Disruption of the PV-PPV Phase Transition by a Dome-like Upwelling Beneath Alaska

Abstract:

The lowermost mantle region, D”, represents one of the most dramatic thermal and
compositional layers within our planet. Global tomographic models display relatively fast patchs
along the circum-Pacific which is generally attributed to slab-debris. Such cold patches interact
with the PV-PPV phase boundary to generate particularly strong heterogeneity at their edges.
Most seismic observations for the D” come from the lower mantle S wave triplication (Scd).
However, the sampling regions concentrated beneath Central America, where intensive studies,
including migration methods and array analysis, have been accomplished. Beneath the central
America, the D” can have a step variation of ~ 100 km, which argues strong lateral temperature
variations or possible chemical variations. However, the common used ray paths between South
American events and seismic stations in US sample such sharp boundary azimuthally, which
make the modeling difficult. Here, we exploit the USArray waveform data to examine one of
these sharp transitions beneath Alaska. From west to east beneath Alaska, we observed three
different type of D”: West region with strong Scd requiring sharp δV_S = 2% increase;
Middle region with no clear Scd indicating lack of D”; East region with strong Scd requiring gradient
δV_S increase. To explain such strong lateral variation, chemical variations must be involved. We
suggested that West region represents a normal mantle. In contrast, the east region is dominated by
subducted slab. At the Middle region, we discovered a strong upwelling structure that disrupts the phase
boundary. A distinct pattern of travel time delays, waveform distortions, and amplitude patterns
reveal a circular anomaly about 5° across which can be modeled synthetically as a dome about
400 km high with a shear velocity reduction of ~5%. Geodynamic modeling indicates that
this structure could be the base of an upwelling and/or a hot Fe-rich oxide hill.