Post-mortem Musings on the Evidence for 65 km Right-Slip in Owens Valley Post 83 Ma
by Arthur Sylvester, 5 May 2005
If a major earthquake had not produced nearly 6 m of right-lateral surface rupture in Owens Valley in 1872, I believe it would be several years more into the future before anyone would undertake a diligent search for either recent or ancient horizontal displacement between the White-Inyo Range and the Sierra Nevada. The evidence for recent strike-slip is meager beyond the displacement of a line of trees, now long gone, and Kim's directed search for it along a small stretch of the Sierran front was in vain.
The early prounouncements of little horizontal displacement by Ross (1962) and Moore & Hopson (1961) were based on information available to them. Don Ross, with little more to go on than petrographic similarities, correlated the granodiorite plutons of Santa Rita Flat and Tinemaha, but now we know that such granodiorite of identical age is widespread throughout the center of the eastern Sierra Nevada as well as in the White-Inyo Range. Therefore, we must discount Ross' correlation without more unique evidence than mere petrographic and temporal similarities.
Moore and Hopson discovered the Independence dike swarm (IDS), and from their discovery point atop the Sierra, they looked across Owens Valley and saw some big black dikes in the Inyo Range and concluded that the IDS extended unbroken across the valley. Brian Carle, together with his companions, eludicated the great extent of the IDS southeastward across the Mojave Desert, but until the density of the dikes within the swarm was determined, one could easily agree with Moore and Hopson that horizontal displacement is negligible in the valley.
Now, two score years since the pronouncements of the early 1960s, we have three pieces of evidence, based on much regional mapping and synthesis by many writers, that give good reason to believe that 65 km of right-slip occurred between the Sierra Nevada and White-Inyo Range since Late Cretaceous time.
1) displacement of the 148 ma IDS;
2) displacement of a Devonian submarine channel; and
3) displacement of an extensive, unique 83 ma granite porphyry dike.
Acceptance of the first piece of evidence requires the assumption that the IDS was originally rectilinear in its geometry. If the overall geometry of the swarm had a right-stepping, en echelon distribution, then we would have to degrade the value of the IDS as a strain marker. Except for the left-lateral displacement of the IDS by the Garlock fault, the IDS is rectilinear from the Inyo and Coso ranges southeastward across the Mojave.
We also have to assume that the density of dikes was regular three dimensionally to correlate the dense domain of dikes in the Coso Range with an equally dense domain in the Tinemaha region of the Sierra. If the dikes are random in distribution and density, then use of the IDS must be suspect as a strain-marker for strike-slip. The available maps reasonably convince me, however, that the IDS is fairly regular along strike, and can be used in the way that we saw on the field trip.
The presence of the Devonian submarine channel at the foot of the Inyo Range and correlation with a similar structure in the Mt. Morrison pendant was new to me, but at first blush, it is a fine story. It seems to be well documented and was certainly well presented. I am only left wondering if there were one such channel, isn't it possible that there were two or more? Is the 65 km right-lateral correlation of the rocks coincidental or a mere artifact of accumulated tectonics and erosion since Devonian time?
The third piece of evidence - the correlation of the Golden Bear and Coso dikes - certainly impressed me. I am satisfied Andy and his associates have elminated the possibility that other parallel, temporally and petrologically similar dikes do not exist in the subject ranges. Bill Hirt and I would be fully convinced if we could eliminate the possibility that the dikes are not the product of the same process operating at the same time in two different places, that is, if the Whitney pluton squirted a dike eastward at the same time as a pluton in the Coso or southern Inyo range squirted one westward. Big long dikes are unique and cry for special explanations in each case. The Golden Bear and Coso dikes are no different, but I think single squirts from two separate big plutons toward each other at the same time stretches credibility.
I would not wish to hang my hat on use of the IDS and the Devonian channel as strain markers by themselves, but taken together with the Golden Bear/Coso dike correlation, I believe the story is a quite credible. All that is left sticking in my craw are correlable Cambrian "framework" rocks NOT displaced across Owens Valley. As we discussed, some slippery slidey tectonics would be required, sending the Cambrian rocks south by left slip before Devonian time, and then bringing them back to their present position by right-slip after, presumably, Cretaceous time. We saw the screen of Campito and Poleta formations at Cardinal and Split Mountains. Although the type localities for the Campito's two members are right across the valley not far from Big Pine, the bulk of the Campito and Poleta formations crop out in the White-Mountains almost to White Mountain Peak, well north of Bishop, for whatever that is worth.
Let us reflect a moment on the nature of the strain markers being used here. Only the Devonian channel represents a piercing line and therefore has kinematic signficance. The IDS and the Golden Bear/Coso dikes are piercing surfaces and have geometric significance only yielding separation, not slip, although it is true that their verticality strengthens their use as kinematic determinators. The Cambrian formations are also surfaces, now folded however, which if considered sensibly flat-lying at the time of strike-slip, are worthless as kinematic indicators for strike-slip because of the phenomenon of trace-slip.
What kind of work would I do to test the hypothesis? I think the Santa Rita Flat pluton has been cut in half, so where is the other half and what properities would be required to prove that the two halves are identical? I wonder if some unique rocks and structures exist in the Jurassic volcanic rocks of the southern Inyos and that can be correlated in pendants in the Sierra?
What else intrigues me from the forum? I would put a student into the Little Lake area right away to test Jeff's observation of right-slip indicators there. Why is Symmes Creek deflected right-laterally right along a fault between basement and alluvium, when no evidence exists anywhere else for right slip on that fault? I would love to know the age and origin of the big benches half way up the Sierran front between Big Pine and Olancha, because I have pondered them many times while eating lunch upon the top of the Inyos. If a downdropped Eocene erosion surface, is it related to the green conglomerate, mentioned so briefly in passing, that was found atop one of the high peaks? Is it a remnant of an Eocene erosion surface? What is the earthquake history of the Owens Valley fault prior to 1872? When did that fault come into existence and what is its cumulative slip? How does it tie into the White Mountain shear zone, or does it? In fact, how does its strike-slip get north of the Poverty Hills if at all? How does it as a strike-slip fault go south of Diaz Lake, and does it make it to the Garlock fault? Or does it step into the Sierra? Will it be the locus of the rupture of Alta California from the North American plate over the next 5 million years as I have been asserting for almost as long?