  We report direct measurements of spatially resolved stress at the boundary of
a shear thickening cornstarch suspension revealing persistent regions of high
local stress propagating in the flow direction at the speed of the top
boundary. The persistence of these propagating fronts enables precise
measurements of their structure, including the profile of boundary stress
measured by Boundary Stress Microscopy (BSM) and the non-affine velocity of
particles at the bottom boundary of the suspension measured by particle image
velocimetry (PIV). In addition, we directly measure the relative flow between
the particle phase and the suspending fluid (fluid migration) and find the
migration is highly localized to the fronts and changes direction across the
front, indicating that the fronts are composed of a localized region of high
dilatant pressure and low particle concentration. The magnitude of the flow
indicates that the pore pressure difference driving the fluid migration is
comparable to the critical shear stress for the onset of shear thickening. The
propagating fronts fully account for the increase in viscosity with applied
stress reported by the rheometer and are consistent with the existence of a
stable jammed region in contact with one boundary of the system that generates
a propagating network of percolated frictional contacts spanning the gap
between the rheometer plates and producing strong localized dilatant pressure.
