Is the Sevier Desert Reflection of West-Central Utah a Normal Fault?: Comment and Reply

The continuing discussion of the Sevier Desert region, almost 20 years after MacDonald’s (1976) classic paper, provides a measure of the significance of the region as well as the non-uniqueness of seismic reflection data interpretation. The article by Anders and Christie-Blick (1994) and the nearly simultaneous publication of similar ideas by Hamilton (1994) raise important questions. The interpretation of a Sevier Desert detachment has always been a hypothesis to be tested rather than a fact. In our opinion, however, Anders and Christie-Blick and Hamilton have ignored a variety of basic geologic data requiring the presence of a major low-angle normal fault on the east side of the Sevier Desert basin. Anders and Christie-Blick (1994) suggested that the Sevier Desert basin is not bounded by significant Cenozoic normal faults. However, they provided no plausible scenario to explain the accumulation of 3.5–4.0 km of Oligocene to Holocene strata in the basin or the geometry of the basement surface. Basement beneath the basin depocenter is as much as 4.5 km higher than basement farther east (see Plate 1 of Allmendinger, 1992). The age of this arch must be Oligocene or younger because east-dipping Eocene beds on the east flank of the Pavant Range parallel the seismically defined, eastdipping Precambrian basement surface. Further support for Tertiary basement uplift comes from fission-track studies of cuttings from boreholes in the Sevier Desert. The apatite fission-track age of Prospect Mountain Quartzite from 6000 to 6500 ft (about 1818–1970 m) in the Cominco well is 8.5 6 2.2 Ma. Zircon fission-track ages from 1635 Ma (K-Ar and Rb-Sr) granitic gneiss from the interval 13,610– 15,537 ft (about 4124–4708 m) in the Arco Meadow Federal well are between 10.8 6 0.9 and 13.0 6 1.0 Ma. Anders and ChristieBlick (1994) would have subsidence in the Sevier Desert Basin occur simultaneously with uplift of the underlying basement surface, while somehow forming and maintaining a planar ‘‘unconformity’’ between Paleozoic and Tertiary. We suggest that basin subsidence and basement uplift are linked; footwall uplift during normal faulting (e.g., Spencer, 1984, and subsequent work) provides the simplest explanation. The crux of the Anders and Christie-Blick (1994) interpretation concerns the use of microfractures in well cuttings to evaluate zones of deformation. Cuttings are notoriously difficult to pinpoint in depth, let alone tie to seismic time sections. Furthermore, Anders and Christie-Blick compared their observations only to sparse data on thrust faults and high-angle normal faults rather than the myriad of well-exposed, low-angle normal faults around the core complexes, particularly those where growth strata overlie carbonate rocks. Also, the role of plastic deformation of salt along the detachment does not seem to have been evaluated. Anders and Christie-Blick (1994) assumed that the Sevier Desert detachment is located precisely at the contact between Tertiary and Paleozoic. However, exposed detachments are commonly composite structures in which different segments have different displacement histories (e.g., Lister and Davis, 1989). The exact location of the samples analyzed with respect to the surface(s) of major movement is very difficult to ascertain. The position of the Canyon and Pavant thrusts beneath and west of the Sevier Desert is a key question for section balancing. Most workers assume that the House and Cricket Range strata are in the upper plate of these thrust faults. The present eastward dip of the exposed Pavant thrust parallels the basement surface and thus was acquired after the Eocene. The exposure of the trailing edges of the thrusts in the ranges east of the Sevier Desert basin require normal fault offset or the selective erosion of a very unusual thrust geometry, which is unsupported by surface, seismic, or well data. A fault contact would also seem to be required by the eastwarddipping Oligocene and pre-Tertiary rocks on the west side of the Sevier Desert. The core of this dip panel, clearly imaged by both COCORP and some industry data, is Paleozoic and Precambrian rocks, not salt. We are unconvinced by Anders and Christie-Blick’s interpretation that the reflection seen on COCORP and industry data is the result of a fortuitous alignment. The estimated 30 to 40 km of displacement was based on the assumption that the hangingwall cutoff of the Canyon Range thrust is in this dip panel. Displacement, however, should decrease up dip to the east. The existing cross sections of the Sevier Desert area are not perfect, but the regional geology constraining the nature of the Sevier Desert detachment were ignored in Anders and Christie-Blick (1994). Underlying this debate is the continuing suspicion that lowangle normal faults are mechanically impossible. Although the Sevier Desert detachment is a serious challenge to ‘‘rolling hinge’’ models, it is only one of many detachment systems in which both hanging-wall and footwall strata nearly parallel the low-angle detachment surface across large distances. The otherwise thought-provoking paper by Anders and Christie-Blick (1994) contains inaccurate and selective referencing of previous work. Allmendinger et al. (1983) explicitly stated that they could not tell whether the Sevier Desert detachment was, or was not, a reactivated thrust. The only published cross sections of the Sevier Desert by the Cornell group (Allmendinger, 1992; Allmendinger et al., 1986) either showed both possibilities or interpreted the Sevier Desert detachment as, in large part, a reactivation of the Pavant Range thrust; neither paper was cited. Anders and ChristieBlick’s unreferenced interpretation of ‘‘a ramp anticline associated with Sevier-aged thrusting’’ was also made by Allmendinger et al. (1983, and subsequent papers) and much earlier still by others on the basis of surface geology alone.