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
Maps, stratigraphic columns, sample location map, cross-section from text page figures 1.3MB pdf file
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