Daniel L. Feuerbach

The Mount Perkins block, northwestern Arizona: An exposed cross section of an evolving, preextensional to synextensional magmatic system

The steeply tilted Mount Perkins block, northwestern Arizona, exposes a cross section of a magmatic system that evolved through the onset of regional extension. New 4oAr/39Ar ages of variably tilted (0-90 o) volcanic strata bracket extension between 15.7 and 11.3 Ma. Preextensional intrusive activity included emplacement of a composite Miocene laccolith and stock, trachydacite dome complex, and east striking rhyolite dikes. Related volcanic activity produced an -18-16 Ma stratovolcano, cored by trachydacite domes and flanked by trachydacite-trachyandesite flows, and -16 Ma rhyolite flows. Similar compositions indicate a genetic link between the stratovolcano and granodioritic phase of the laccolith. Magmatic activity synchronous with early regional extension (15.7-14.5 Ma) generated a thick, felsic volcanic sequence, a swarm of northerly striking subvertical rhyolite dikes, and rhyolite domes. Field relations and compositions indicate that the dike swarm and felsic volcanic sequence are cogenetic. Modes of magma emplacement changed during the onset of extension from subhorizontal sheets, east striking dikes, and stocks to northerly striking, subvertical dike swarms, as the regional stress field shifted from nearly isotropic to decidedly anisotropic with an east-west trending, horizontal least principal stress. Preextensional trachydacitic and preextensional to synextensional rhyolitic magmas were part of an evolving system, which involved the ponding of mantle-derived basaltic magmas and ensuing crustal melting and assimilation at progressively shallower levels. Major extension halted this system by generating abundant pathways to the surface (fractures), which flushed out preexisting crustal melts and hybrid magmas. Remaining silicic melts were quenched by rapid, upper crustal cooling induced by tectonic denudation. These processes facilitated eruption of mafic magmas. Accordingly, silicic magmatism at Mount Perkins ended abruptly during peak extension -14.5 Ma and gave way to mafic magmatism, which continued until extension ceased.

Eruptive probability calculation for the Yucca Mountain site, USA: statistical estimation of recurrence rates

Investigations are currently underway to evaluate the impact of potentially adverse conditions (e.g. volcanism, faulting, seismicity) on the waste-isolation capability of the proposed nuclear waste repository at Yucca Mountain, Nevada, USA. This paper is the first in a series that will examine the probability of disruption of the Yucca Mountain site by volcanic eruption. In it, we discuss three estimating techniques for determining the recurrence rate of volcanic eruption (λ), an important parameter in the Poisson probability model. The first method is based on the number of events occurring over a certain observation period, the second is based on repose times, and the final is based on magma volume. All three require knowledge of the total number of eruptions in the Yucca Mountain area during the observation period (E). Following this discussion we then propose an estimate of E which takes into account the possibility of polygenetic and polycyclic volcanism at all the volcanic centers near the Yucca Mountain site.

Lead isotopic evidence for synextensional lithospheric ductile flow in the Colorado River extensional corridor, western United States

Temporal changes in the Pb isotopic compositions of Miocene lavas erupted in the northern Colorado River extensional corridor suggest that lithospheric mantle and middle to deep crust migrated from beneath the Colorado Plateau into the corridor during extension. Basaltic to rhyolitic lavas erupted in the extensional corridor prior to 12.2 Ma have Pb isotopic values that are similar to those of Tertiary to Quaternary lavas erupted through Proterozoic Mojave crust, which comprises surface exposures of basement in the corridor and much of the extended territory to the west. In contrast, most post-12.2 Ma lavas from the same region have Pb isotopic compositions similar to those of lavas erupted through Arizona crust, which forms the basement of the Colorado Plateau. The changes in isotopic compositions of the basaltic lavas, and perhaps a portion of the changes in isotopic compositions of silicic lavas, are attributed to a change in the composition of the mantle source. However, the 2o6pb/2o4pb ratios for lavas erupted before and after 12.2 Ma in the corridor decrease with decreasing MgO concentrations, suggesting that the Pb isotopic com- positions of crustal assimilants changed at about the same time as the composition of the mantle. In the area of the Black Mountains accommodation zone, the surface boundary between the Arizo- na and Mojave crustal provinces lies a minimum of 60-80 km to the east of the westernmost lava with an Arizona Pb isotopic signature. This distance cannot be accounted for by displacements along nearby major faults, suggesting that middle to deep Arizona crust flowed a significant dis- tance to the west during extension.