Nature and significance of the Inyo thrust fault, eastern California: Discussion and reply Discussion GEORGE C. DUNNE Department of Geosciences, California State University, Northridge, Northridge, California 91324 RACHEL M. GULLIVER Envicom Corporation, 4521 Sherman Oaks Avenue, Sherman Oaks, California 91403 INTRODUCTION Olson (1972, Fig. 2). Also included in Fig- caught between two downward-convergent ure 1 are our alternative interpretations of faults, one of which truncates the other; Stevens and Olson (1972) proposed that the geology along their cross-section lines. folding is observed in one area, but it seems a complexly faulted area at the west base of We base our interpretations on 11 days of to result from drag along one of the faults. the northern Inyo Mountains is a window field work in the Tinemaha area. Locations 2, 3: The elongate mass of Or- in a folded, large-slip fault they named the We have few disagreements with the dovician chert and an adjacent elongate ex- Inyo thrust fault. On the basis of their rec- more objective aspects of the geologic posure of Mississippian rock are essentially ognition of this window and earlier struc- mapping of Stevens and Olson. We agree in homoclinal, separated by steep faults that tural interpretations by Stevens (1969, general with their identification and map dip toward the younger sequence (see cross 1970), Stevens and Olson proposed a distribution of rock units, although we sus- section C, Fig. 1). Location 5: The tectonic model in which the Inyo thrust pect that some units on hills 1 and 4 (Fig. 1) arrowhead-shaped contact between older fault played a major role in the Mesozoic are misidentified. We also agree that the and younger sequences is shaped by two structural evolution of eastern California. Ordovician-Cambrian rock sequence is sets of steeply dipping faults, one set having We developed an interest in the work of everywhere in fault contact with the a northeast trend, the other a northwest Stevens and Olson for two reasons. First, Permian-Mississippian sequence. trend. Location 6: Two "klippen" of Or- we were intrigued by the contrast between We disagree, however, with the major in- dovician rock are brecciated near their the map and cross sections of the proposed terpretive aspect of Stevens and Olson's bases, and we interpret them to be landslide window by Stevens and Olson and mapping mapping of the Tinemaha area. They con- blocks; the underlying Mississippian se- of the same area by Nelson (1966), who tended that the faulted contact between the quence is poorly exposed and appears to be showed the structure to be dominated by Ordovician-Cambrian sequence and the homoclinal (no evidence for a large syn- normal faulting. Second, and more impor- Permian-Mississippian sequence is a tightly form). Location 7: An older rock sequence tant, the tectonic model of Stevens and Ol- folded thrust fault that separates older appears to rest upon a younger rock se- son, which relies heavily on their interpre- allochthonous units above from younger quence across a flat-lying movement sur- tation of the proposed window, differs autochthonous units below. We believe that face; the older rock mass contains numer- markedly from previous tectonic interpre- many field relationships in the Tinemaha ous brecciated zones, including contact tations of this part of eastern California area cannot be readily reconciled with this with the underlying sequence; we interpret (Stewart and others, 1966). If correct, the thrust-fault hypothesis and that most field the older rock mass to be an old landslide Stevens and Olson model would place im- data are supportive of an alternate model deposit, emplaced from steep slopes (per- portant constraints upon interpretations of that closely follows the earlier work of Nel- haps a fault scarp) to the east. Location 9: the tectonic evolution of this part of the son (1966) — that is, principal structures in The west-trending contact between the Cordilleran orogen. This regional implica- the Tinemaha area have been shaped by older and younger sequences is controlled tion prompted this discussion. normal faulting and subsidiary landsliding. by a set of subvertical, N80°W-trending We comment briefly below on what may faults. Location 10: The north-pointing re- GEOLOGY OF THE be observed (or reasonably inferred where entrant in the contact is controlled by two TINEMAHA AREA exposures are poor) at the major outcrops sets of steep faults having different north- of this faulted contact. Locations are shown ward trends. Locations 11, 12: A faulted Figure 1 is a simplified version of the in Figure 1. contact dips 75°W to 75°E. Locality II: The geological map and cross sections of the Locality I: The fault contact is poorly ex- fault contact is poorly exposed and seems to Tinemaha area presented by Stevens and posed, but the approximate trace of contact have a near-vertical dip. Locality III: The relative to topography suggests it dips fault contact is well exposed at one point and dips 72°SW. Locality IV: The fault is The article discussed was published in the steeply westward. Location 1: The Bulletin, v. 83, p. 3761-3768. "klippe" of Ordovician chert is a wedge well exposed at one point and dips 75°SW.; Geological Society of America Bulletin, v. 89, p. 1787-1792, 3 figs., December 1978, Doc. no. 81212. 