Geology Reference
In-Depth Information
The gist of the tw o-stage hypothesis in the conte xt of fracture density is that those compar t-
ments that are riddled with open fractures are more rapidly and intensely weathered because water
can more readily penetrate the mass. They are reduced to plains underlain either b y grus or by
corestones set in grus. The massive compartments, on the other hand, remain essentially fresh and
are resistant to erosion. The initial contrast in fracture density need not be pronounced , but once
the contrast has been e xploited, it is reinforced. Even at the weathering front, projections would
shed water, and when exposed, the topography would be accentuated, because the residuals shed
runoff and tend to remain dry, whereas the plains receive water and, thus, the rocks beneath them
become more and more weathered: another example of a reinforcement or positive feedback effect.
Discussing the explanation of bornhardts based on v ariations in lithology, it was pointed out
that the relevant evidence has gone, for it has been eroded, and the same objection can be levelled
at the fracture density hypothesis. In principle it can, but in practice it has been sho wn that frac-
ture patterns at the surf ace provide a good guide to fracture patter ns and spacing at depths of a
kilometre or so. This being the case, extrapolations upwards into the mass of rock removed by ero-
sion surely also are valid? If this extrapolation is correct, the contrasts in fracture density at the
surface provide a valid comparison of densities betw een the massive compartments that remain
inselbergs and the compar tments at the same topo graphic level that ha ve been w eathered and
eroded. Thus, the suggestion that man y bornhardts are coincident with relati vely massive com-
partments appears to be soundly based. Even in areas that appear to be well fractured, like the Mt.
Sinai area of the Sinai P eninsula, northeastern Egypt, domical shapes e volve if the par tings are
tight and impenetrable to water.
Fracture density is evidently critical to the development of some bornhardts. Several possible
explanations have been suggested for such variations. According to Lamego (1938), for example,
the distribution of fractures is directly related to the distribution and sense of stress and strain in
folded crystalline sequences, with some zones in compression, others in tension. The deeper zones
of antiforms are in compression, the shallower ones in tension and the converse applies in synfor-
mal structures (Fig. 6.15). Similar stress variations are associated with offset or en echelon tran-
scurrent faults, and recurrent shearing and dislocation of or thogonal sets of regional magnitude
could result in distor tion of the cubic or rectangular b locks with a tendenc y to stretching along
axes aligned at roughly 45° to the direction of stress. Compression and tension do not cancel out
but are additive, and are of the same order of magnitude as the stresses applied. Continued dis-
location causes fracture propagation in the zones adjacent to the primar y fractures so that each
major block comes to consist of a stressed core set in a fractured zone (
Fig. 6.16)
.
Part of this hypothesis is susceptible to testing, for strain can be measured. This has been done
on several of the Eyre Peninsula, South Australia, inselbergs, by drilling shallow holes, inserting
strain gauges, and measuring the sense and amount of distotion for several hours. Results are con-
sistent with the shear theory, with the measurements from some sites, especiall y in the Wudinna
district, indicating contrasted stresses along different horizontal axes.
Of course, the two-stage explanation can, and ought to be, as is implied in the discussions out-
lined above, extended to include not only contrasts in susceptibility based in fracture density, but
Figure 6.15. Section through the Rio de Janeiro, southeaster n Brazil, area showing suggested relationship
between morros or bornhardts, and antiforms and synforms developed in the crystalline rocks
of the region (Lamego, 1938).
