DEBLOBBING OF MANTLE LITHOSPHERE FROM BENEATH TIBET: PLATE MOTION, PALEOALTIMETRY, PLATEAU GROWTH, VOLCANISM, AND UPPER MANTLE STRUCTURE

Normal faulting and crustal thinning, not processes that build high plateaus, dominate active deformation of Tibet today; thus, some change must have occurred since the Tibetan Plateau formed. Northern Tibet is underlain by thinner crust, by > 10 km, and apparently hotter upper mantle than southern Tibet, for both low seismic wave speeds and widespread, if low volume, volcanism suggest a warm uppermost mantle. Thus mantle lithosphere beneath northern Tibet seems to be thin, if present at all. If mantle lithosphere has thinned, a bunch of facts suggest that removal and thinning occurred since ~ 15 Ma, but with no shortage of interpretations that do not require lithospheric removal at that time. Some think that mantle lithosphere was not removed, but was simply heated by a thick radiogenic crust. Layers of high seismic-wave speeds deeper in the upper mantle offer plausible resting places for foundered mantle lithosphere, but proponents must appeal to peculiarities that are not easily explained, or ignore an arguably simpler explanation for the high-speed layers. Paleoaltimetry remains blind to changes in mean elevations of northern Tibet and, within certainties, all estimates allow for removal of some mantle lithosphere beneath not just northern but also southern Tibet. The volcanism suggesting removal of mantle lithosphere beneath northern Tibet since 15 Ma shares characteristics of volcanism elsewhere in Tibet earlier in its history, as if mantle lithosphere was removed from other parts of Tibet, unless volcanism owes its occurrence to other processes. Proponents of convectively and rapidly removed mantle lithosphere beneath Tibet see the idea passing tests; opponents see failure. Yet, if this process did occur beneath Tibet, surely it has occurred beneath other belts and must be an element of the geodynamic canon. If not, then maybe it requires a thick eclogitized lower crust, found only in subduction zones, and hence in Andean settings.