The Heterogeneity of Southern California Mantle: Insights From Basalts and Mantle Xenoliths Along the Eastern Margin of the Salton Trough

Southern California has been in an active plate margin setting since the Permian, and has possibly had a very dynamic mantle history. Episodes of normal subduction, shallow subduction, slab breakoff, and the transition to transtension could have imparted different geochemistry and scales of heterogeneity to the lithospheric mantle and asthenosphere. A series of xenolith-bearing basalt dikes and flows exposed along the San Andreas fault in Joshua Tree National Park (JTNP) provide a northwest to southeast transect of southern California mantle in Pliocene time. Non-xenolith bearing alkali basalts exposed on the eastern side of the park provide an east to west transect of the mantle. The xenolith and non-xenolith bearing basalts differ based on their Fe, Mg, and Sr content suggesting that the mantle source of these basalts becomes more enriched along the transect from east to west. The differences in chemistry and the presence and absence of xenoliths suggest that there may have been a boundary in the mantle between the basalt reservoirs. This boundary may have been emplaced in the Mesozoic to Cenozoic due to normal and flat-slab subduction. The question becomes, does this trend and subducted slab boundary continue along the northwest to southeast transect through the xenolith-bearing basalt dikes and flows. The five xenolith-bearing basalt sites contain Group I xenoliths, dominated by spinel lherzolite (52%), but are multi-modal including harzburgite (29%), dunite (5%), and wehrlite (14%). Mantle-derived olivine ranges Fo87-91, and clinopyroxene (cpx) ranges Wo42.51-50.91 En47.39-53.30 Fs1.72-4.83. Due to the Fo content and modal mineralogy, these samples more closely resemble previously studied Proterozoic mantle than Archean mantle (cf. Lee, 2001). The cpx compositions yield a quenching temperature range of 900 1100 °C at 10 kb using the geothermometer projection scheme from Lindsley (1983). The quenching temperatures increase to the southeast approaching the central Salton trough. This implies that the southeastern xenolith-bearing basalts may have originated at a greater depth consistent with a deeper depleted mantle source, whereas the northwestern xenolith-bearing basalts may have originated at a shallower depth consistent with a higher Fe content in the basalt. A second model implies that the increasing temperatures to the southeast represent a lateral thermal gradient reflecting a heat source associated with early rifting in the Salton trough consistent with a higher Mg content in the southeastern basalts. Preliminary results of Principle Components Analysis detect few differences in cpx geochemistry. This implies that the subducted slab boundary evidenced by the basalt geochemistry must be east of the xenolith-bearing basalt sites. This boundary must have a depth component which suggests that the flat slab subduction did not completely destroy the cratonic mantle.