A PROMINENT BASEMENT FEATURE IN NORTH-CENTRAL NEVADA AND ITS TECTONIC IMPLICATIONS

We consider the origin and character of a large-scale gravity feature in north-central Nevada that is coincident with the western margin of the northern Nevada rift. The northern Nevada rift itself, is defined by a prominent NNW-striking magnetic high that reflects a mid-Miocene mafic dike swarm inferred to be emplaced normal to the mid-Miocene least principal stress direction (e.g., Zoback et al., 1994). The gravity feature also correlates to gold mineralization in north-central Nevada. Notably, a belt of mid-Miocene epithermal gold deposits occur along this trend (e.g., John et al, 2000), and the Battle Mountain-Eureka mineral trend is partly coincident with this feature along the western part of the Shoshone Range. Reese River Valley lies along the western margin of this feature, a 2-km deep Cenozoic basin inferred from the inversion of gravity data, and may be structurally controlled by the source of the geophysical anomaly.

This gravity feature may represent a basement structure separating relatively lower-density rocks to the southwest with higher-density rocks to the northeast. However, geophysical modeling indicates that shallow crustal sources alone cannot account for the gravity anomaly, for example, eastern and western facies lower Paleozoic rocks only have a density contrast of about 0.1 g/cm3 (Mabey, 1966). Instead, the source of the gravity feature must extend to deeper crustal levels, perhaps reflecting a transition from Paleozoic crust in the southwest to cratonic crust in the northeast, or the edge of mid- to sub-crustal mafic intrusions associated with Tertiary magmatic underplating, possibly associated with hotspot magmatism.

These two cases offer very different possibilities for the age, depth, and origin of the source of the geophysical anomaly, and present distinct implications for crustal evolution in the northern Great Basin. For example, if the anomaly is due to a basement structure, then its coincidence with the northern Nevada rift suggests that the trend of the rift may have been guided by a pre-existing crustal structure. On the other hand, if the anomaly arises from mafic Tertiary intrusions, then the western limit of this magmatism may have been influenced by hotspot fracturing of the crust.