Shallow geophysical and geological evidence for a regional-scale fault duplex in the Lower Palaeozoic of the Isle of Man

Abstract

A distinctive set of linear anomalies is seen on potential field data crossing the Isle of Man in a northeast-southwest belt, 5-6 km wide. The lineaments occur in an imbricate pattern with three constituent trends: northeast-southwest, east-west (to east-northeast-west-southwest) and north-south. The belt can be traced into the offshore where it ties with a northwest dipping set of anomalous high-amplitude seismic reflections interpreted as fluid-filled fractures or intrusions along fractures. In the field, the lineaments coincide with northeast-southwest reverse faults, east-west dextral strike slip faults and north-south sinistral strike slip faults interpreted to have formed during northwest-southeast compression in the late Caledonian. Several of the strike-slip faults were later sites of mineralization. In addition, there is limited kinematic evidence for an earlier period of sinistral transpression on east-west ductile shear zones. A tentative model is proposed where the Manx Group is located on the eastern side of an inverted Lower Palaeozoic basin (the Manannan Basin) forming an embayment on the northwest margin of Eastern Avalonia. During closure of Iapetus, the direction of maximum principal stress (~1) rotated from northnortheast-south-southwest to northwest-southeast as Eastern Avalonia docked and then locked against Laurentia. The imbricate belt developed as a duplex at the eastern edge of the basin during the later stages of contraction. The implications of the model is that the stratigraphy of the Manx Group is telescoped.