The Lems Ridge olistostrome (LRO), located within the western Jurassic belt of the Klamath Mountains geologic province, is a melange unit showing a distinct block-in-matrix fabric. The conformable relationship to both an igneous basement and the overlying flysch of the Late Jurassic Galice Formation points to a sedimentary origin. The lack of pervasive deformation and the internal stratification also support a predominantly olistostromal origin of the LRO. The matrix of the LRO consists of pebbly mudstone which interfingers with tuffaceous greenstone and tuff-breccias. Subordinate beds of chert, argillite, and sandstone are also present. The matrix contains abundant ophiolitic clasts, fragments of porphyritic and vesicular volcanics, as well as a variety of sedimentary and metamorphic rocks. A block supported talus deposit is locally dominant and consists of serpentinized ultramafics, diabase, and sparse gabbroic rocks.
The LRO has been previously interpreted as a remnant of an ancient fracture zone within the Josephine ophiolite (JO). Basement and clasts were thought as locally derived from the JO which is of Late Jurassic age and has a distinct supra-subduction zone chemistry. Geochemical and geochronological data were obtained to test this hypothesis.
The major and trace element geochemistry of the tuffaceous matrix and the ophiolitic lithologies indicates the presence of several petro-tectonic regimes in the source areas of the LRO: the tuffaceous rocks are transitional between island-arc tholeiitic and calc-alkaline; the basal pillow lavas range from within-plate basalts to enriched MORB compositions; olistoliths of pillow basalt range from depleted to enriched MORB; clasts of sparse mafic scorias are alkaline within-plate basalts. Diabase talus blocks are transitional between N-MORB and island-arc tholeiites, but also include very primitive arc-related rocks.
40Ar/39Ar hornblende ages of large gabbro blocks cluster consistently around 190 Ma; a basement gabbro yields the same age within error limits. The metamorphic age of a mica-schist clast is found to be 280 Ma; the total gas age for an andesitic boulder is 173 +/- 6 Ma. Calc-alkaline dikes that cross-cut block-matrix boundaries show ages of 148 Ma and 149 Ma, respectively, and constrain a minimum age for the assembly of the LRO.
It is concluded that the LRO overlies an older basement than previously assumed and contains ophiolitic lithologies, the age and chemistry of which is not compatible with a derivation from the JO. On the other hand, the chemistry of diabase blocks as well as the lithologic assemblage in a talus breccia suggests an affinity to the JO. The conformable contact to the Galice Formation implies the same genetic link.
A model that integrates this diversity is suggested: the JO formed in a transform dominated marginal basin; its spreading geometry requires the presence of fracture zones parallel to, and projecting into the passive margin of the Hayfork-Rattlesnake Creek remnant arc. The ensuing boundary tranform fault is floored by older rocks of the margin, whereas the infill into the transform trough comprises three sources: the passive margin, an active arc that rifted off the older arc, and the incipient JO. The LRO and its basement are thus interpreted as a preserved rift-edge sequence of the Josephine ophiolite.
Ohr, M., 1987. Geology, geochemistry, and geochronology of the
Ridge olistostrome, Klamath Mountains, California.
thesis, State University of New York at Albany. 278 pp., +xi
University at Albany Science Library call number: SCIENCE Oversize (*) QE 40 Z899 1987 O57
Maps, stratigraphic columns, sample location map, cross-section from text page figures 1.3MB pdf file
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