4-D IMAGES OF THE LITHOSPHERE BENEATH THE ROCKY MOUNTAINS AND CHALLENGES FOR UNDERSTANDING THE EVOLUTION OF CONTINENTAL LITHOSPHERE

The CD-ROM experiments, combined with other regional seismic surveys, provide new 4-D images of the lithosphere in the Rocky Mountain region. Combined depiction of surface geology and mantle tomography indicate modern-day preservation of an initially segmented lithosphere. Ancient crustal features that extend 100s of km into today’s mantle include Proterozoic paleosubduction zones at the Cheyenne belt (Archean-Proterozoic boundary), Lester-Farwell Mountain area of the Park Range (arc-arc suture), Colorado Mineral belt (arc-arc suture), and Jemez lineament (arc-arc suture). Multiple teleseismic lines transect these province boundaries and reveal profound mantle velocity contrasts that seem to correspond with the ancient sutures. 1) A N-dipping high velocity mantle “slab” is equated with the Cheyenne belt and can be seen on CD-ROM, DeepProbe and Snake River Plain lines. 2) The Lester-Farwell Mountain zone projects at depth as a “tee-pee” structure and bivergent suture on CD-ROM, DeepProbe and Snake River Plain lines. 3) The Aspen anomaly is prominent on the CD-ROM and La Ristra lines as a S-dipping low velocity “slab” that projects down from the Colorado Mineral belt. And 4) the Jemez lineament can been seen as a S-dipping gradient (fast on the south) on the CD-ROM and La Ristra lines. These velocity anomalies are tabular, contain dipping mantle anisotropy, and extend from the base of the crust to depths > 150-200 km. After nearly 2 billion years these zones have retained compositional variations in terms of degree of hydration and depletion; we interpret them as paleosubduction scars that were frozen into the lithosphere following collisions of oceanic terranes in the Proterozoic. Two models for the anomalies are: 1) the marked velocity anomalies represent upwellings of asthenosphere and downwellings of lithosphere. However, 2) we interpret them as old lithospheric structures that are being variably melted in the Cenozoic by differential heating caused by transient plumes or small-scale asthenospheric convection. This debate highlights a continuing challenge for EarthScope, which is to provide integrated geologic and geophysical studies at a range of scales with high enough resolution to distinguish between tectonic models.