Geology Reference
In-Depth Information
Fig. 11.17. Area-depth relationships for compressional and extensional detachment structures. a Area-depth
graph. b Relationship between the slope of the area-depth line and the amount and type of deformation
ment. The elevation of the reference level is arbitrary. If the reference level is at the
detachment, H e = 0. If the reference level is chosen to be sea level, then H e will be the
elevation of the detachment relative to sea level. The sign convention for area is that
excess area and contractional displacement are positive and lost area and extensional
displacement are negative. If deformation causes both uplift and subsidence from the
regional, the correct value of S is the algebraic sum or net area (Groshong 1994). The
slope of the area-depth line, 1 / D , is the inverse of the displacement on the lower de-
tachment. To be a valid cross section, the points on the area-depth graph (Fig. 11.17a)
must all fall on or close to the area-depth line.
Each horizon has its own separate regional. The reference level should be at least
approximately parallel to the directions of the regionals and the elevations measured
perpendicular to it. If the regionals are tilted with respect to the reference level, the
elevation measurements should be taken at the center of the structure (equivalent to
the average value).
Equation 11.23 and the graph in Fig. 11.17a are modified from the forms origi-
nally given by Epard and Groshong (1993) in order to show elevation as the vertical
axis on the graph. This makes the graphical relationship between area and depth more
intuitively obvious, with the area on the area-elevation line decreasing downward on
the graph to zero at the lower detachment (Fig. 11.17a), as it does in the structure
itself (Fig. 11.16). In this form, a vertical area-depth line represents differential ver-
tical displacement (Fig. 11.17b) and the lower the slope of the area-depth line, the
greater the amount of extension or contraction. A positive slope of the line indicates
contraction and a negative slope indicates extension. The slope of the area-depth line
is the inverse of the displacement, instead of the displacement itself as in the previous
formulation (Epard and Groshong 1993). For any given area-elevation data point, the
range of h values caused by variations in measurement and interpretation will usu-
ally be significantly less than the range of the S values, and so a least-squares fit to the
data should be a regression of S onto h , rather than h onto S as the form of Eq. 11.23
might suggest.
Search WWH ::




Custom Search