INTERPRETATIONS OF THE SEISMIC STRUCTURE AT CIMA VOLCANIC FIELD FROM MANTLE XENOLITHS: AN INTEGRATED STUDY OF GEOLOGY AND GEOPHYSICS IN WALKER LANE

Ultramafic xenoliths from Cima volcanic field are used to interpret the mantle seismic structure in the Walker Lane, an active strike – slip system that trends NNW from southern CA to OR. The structure of the upper mantle is: (1) Vp = 7.8 km/s from the Moho (28 km) to 40 km, (2) a reflector with Vp = 7.7 km/s from 40 – 60 km, followed by (3) a layer with Vp = 8.0 km/s that continues to 75 km depth, where the model is no longer resolvable (Hicks, 2001, MS thesis, UTEP). Temperatures are calculated from Brey and Kohler (1990, JPetrology 31, 1353 - 1378) and seismic velocities (Vp) are from spreadsheets in Hacker and Abers (2004, G3, V5, No.1). Two plagioclase peridotite samples record temperatures of 1000 – 1010 ˚C and show spinel dissolution and plagioclase precipitation in the plane of foliation. (All peridotite samples from Cima exhibit much stronger foliation than the xenoliths we have studied from the San Carlos volcanic field (AZ) and Potrillo volcanic field (NM), which are substantially more inboard from the active plate boundary than the Cima field.) Plagioclase bearing samples contribute to the seismic structure of the Moho: Vp is 7.85 to 7.89 km/s. Temperatures of spinel peridotite samples range from 1015 – 1020 ˚C and have Vp of 7.7 – 7.8 km/s. Thus, the velocity inversion observed at 40 – 60 km may simply be explained by variation in Vp for different peridotite compositions or, potentially, two other factors. Pyroxenites are found in abundance at Cima and have seismic velocities 0.2 - 0.3 km/s slower than peridotite at the same temperature. A second possibility is presence of partial melt. We find interstitial glass in some samples. We do not yet know the age of this partial melt event. The mantle in this region has undergone modification since the subduction of the Farallon plate through Basin and Range extension and continues today with the transtensional tectonics of Walker Lane. However, if the glass represents recent melting, the presence of silicate liquid would slow seismic waves significantly and contribute to the velocity inversion. Continued characterization of peridotite, pyroxenites, composite xenoliths, and glass compositions is underway in order to interpret these possibilities.