What happened to the crust under the High Lava Plains, Oregon?

Thin crust, 30-35 km thick, underlies the High Lava Plains (HLP) of southeastern Oregon. Subcontinental lithospheric mantle is minor or absent based on equilibration depths of low-K, high-Al basalts characteristic of the region. The crust thickens to ~38 km to the east under the Owyhee Plateau and to as thick as 45 km under the Cascades arc to the west. Thin crust extends to the south, under the northern Basin and Range, but also under the unextended Blue Mountains. Why is the crust thin in the absence of major extensional faults? In part, because the crustal collage of tectonic terranes under the HLP has been changed by addition of basaltic material: 1) At least 0.5 km of basalt lavas intercalated with similarly thick sections of tuffs and volcaniclastic sediments mantle the area. (2) Most of the basalts are differentiated mainly through crystal fractionation (tholeiitic) accompanied by some crustal contamination (calcalkaline), implying at least 1 km of mafic to ultramafic crustal residuum. (3) The planar topography of the HLP, albeit cut by a swarm of steep faults with modest offsets of a few tens of meters, indicates that the extension expressed in large, Miocene to recent normal faults to the south ( 10 %) is accommodated by invasion of basalt, akin to the Snake River Plain. (4) Ignimbrites and rhyolite domes of this bimodal province are mainly partial melts of mafic protoliths and the thermal driver was presumably basaltic material added to the crust. (5) Plain-ward dips of ignimbrites and the directionality of folds in rheomorphic welded tuff indicate the HLP has subsided by gravitational loading (~ 1 km of mafic constituents). If closed system extension were perpendicular to the orientation of the HLP, then present crustal thickness implies 20 to 25% extension. Either the deep crust has been eroded (delaminated), or there was a major, older, and cryptic extensional event.