INTERPRETATION OF PS/SP SUB-MOHO NEGATIVE POLARITY ARRIVALS FROM DULUTH TO FLAGSTAFF, FINGERPRINTING THE NAVAJO VOLCANIC FIELD SOURCE REGION, AND THE COLORADO PLATEAU AND ROCKY MTS CURRENT THERMAL THICKNESS

Mantle xenoliths from the 28-19 Ma diatreme eruptions in the Navajo volcanic field record cool temperatures (1050° C) at 110-145 km depths. At the same time, a swarm of minette dykes were emplaced within the SE Colorado plateau. These two observations suggest that a pre-existing metasomatic layer containing stable H₂0/C0₂ phases went supersolidus due to volatile and heat ingress to produce the minnete magmas internal to a >145 km lithosphere.

To test this proposition, Ps and Sp seismic imaging has been performed and finds a negative shear velocity contrast @ 75-105 km depth in this region today. Two different interpretations of this –Sp event are possible: 1) the –Sp arrival demarks the base of the thermal boundary layer (e.g., the 1300-1100° C). This implies that >50 km of the lower TBL has been advected aside and/or downwards after the ca. 25 Ma xenolith bearing eruptions; 2) The –Sp event demarks a metasomatic layer that was the minette source region and this layer remains a stockwork of supersolidus magma today. In the seismic argot, the –Sp arrival is interpreted to demark a mid-lithospheric discontinuity arrival internal to the TBL, not a lithosphere asthenosphere boundary arrival.

To constrain the –Sp interpretation, the statistics of –Sp arrivals in a Ps/Sp image volume from Duluth, MN to Flagstaff, AZ is analyzed and finds a probability peak for negative velocity contrasts in the 75-110 km depth range. The mantle temperature field is estimated via mapping of a surface wave shear velocity model volume (Shen and Ritzwoller, pers. com.) to temperature using the anelastic olivine phenomenology (Jackson and Faul, 2010). The estimated temperature field find <1100° C temperatures beneath the Navajo volcanic field consistent with near-solidus metasomatic phases. The T-field is checked against temperature estimates from P_n tomography (Schutt, pers. com.) and Kilbourne hole xenolith thermometry (Gao et al., 2011). We conclude that the Navajo field –Sp arrival demarks the source layer and does not map the base of the thermal boundary layer. Additionally, the late Tertiary volcanism in the Colorado Rocky Mts suggests a mixing between metasomatic layer and asthenospheric magma inputs. The Sp image finds a much wider depth range of negative shear velocity contrast consistent with a more extensive lower lithosphere stockwork.