AREA BALANCE, DEPTH TO DETACHMENT, AND STRAIN IN EXTENSION

A new ''lost-area'' cross-section construction and validation techniqu e is based on the area balance of a graben system formed above a detac hment. Area balance requires that the area displaced below the origina l regional level of each bed in a graben (the lost area) be balanced b y the displaced area at the boundary of the system, given by the produ ct of the displacement times the depth to detachment. This relationshi p is a straight line on a plot of lost area versus depth. The slope of the line is the displacement, and the depth intercept is the depth of the detachment. In general, beds within the graben system undergo bot h visible and homogeneous layer-parallel extension. The homogeneous co mponent can be determined from the width of the graben system, the los t area, and the depth to detachment. Conversely, the detachment depth may be calculated from the total layer-parallel extension and the lost area. The techniques are illustrated by application to models and to very well constrained natural examples, an earthquake-induced slope fa ilure in Alaska and kilometric-sized horsts and grabens in the Black W arrior foreland basin of Alabama. A half graben without footwall uplif t and a full graben are the same on a lost-area diagram. A half graben with footwall uplift linked to the hanging wall extension requires th at both the hanging wall and footwall areas be included to determine t he detachment depth and displacement. All the models show layer-parall el extension e in the upper levels that are less than the crustal-scal e strain beta.