Evidence for rapid displacement on Himalayan normal faults and the importance of tectonic denudation in the evolution of mountain ranges : Comment and Reply

In the Rampino and Volk (1996) study of the eight structures in the central U.S. chain, we reviewed all previous work and concluded that the ages of all are very poorly constrained, based as they are on perfunctory geologic mapping and crude radiometric dating (mostly K/Ar and Rb/Sr) of uncertain materials; most were published more than 30 years ago. This early work was done before impact was a respectable alternative to endogenous processes, and thus prior to the many studies that have determined the diagnostic characteristics of impact-induced deformation, impact breccias, impact melts, and pseudotachylites (e.g., French, 1990). We also noted that shock features unique to hypervelocity impact (shocked quartz and shatter cones) were known from Decaturville and Crooked Creek (e.g., Short and Bunch, 1968; Offield and Pohn, 1977), but careful searches for such features have never been attempted for the other six structures. We did not mean to imply that other features (e.g., shattered quartz grains, breccia dikes) found at these structures were diagnosticevidence of impact, only that they were suggestive. Detailed mapping of the structures and breccia distribution, a search for diagnostic shock features, and reliable radiometric dating would be useful. Luczaj (1998), however, presented no new data, and reviewed the same largely pre-1970s literature that we used in the Rampino and Volk (1996) paper. Luczaj referenced French (1990) to imply that a controversy still exists with regard to the process that produces “cryptoexplosion” structures. The reality is just the opposite. To quote French’s conclusion, “During the last 25 years of this debate, numerous studies have reinforced the view that shock-metamorphic features uniquely indicate shock pressures of from about 5 to >50 GPa (50 to 500 kbar); no generally accepted shock-metamorphic features have been found in unquestionable volcanic structures; and no internal process has been demonstrated to produce the required shock pressures. This history therefore supports an impact origin for . cryptoexplosion structures. ” Experimental studies have shown that shocked quartz showing various planar deformation features requires at least 5 GPa (more commonly >20 GPa) (Grieve et al., 1996), and shatter cones >4.75 GPa (Schneider and Wagner, 1976). Gibson and Spray (1998) have shown that some shatter cone surfaces display evidence of shockinduced melting and vaporization, indicating localized shock pressures in excess of 60 GPa. These shock pressures contrast markedly with the conditions during volcanism or “cryptoexplosion,” where the main control on the maximum overpressures is the tensile strength of the rock surrounding the magma chamber-conduit system (de Silva et al., 1990). This limits maximum explosion pressures to less than 0.05 Gpa (0.5 kbar), orders of magnitude less than the experimentally determined pressures required to form shocked quartz and shatter cones. Luczaj (1998) argued that if the structures are shown to be of various ages, then the impact chain theory is ruled out and no eof these structures can be impact-related; therefore the characteristic features of shatter cones and shocked quartz (as found at Decaturville and Crooked Creek) can be produced by internal processes. The situation instead is simply this: Crooked Creek and Decaturville are unequivocally known to be impact structures, on the basis of shock criteria. If it turns out that the other structures were formed at different times (and Luczaj provided no new absolute dating to support this conclusion), then some other mechanism besides an impactor string must be found to explain why the structures occur in a straight line. If any of the other six structures are non–impact-related in origin, then there are several possibilities, including localization of stresses by the two known craters, tectonic effects of impacts (including igneous intrusions that might be associated with impact deformation or impact-induced faulting), and of course possible coincidences. If they are all impact structures, but really of different ages, then the apparent alignment could be a result of an unlikely coincidence, a repetitive capture process, or some still unknown mechanism. Some of Luczaj’s (1998) interpretations of age constraints are not supported, in my view, even by the poorly known geology. For example, the Decaturville structure was first dated within narrow limits as latest Cambrian to early Ordovician by Bucher (1936), on the basis of supposed constraints in the internal stratigraphy, and then revised to post-Silurian when Silurian blocks were discovered within the breccias (Snyder and Gerdemann, 1965). Because undeformed Pennsylvanian cover rocks are fo nd in the vicinity, the event can most likely be bracketed between the Silurian and Pennsylvanian, not (as Luczaj concluded) younger than P nnsylvanian (for an impact origin) or younger than Silurian (for a volcanic origin), with no lower age limit given in either case. Luczaj (1998) also suggested that origin of a 700-km-long crater line through breakup of a parent object by terrestrial or lunar tidal forces or from bodies in orbit around the earth is unlikely. However, the recent discovery of a 4500-km-long row of Late Triassic impact craters (Spray et al., 1998), has led to a reconsideration of the probability of asteroid fragmentation and capture into Earth orbit through aerobraking in the upper atmosphere. The published age determinations, geologic mapping, and search for shock effects are still insufficient to rule out the contemporaneous impact or impact-related origin of all eight structures (Rampino, 1997). In any case, whatever the outcome of future studies, the Decaturville and Crooked Creek s ructures clearly contain shock features unequivocally linked to the enormous pressures created only by hypervelocity impact; Luczaj’s (1998) arguments provide no reason to question shatter cones or shocked minerals as clear indicators of impact. On the other hand, numerous studies argue against “cryptoexplosion” as a real phenomenon capable of producing the kinds of features seen in hypervelocity impact structures.