THE RELATIONSHIP BETWEEN JOINTS AND FAULT PATTERNS IN THE ESKDALE GRANITE (CUMBERLAND) AND THE ADJACENT BORROWDALE VOLCANIC SERIES

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Abstract
Minor fractures in the Eskdale Granite and adjacent Borrowdale Volcanic rocks have been investigated using a random sampling technique. One hundred joints were measured at each station and the data plotted on equal-area nets. The major concentrations are represented on maps in this paper. The distribution, dip and strike of the low-angled joints corresponds with that of the aplites and pegmatites and suggests that most of the low-angled joints were formed at an early stage in the emplacement of the granite. Very few of the high-angled joints show a similar correlation with aplites and pegmatites and the majority are thought to have been formed by a later north-to-south stress. Two types of joint pattern, with predominating high-angled joints, occur in the granite. One type is considered to consist of two near-vertical, shear-joint concentrations intersecting at about 60 o and bisected by two sets of tension-joints. These are attributed to irrotational fracturing resulting from an approximately north-to-south stress. The hypothetical shear-joints are parallel to sinistral and dextral faults and haematite veins. The other type is restricted to areas near the wrench-faults and haematite veins. Evidence is presented which suggests that these patterns are developed in zones of dextral and sinistral shearing and are due to rotational shear. Diffuse patterns in which low-angled joints predominate occur in both the granite and Borrowdale Volcanic rocks, only a few yards from normal faults. Although no satisfactory mechanical explanation of these patterns has been deduced, they are capable of empirical interpretation to give the approximate strike and direction of hade of the fault-plane. The structural history of the area is outlined.

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