A mantle diapir of 8-km radius has been recognized by systematic structural mapping of the Oman ophiolite in the Maqsad district. This diapir chilled while still active under the ridge crest. Streamlines rotated in the diverging part of the diapir a few hundred meters under the Moho. This implies a decrease by several orders of magnitude in the effective viscosity of the peridotites to a depth of 1 km beneath the Moho. We present dynamic models for diapiric flow across such an interface. A step of 0.5 MPa in dynamical pressure is found along the high/low viscosity interface; the highest pressure occurs within the uppermost low-viscosity layer. This implies that magma percolating through the porous peridotite network is retained below this interface. This promotes a rapid increase of the melt/rock ratio, hence also a dramatic decrease in the effective viscosity of the peridotites. The feedback between magma percolation and decreasing viscosity produces the steep rotation of the mantle flow under the Moho. ¿American Geophysical Union 1987