Full Title: Geological map and cross section of the southern part of the northern Coast Ranges and Sacramento Valley, California
Author: John Suppe
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One 36.5" x 28" sheet. Abstract: The structure of the southern part of the northern Coast Ranges exhibits a general southeasterly plunge; thus, much of the shallow structure shown on the cross section may be seen directly, albeit in somewhat distorted form, if the map is viewed down plunge. At greater depth the structural solution of the cross section is constrained by the requirement that it be possible to undeform any proposed solution, a constraint that Ieads to solutions involving large-scale southwest-vergent thrusting within the Coast Ranges in Tertiary time. The thrusting postdates both the emplacement of the upper Mesozoic Great Valley sequence (KJg) over the coeval Franciscan Complex (KJf) and the emplacement of the Mesozoic Franciscan (KJf) over the Upper Cretaceous and Tertiary Coastal Belt Franciscan (TKfc). Prominent structural relief on the contacts between these major tectonostratigraphic units is produced by imbrication. The Cenozoic horizontal shortening that accompanies imbrication is about 175 km in the present structural solution. This distance is large relative to the present width of the Coast Ranges, which is about 100 km. Solutions that involve less horizontal shortening require thicker thrust sheets in the middle and western Coast Ranges. The cross section was tested with detailed two-dimensional gravity models and is consistent with observed gravity. In contrast, the traditional view of the structure, which places mainly higher-density Franciscan rocks (KJf) at depth in the Coast Ranges instead of substantial Great Valley sequence (KJg) and low-density subducted marine sedimentary material (TKfc), appears to be inconsistent with observed gravity. The Great Valley gravity high, the source of which is buried under the Sacramento Valley, is modeled as a west-dipping slab of Mesozoic oceanic crust and mantle. The shallow top of the low-velocity zone, high heat flow, Pliocene-Pleistocene postsubduction magmatism in the Coast Ranges, and uplift appear to be effects of northward motion of the Mendocino triple junction.
Published: 9/02/1978
Pages: 7, plus 1 sheet
Product Category: Maps and Charts