1787 Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/89/12/1787/3418788/i0016-7606-89-12-1787.pdf by guest on 26 September 2021 1788 DUNNE AND GULLIVER the relation of fault trace to topography with the interpretation of Stevens and (fan folds), an unusual fold style not ob- suggests a southwest dip along this whole Olson that depicts these older sequences served in mesoscopic folds of this area, stretch. lying in the troughs of synforms, unless it finally, we carefully examined all areas Faults and folds in the Tinemaha area could be shown that these inferred folds where Stevens and Olson contended that have several chara«eristics that are perti- have been tightened past isoclinal geometry the distribution of older and younger rock nent to the interpretation of their origin. First, neither the style nor the attitude of faults separating older and younger rock sequences is consistent with the folded Quaternary units thrust model of Stevens and Olson. Almost unconformity all of the faults juxtaposing older and younger rock sequences display brittle de- formation characteristics, notably abun- Permo-Penn. units, undiv. dant brecciation. This predominantly brittle behavior is difficult to reconcile with the model of Stevens and Olson, which postu- lates that this fault developed beneath an allochthon many kilometres thick. Such a Mississippian units,undiv. tectonic setting would favor the develop- unconformity ment of high confining pressure that would in turn promote ductile deformation fea- tures such as abundant tight folds, foliation, Ordovician Ely Springs Dolomite and flowage in rocks adjacent to the fault — phenomena that are almost universal for rocks adjacent to major thrust faults in eastern California. We believe that abun- dant brecciation adjacent to faults in the our irnterpretation« J/ Ordovician, Cambrian units, undiv. Tinemaha area reflects their environment of contact formation, namely relatively low effective east-west high-angle fault confining pressure and/or high strain rate, cross sections of^ conditions characteristic of normal faults Stevens,Olsoiv-t,^ \ V V 7" and landslide slip surfaces. unnamed reverse, thrust fault The attitudes of faults between older and younger rock sequences also are more com- north end, hill Inyo thrust fault patible with origins as normal faults than as — a tightly folded thrust fault. We measured antiform, anticline the dip of the faulted contact separating older and younger rock sequences at 17 points over the entire Tinemaha area. The synform,syncline south end, hill lowest dip recorded was 40°; the mean dip is 69° and the median dip is 67°. The fault was nowhere observed to dip 25° to 40° east along its eastern exposures as suggested by Stevens and Olson. We think it is more than coincidence that the mean and median dips between pts. 2,3 for these faults are reasonably representa- tive of dips of normal faults in the Great Basin. The pattern of dip direction of these Figure 1. Slightly simplified faults also is more consistent with the nor- between pts. 5,6 version of geologic map and mal fault model than with the folded cross sections of Tinemaha thrust-fault interpretation. In areas where area by Stevens and Olson elongate masses of older rocks are bounded (1972) and our alternative in- on both sides by younger rocks, as at loca- ! terpretations of geology along tions 2, 3, and 5, faults bounding the older y their cross-section lines. Loca- rock mass consistently dip in opposite di- tions 1 through 12 on map are rections away from the older rock sequence / those of Stevens and Olson; (for example, see our cross section C, Fig. r->>3 0 m|le te localities I through IV are de- 1). This observation is difficult to reconcile 0 km '/2 scribed in our discussion. Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/89/12/1787/3418788/i0016-7606-89-12-1787.pdf by guest on 26 September 2021 DISCUSSION AND REPLY 1789 sequences is controlled by large tight folds stratigraphic trend, it is likely that in the folded and cleaved rocks of Cambrian to (locations 1, 2, 3, 5, 6, 10). Study of at- Tinemaha area Mississippian strata were Permian age which had one or more uncon- titudes of bedding, bedding-cleavage rela- deposited unconformably directly upon formities between Mississippian and Or- tions, and repetition (or lack thereof) of Ordovician rocks, or that the two were dovician units.