Found in late afternoon resting in the shade on a large chunk of volcanic rock (welded tuff), a bit above the base of Chalk Bluffs just north of the Owens River. This mating pair stayed absolutely motionless for quite some time while I walked back to the vehicle to get my camera, and also for many minutes more after I returned. Then all of a sudden it darted off in a flash, spooked by some relatively minor movement compared to our proceeding movements while positioning to get photos.

Appeared to be a therevid in the field, and though I couldn't positively verify all the characters in keying it to family in the MND(1), it did seem to go best to Therevidae.

Similarly, in the MND(1) key for family Therevidae, I don't have enough detail in my photos to navigate all the detailed character couplets...but going "both ways" at unclear junctures, the best candidate I got was Ammonaios. And consulting:

Hauser, Martin & Michael E. Irwin (2003). The Nearctic Genus Ammonaios Irwin and Lyneborg 1981 (Diptera: Therevidae). Ann. Entomol. Soc. Am. 96(6): 738-765. (Full Text)
...my best guess would be A. confusus, which also has nearby records (e.g. at Fish Slough).

Alice Abela also got some photos of the pair before it darted off, perhaps hers may show more diagnostic characters.