Abstract
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
Lems Ridge olistostrome, Klamath Mountains, California.
Unpublished MSc. 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
thesis (scanned
text) - 21.5MB pdf file
Maps, stratigraphic columns, sample location map, cross-section from text page figures 1.3MB pdf file
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