A tomographic glimpse of the upper mantle source of magmas of the Jemez Lineament, New Mexico

Abstract

The 800-km-long Jemez lineament is the most active volcanic feature in the southwestern United States. It is the southeastern tectonic boundary of the Colorado Plateau and crosses the Rio Grande rift at the Jemez Mountains. The primary volcanism of the lineament is basaltic and has occurred in the last 4.5 m.y. To infer spatial distributions of partial melt in the upper mantle source zones for the Rio Grande rift and the Jemez lineament, we investigated the lateral variations of P wave velocity in the upper mantle beneath these features. We used teleseismic P wave delays recorded at a 22-station network to perform a damped least squares, three-dimensional inversion for these lateral variations. Our technique employed velocity interpolation within a three-dimensional grid of points, rather than using blocks of constant P wave velocity. This method allows highly realistic computation of seismic ray paths as well as accurate computation of the matrix elements in our system of equations. Determinations of resolution of results were done in two independent ways, both of which gave consistent estimates of resolution. In our best resolved volume the inversion showed no significant concentration of relative low velocity for P waves beneath the Rio Grande rift. However, directly beneath the Jemez lineament there is a -• 100-km-wide, 1-2% low-velocity feature in the depth range of 50-160 km. Because of the association of the low P wave velocity with the Jemez volcanic lineament but not with the Rio Grande rift, because lowered P wave velocity can be associated with increased partial melt, and because the volume of recent volcanism at the lineament greatly exceeds that at the rift, we infer that a large magmatic source zone exists beneath the Jemez lineament but not beneath the Rio Grande rift. This implies that the volcanic potential of the Jemez lineaments continues to greatly exceed that of the Rio Grande rift. The mantle source zones for volcanics of the Jemez lineament are not overridden by, but rather track, the motion of the North American plate; this implies that these sources are within the lithospheric plate, as is clarified in the discussion. The magmatic source zones of the Jemez lineament are modeled as due to clockwise rotation of the Colorado Plateau about a pole in northeastern Colorado. This rotation caused extension of the lithosphere beneath the Jemez lineament, permitting concentration there of partially melted rock in the upper mantle. ], crossing the Rio Grande rift at the Jemez volcanic center. Most of the volcanism along the lineament has occurred within the last 4.5 m.y. [Lipman, 1981; Aldrich and Laughlin, 1984; Smith and Luedke, 1984; Baldridge et al., 1987; Perry et al., 1987, 1988]. Hamilton and Myers [1966] indicate that changes in direction of extension within the Basin and Range led to a clockwise rotation of the Colorado Plateau, and Aldrich and Laughlin [1984] indicate that the timing of these changes corresponds to the timing of volcanism along the Jemez lineament. The Jemez lineament is t...

Status of volcanism studies for the Yucca Mountain Site Characterization Project

“…It could represent a large heat source possibly capable of generating significant future volumes of presumably basaltic magma. The east/west to north/northeast trend of the proposed anomaly is parallel to volcanic features of the southwest United States that have been interpreted to track motion of the North American plate Spence and Gross 1990;Humphreys and Dueker 1994qb). The presence of sites of Quaternary basaltic activity at the ends of the anomaly are analogous to the patterns of magmatism in the Great Basin, which are concentrated at the east and west margins (Best and Brimhall 1974).…”

Middle Tertiary buoyancy modification and its relationship to rock exhumation, cooling, and subsequent extension at the eastern margin of the Colorado Plateau

“…We conclude that lithospheric buoyancy modification during middle Tertiary magmatism influenced regional rock uplift, exhumation, and cooling, and possibly surface uplift and Neogene extension. Important directions for future work include coupled regional thermal isostatic-erosion models and understanding the relationship between middle Tertiary lithospheric heterogeneity and the present-day anomalous mantle of the southern Rocky Mountains-Rio Grande rift (Humphreys et al, 2003;Lee and Grand, 1996;Slack et al, 1996;Spence and Gross, 1990).…”

“…In the southern Rocky Mountains and Rio Grande rift regions, the largest caldera complexesthe San Juan and Mogollon-Datil volcanic fields ( Fig. 1)-probably represent eroded remnants of a zone of ignimbrites extending from Mexico into southern Colorado (Lipman et al, 1972;Steven, 1975). This paper combines traditionally distinct geologic and geophysical data sets to constrain the effects of the middle Tertiary ignimbrite flare-up on the buoyancy and thermal state of the lithosphere and on subsequent rock uplift.…”

The Rio Grande rift: A geological and geophysical overview

“…2) suggests that 7.6-7.7 km/s is a reliable figure there. Wleseismic delays in the rift region show slow mantle velocities, but the pattern is complex and does not follow the surface expression of the rift exactly (Spence and Gross, 1990;Slack et al, 1996). In the vicinity of the Jemez lineament, the velocity anomaly in the upper mantle shows a northeast trend suggesting that the Rio Grande rift might be secondary to this feature in some way.…”

“…These volcanoes partially define a north-northeast trending linear array of volcanic centers referred to as the Jemez lineament (Mayo, 1958). The Jemez lineament probably corresponds to a major boundary or zone of weakness in the lithosphere (Karlstrom and Humphreys, 1998) and appears to be correlated with a low-velocity zone in the mantle (Spence and Gross, 1990;Slack et al, 1996). However, it is not an expression of a known fault or fracture zone and does not seem to correspond to any single, simple structure in the upper crust (Baldridge et al, 1983).…”