Ropy flow structures: a neglected indicator of magma-flow direction insills and dykes

Ropy flow structures that form in dikes and sills are very similar to ropy lava structures (pahoehoe), but are of intrusive origin. They are unusual among magma-flow indicators in that they provide a frozen record of the magma flow. Our observations suggest that they occur in large vesicles, which form as a result of local repeated pressure drops during propagation and emplacement of the magma. Adiabatic gas expansion causes a ductile rim to form around a vesicle; this rim is then sheared by the underlying flow. Anisotropy of magnetic susceptibility data from the contact zones confirm the flow data from the ropy flow structures, but the results from deeper within the body, both locally and regionally, yield different, yet consistent flow orientations. This discrepancy suggests that a distinction can be made between the magma flow close to the contact and the deeper regional flow patterns in sills and dikes.