Structure of Tertiary and Pleistocene normal faults in Iceland

Abstract

Normal faults were measured in three selected areas of the late Tertiary and Pleistocene lava pile in Iceland. The arithmetic mean throw of 315 faults is 8.2 m, the minimum being 0.5 m and the maximum 150 m. About 93% of the faults have throws of less than 20 m. The mean strike of the faults is N31°E, but the mode is N45°E. The faults dip from 42° to 89°, nearly 80% dip between 65° and 79°, and the mean dip is 73°. Fault breccia is common and varies in thickness from 0.05 m to 2 m. There is a positive linear relation between breccia thickness and throw, the linear correlation coefficient being 0.78. It is proposed that the normal faults in Iceland form by two basic mechanisms. One is the development of faults from sets of inclined joints in the lava pile. The initially vertical columnar joints in the lavas gradually decrease in dip with depth in the crust because the regional dip of the lavas increases by about 1° for every 150–170 m depth. The inclined joints form sets of en echelon cracks that may link up through tension cracks that propagate from near the ends of the joints. The other mechanism involves formation and growth of normal faults by coalescence of smaller tension fractures. The great majority of fractures within the active rift zone are tension fractures, but when they attain the length of several hundred meters they normally change into normal faults. It is concluded that normal fault development in Iceland commonly starts at depths of about 1 km in the crust.