Formation and thrusting in some great valley rocks near the Franciscan Complex, California, and implications for the tectonic wedging hypothesis

Abstract

On the western side of Sacramento Valley in northern California, the Upper Jurassic and Cretaceous Great Valley sequence dips east at 40° to 70° in what has been regarded as a tilted but otherwise undeformed east dipping homocline containing strata collectively up to 15 km in thickness. This paper reports the results of structural mapping in the Paskenta area which suggests that, in contrast, steep dips are caused by westward directed thrusting and folding which led to tectonic thickening of the sedimentary pile. In a mainly east dipping sequence, thrusts are recognized by changes in the structural facing and vergence of both folds and cleavage. Although mainly affecting undated rocks of the Great Valley sequence, one major thrust rides on a serpentine‐matrix melange which forms the substrate of the Great Valley sequence. Differential shortening occurring in different blocks during thrusting was accommodated by left‐lateral movement on the Paskenta and Elder Creek fault zones which acted as tear faults. Ductile and brittle faulting have also occurred throughout the sequence, in the melange, and on the tear faults. From work described in this paper and from other reports of thrusts and deformation in the Great Valley sequence in the foothills west of Sacramento Valley, the hypothesis is advanced that these strata have all been tectonically thickened by westward directed thrusting of possible early Tertiary age. While the regional applicability of this hypothesis awaits future detailed structural work, it is no longer sufficient to assume that east dipping strata have been merely tilted. The proposed thrusting model is compatible with the recent tectonic wedge model proposed for the Coast Ranges‐Sacramento Valley by Wentworth et al. (1984). With respect to such a crustal model in which a tectonic wedge of Franciscan Complex was thrust east above older continental basement but below rocks of the Coast Range ophiolite and Great Valley sequence, thrusts of this paper are back thrusts developed above a gently west dipping floor thrust. Back thrusting, shortening, and structural elevation of the Great Valley sequence are direct consequences of this wedging. The uplifted strata lie east of a poorly defined triangle zone and define the western limb of a regional synclinorium that has some analogies with the Alberta syncline of the Canadian foothills.