently

Detailed mapping of extensional structures and synextensional volcanic rocks exposed in the eastern Panamint Mountains, southeastern California, place new constraints on the rates and geometry of late Miocene extension in the Death Valley area. At the eastern edge of the central Panamint Mountains, the Burro Trail and Amargosa “thrust” faults of Hunt and Mabey (1966) form subparallel roof and sole faults, respectively, of a kinematically related system of currently low-angle normal faults and subvertical strike-slip faults named here the Eastern Panamint fault system. The roof and sole faults initiated as 40 to 60°W-dipping normal faults in late Miocene time and were subsequently rotated to shallow dips by later, structurally lower, normal faults. Roughly 150 to 160 percent cumulative extension can be demonstrated for Eastern Panamint structures exposed in Trail Canyon. Late Miocene tuffs and andesites exposed in the eastern Panamint Mountains are correlative with the Sheephead Andesite and Rhodes Tuff in the southern Black Mountains. Reconstruction of these sequences indicates 25 to 55 km of post-9 Ma extension between the eastern Panamint and southern Black Mountains along an azimuth of N55° ± 3 °W. This extension was dominantly accommodated by movement on the Amargosa fault system and late Neogene faults exposed along the western front of the Black Mountains. These data constrain the extension rate between the Black and Panamint Mountains to be between 6.4 and 2.7 mm/yr over the past 9 m.y. Existing data on the direction and age of initiation and cessation of extension for faults in the Death Valley area show two periods of extension. The earlier period, from 15 to 10 Ma, appears to have widely varying extension directions; the later period, from 10 to 0 Ma, is characterized by consistently northwestern extension directions.

Greenschist and amphibolite facies metamorphic rocks within the core of the Panamint Mountains of southeastern California were brought to the surface largely by movement on diachronous systems of west-dipping normal faults. Much of the unroofing can be attributed to displacement along the low-angle Harrisburg detachment, which placed weakly to unmetamorphosed upper Precambrian-Paleozoic strata on upper Precambrian metasedimentary rocks in Miocene time, prior to 10.6 ± 0.9 Ma intrusion of the Little Chief quartz monzonite porphyry. The Eastern Panamint normal fault system (late Miocene) initiated at a moderate angle (40 to 60°), juxtaposing amphibolite facies(?) Precambrian crystalline basement and upper Precambrian-Paleozoic sedimentary rocks. Range-scale anticlinal folding of the Harrisburg detachment and eastward tilting of the Eastern Panamint fault system are attributed to reverse-drag flexure induced by movement on the west-dipping Amargosa fault system (late Miocene?), which is exposed in the Black Mountains to the east of Death Valley and is inferred to dip beneath the Panamint Mountains. The low-angle Emigrant detachment (late Miocene to early Pliocene) incised the Harrisburg footwall and acted as the growth fault for the Neogene Nova Basin, which is dominated by material eroded from the metamorphic core.