structural geology, propagation mechanics and - Stanford School of ...

otated all of the CB orientation data 25° counterclockwise about an axis trending 315°
(northwest) and plunging 0°, in keeping with available bedding orientation data for the
Cretaceous units (Bohannon 1977, 1983). The restored data are presented graphically in
Figure 1.10 and summarized in Table 1.2.
The most obvious result of the restoration is that the 22 measurements originally
attributed to a distinct third set of CBs trending northwest and dipping steeply southwest
(also previously identified by Hill, 1989 and others) can reasonably be categorized as
falling within the periphery of the primary (dominant) orientation group. This leaves just
two sets of CBs with mean orientations that are essentially orthogonal (dihedral angle of
88.8°). On average, the dominant set trends due north and dips 52° east, while the
secondary set trends almost due northeast and dips 51°northwest. The axis of intersection
between these mean CB orientations—azimuth of 24° and plunge of 27°—represents the
approximate line of σ3, which turns out to be more nearly horizontal than vertical. The
inferred azimuth/plunge directions of all three paleostresses are: 270°/38° (σ1), 138°/39°
(σ2) and 24°/27° (σ3) (Figure 1.11a). These inferred paleostress directions represent
approximate estimates, both because σ1 and σ2 (the poles to the dominant and secondary
mean CB orientations) are not quite orthogonal (88.8°), and as a result of the scatter in
the raw orientation data (± 17° in azimuth and 12° in dip) (Table 1.2).
6.3. Geomechanical implications
The radical departure of our inferred paleostress state from the classic Andersonian
expectation that one of the principal stress components is always subvertical, σ3 in thrust-
faulting environments (Anderson, 1951) means one of three things: the reconstruction of
the paleo CB orientations is faulty and the two sets were actually subvertical as well as
orthogonal; the underlying interpretation of CBs as anticracks is incorrect and they do not
reflect the orientations of the principal paleostresses; or the analysis is correct and points
to an as yet unrecognized regional tectonic explanation. We address the two former
possibilities first.
While our first-order approach to the reconstruction is certainly approximate, it is
fundamentally sound and anything more complex is currently unwarranted given the
available data. The best way to refine the reconstruction would be to collect a high
density of bedding orientation data as stratigraphically low in the Cretaceous deposits as
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