LOW-ANGLE NORMAL FAULTS: UNANSWERED QUESTIONS AND RESEARCH NEEDS

Low-angle normal faults (detachment faults) have long been enigmatic with respect to fracture mechanics that argue against their development and seismotectonics that tend not to identify currently active examples. A lack of clear answers to critical questions adds to the enigma, including: 1) Why are large-displacement strike-slip accommodation faults and/or oroclinal bends generally absent along transverse boundaries between large-strain detachment-fault terrains with opposed vergence? 2) Why are there generally no examples of upper-plate rocks imbedded into lower plates as a result of the excision tectonics demanded by bedding/fault intercept angles? 3) Why are foliated shear fabrics absent or only weakly developed at several major well-studied detachment faults? 4) If the dip of detachment faults decreases with maturity, how do the faults grow (lengthen) in their down-dip direction? 5) How does tectonic unloading of relatively low-density rocks by detachment faulting result in isostatic adjustments that put higher density rocks in their place? 6) What constraints do huge heave and little or no throw place on the mechanics of detachment faults? 7) Why are the rake-angle distributions of upper-plate faults bimodal, tending to cluster at low (0º-30º) and high (69º-90º) angles? 8) How does the fluid-flow history of detachment faults affect their development? 9)How can the cycling of fluid pressure through the high values required to allow slip be rationalized with massive chemical changes and dissolution documented in rocks above and below some faults? To deal with these questions requires research focused on identifying additional processes involved in the uplift and extension history of highly extended areas. Processes such as 1) sub-detachment, extension-normal inflow of viscoelastic material, 2) extension-parallel tectonic rafting, and 3) protracted dissolution will be key to answering many of the questions.