The palaeomagnetic properties and magnetic fabric of clay-rich fault gouge in a prominent fault at Porth-y-pistyll on the northern coast of Anglesey, North Wales, have been investigated in an attempt to constrain the age of last movement on this fault. The texture and mineralogy of the gouge has been examined by scanning electron microscopy. The Porth-y-pistyll fault is one of a set of E-W-trending faults that cut metasedimentary rocks of the late-Precambrian/early-Cambrian Monian Supergroup on the island of Anglesey. A high-coercivity component of magnetization with a reverse polarity has been identified in the gouge. This is believed to be a chemical remanent magnetization resulting from fluid movement and oxidation which post-dated the last major movement on the fault. This strongly suggests that movement has not occurred during the present normal polarity chron, thereby establishing a minimum age of 0.73 Ma for the last movement on the fault. A well-defined planar magnetic fabric is present in the gouge and this corresponds in geometric orientation to a planar tectonic fabric observed from SEM studies. These fabric data provide additional information on the kinematics of motion on the fault.

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