Intraplate Basaltic Volcanism in the Basin and Range, USA: Relationship to Low-Velocity S- wave Anomalies and Asthenospheric Dynamics
Abstract
Pliocene to Recent intraplate mafic volcanic rocks of the Basin and Range Province mostly formed by asthenospheric melting, as can be seen by melting temperatures. Here asthenosphere is defined by mantle rheology and temperature and not by geochemistry. The duration of melting in a volcanic field may be related to the size and shape of pockets of low velocity asthenosphere moving under the areas of volcanism. Seismic S- wave velocity profiles constrained by ambient noise and earthquake tomography of the mantle (Yang et al., 2008) show low velocity pockets, which may correspond to higher temperatures and/or higher water contents. By applying an asthenospheric flow velocity of 5 cm/yr east (Silver and Holt 2002, Conrad et al., 2007), the distance the mantle has moved since the time of volcanism can be calculated for basalts of known age. Past positions of low-velocity anomalies in the asthenosphere combined with depths and temperatures of melting calculated using the silica-liquid geobarometer (Lee et al., 2009) were used to determine if a low velocity anomaly existed under an area of volcanism at the depth of melting and time of eruption. The data constraints used for calculating depths and temperatures of melting are dry, MgO > 7.5 wt. %, SiO2 > 44 wt.%, and Fe as 90% Fe2+. Depths and temperatures of melting were calculated for several basalt fields of known age. Ages, temperatures, and depths are as follows: Death Valley 4 Ma, 1295-1350°C, and 42-63 km; Crater Flat 80 ka, 1 Ma, 3.8 Ma, 1388-1415°C, and 80-90 km; Lunar Crater 2.9-5.7 Ma, 1414-1480°C, and 80-121 km; Reveille 3.8-4.6 Ma, 1458-1516°C, and 110-140 km; Coso 0.23-5.3 Ma, 1244-1399°C, and 39.6-72 km; Big Pine 0.9-1.8 Ma, 1276-1356°C, and 42-72 km; Long Valley 0.4-3.2 Ma, 1289- 1323°C, and 44-50 km; Cima 0.3-8.3 Ma, 1330-1376°C, and 53-82 km; Snow Canyon <10,000 years, 1470-1485°C, and 75-85 km. Ages were converted to km of mantle motion and to degrees of longitude and plotted on seismic profiles for the appropriate latitude. Using the examples of Coso, Big Pine and Crater Flat, calculated depths correspond to low velocity pockets at the time of volcanism. A low velocity pocket entered the area under Coso at 5.3 Ma and will exit this area in about 0.4 m.y. explaining the span of volcanism in this field (5.3 to 0.23 m.y. and still active). At Big Pine the low velocity pocket entered at 1.8 Ma and exited about 0.9 Ma again similar to the span of volcanism (1.3 - 0.03 Ma, from Blondes et al., 2008). Crater Flat has undergone several episodes of volcanism, though the distance covered by the seismic profile only includes the most recent episodes. The leading edge of a low velocity pocket reached Crater Flat ∼ 6.75 Ma but the pocket is still beneath the area suggesting the possibility of future activity. It is possible that the concurrence of basaltic volcanism and low velocity seismic anomalies (hot and/or hydrous asthenosphere) at melting depths extends to the entire Basin and Range and that asthenospheric dynamics are an important driving factor in producing intraplate mafic volcanism.
- Publication:
-
AGU Spring Meeting Abstracts
- Pub Date:
- May 2009
- Bibcode:
- 2009AGUSM.V23B..01T
- Keywords:
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- 7208 Mantle (1212;
- 1213;
- 8124);
- 8410 Geochemical modeling (1009;
- 3610);
- 8415 Intra-plate processes (1033;
- 3615)