The Relationship Between Joints and Fault Patterns in the Eskdale Granite (Cumberland) and the Adjacent Borrowdale Volcanic Series

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.

The English Lake District batholith – Ordovician, Silurian, Devonian or…?

“…Nevertheless we feel that it is more than a coincidence that the orientation of the maximum principal stress deduced from the study of joints in the Eskdale Granite (Firman, 1960) is closely similar to that deduced from the axial traces of the Ulpha Syncline as mapped by Numan (in Soper & Moseley, 1978) and markedly oblique to that required for structures with 'Caledonoid' (NE-SW) trends. We are not yet dissuaded from our view that both are due to pre-Ashgill stress, but hope that Webb, Millward, Johnson & Cooper will agree that more research is needed, not only into possible effects on pre-existing structures of the intrusion of early parts of the batholith, but also of the likely effect of later folds (e.g.…”

“…(b) Measurement of 8600 Eskdale Granite joints (Firman, 1960) failed to reveal the patterns of radial and concentric joints which typify many intrusions which cool in tensional regimes. Instead the study showed a singular paucity of joints which could be ascribed with confidence to cooling cracks, and a wealth of joint patterns consistent with a stress system in which the principal compressive force was northsouth.…”

“…Instead the study showed a singular paucity of joints which could be ascribed with confidence to cooling cracks, and a wealth of joint patterns consistent with a stress system in which the principal compressive force was northsouth. That this stress system was probably operative before the granite had completely consolidated is suggested by the development of zones of rotational shear adjacent to wrench faults within the granite, but not in the country rocks (Firman, 1960).…”

The succession and structure of the Borrowdale volcanic rocks Northwest of Ullswater

“…It is concluded that in the Ullswater-Helvellyn region basalts, basic andesites and andesites (the Ullswater Group) are followed by felsitic rocks (the Birk Fell Group). Further to the west there are similar successions with a thick development of andesites at or near the base of the Borrowdale Volcanks, followed by felsitic rocks (Firman 1957, Mitchell 1956, Oliver 1962. It seems probable that these'successions can be broadly related to that of the Ullswater region, although it is unlikely that individual formations can be correlated in rocks so variable as the Borrowdale Volcanics.…”