Fragmentation of island arcs and related dispersion of the resulting arc

segments is common in modern continental margin environments. Inasmuch as a

majority of Cordilleran accreted terranes show island arc affinities one would

expect to find remnants of arc-related transforms and rifts within such

terranes. Jurassic arc and ophiolitic terranes of the western Sierra Nevada

and Klamath Mountains have regional age and structural patterns which suggest

multiple rifting and transform faulting episodes, and some areas offer

remnants of the actual rift or transform complexes

Saleeby, Jason

Caltech Authors - Main

Recognition and significance of boundary transforms and rift edges within island arc terranes of the western Sierra Nevada

52. Recognition and Significance of Boundary Transforms and Rift Edges 
within Island Arc Terranes of the Western Sierra Nevada 
Jason Saleeby 
Division of Geological and Planetary Sciences 
California Institute of Technology 
Pasadena, California 91125 
Fragmentation of island arcs and related dispersion of the resulting arc 
segments is common in modern continental margin environments. Inasmuch as a 
majority of Cordilleran accreted terranes show island arc affinities one would 
expect to find remnants of arc-related transforms and rifts within such 
terranes. Jurassic arc and ophiolitic terranes of the western Sierra Nevada 
and Klamath Mountains have regional age and structural patterns which suggest 
multiple rifting and transform faulting episodes, and some areas offer 
remnants of the actual rift or transform complexes. 
Regional age patterns show a strong correlation between island arc 
volcanism and ophiolite generation (Saleeby, 1982; Saleeby and others, 
1982). In particular ophiolite generation at two distinct times (200 m.y. and 
160 m.y.) coincides with the termination or waning of volcanism and magmatism 
along major arc segments and the emplacement of dike swarms across the arc 
framework. Such relations suggest that ophiolite generation represents the 
disruption of arc constructional patterns which is a phenomenon seen in modern 
environments with the formation of inter-arc basins (Karig, 1971, 1972). The 
edges of disrupted arcs consist of rift or boundary transform contacts with 
juvenile ophiolitic crust. 
In the central Sierra Nevada between the American and Merced Rivers 
remnants of early Jurassic arc rocks (210-200 m.y.) are exposed along with 
their ophiolitic melange basement in the core areas of large late Jurassic 
anticlinal or upright nappe structures. The arc sequences are clearly resting 
with primary relations on a previously disrupted ophiolitic substrate. This 
ophiolitic basement comprises a distinct tectonostratigraphic package with 
distinct geochronological patterns, and can be shown to extend southward into 
the southern Sierra Nevada (Saleeby, 1982). Here distal arc-related tuffs and 
tuffaceous cherts are interbedded with aphyric pillow lava, sedimentary 
serpentinite and ophicalcites (Saleeby, 1979). This complex volcanic-
sedimentary sequence lies depositionally .above the ophiolit ic melange of the 
arc basement. A dike swarm which appears to have fed volcanic rocks of the 
superjacent sequence has been dated at 200 ±10 m.y. by Pb/U zircon. The 
nearby Kings River ophiolite has yielded 200 m.y. zircon dates from 
plagiogranite within sheeted dikes and from diorite layers within its gabbroic 
intrusive sequence (Saleeby, 1982). An outstanding feature of this ophiolite 
is major mylonite and flaser gabbro zones with minimum structural thicknesses 
of 5 km (Saleeby, 1978). Such mylonitic rocks formed during ophiolite genesis 
with flaser gabbros showing deformational fabrics developed under magmatic and 
hot sub-solidus conditions. Furthermore, syngenetic mylonitization 
accompanies juxtapositioning of the ophiolite against older ophiolitic melange 
of the arc basement along a structurally complex dynamothermal aureole. This 
aureole shows pervasive amphibolite facies metamorphism superimposed across 
pre-existing melange structure in the arc basement. 
167 
Capping of ultramafic basement uplifts with pillow lava, sedimentary 
serpentinite and ophicalcite sequences, and the development of major flaser 
gabbro-mylonite zones under magmatic and hot sub-solidus conditions in 
ophiolitic crust are processes that have only been documented within transform 
fracture zones. The 200 m.y. fracture zone complex of the southern Sierra 
formed along and across the basement of the early Jurassic island arc. Arc 
act1v1ty appears to have ceased at about 200 m.y .. It is suggested that the 
southern Sierra fracture zone complex comprises the remnants of a boundary 
transform which formed during the rifting disruption of the early Jurassic 
arc. 
The remnants of a mid- to Late Jurassic inter- or intra-arc rift edge are 
preserved in the American River area of the northern Sierra Nevada. Here 
Callovian to Oxfordian arc rocks are built on a complex basement which 
includes lower Jurassic arc rocks and their ophiolitic melange basement 
(Behrman, 1978; Behrman and Parkison, 1978; Saleeby, 1982). These rocks are 
cut by a layered pyroxenite-gabbro-diorite plutonic complex (Springer, 1980) 
and by a sheeted dike complex, both of which have been dated at 163 m.y. by 
Pb/U zircon techniques (Saleeby, 1982). The layered intrusive complex is the 
same age as, and can be petrologically linked to the overlying arc volcanic 
pile. The ophiolitic melange and early Jurassic arc remnants are contact 
metamorphosed and occur as local screens within the intrusive complex. The 
163 m.y. intrusives along with their wallrocks were deformed while still hot 
from primary heat in the core area of a Nevadan (155 to 150 m.y.) anticline or 
upright nappe. 
North of the American River complex occurs a large sheet with ophiolitic 
affinities referred to as the Smartville complex. This complex represents a 
rifted arc (Xenophontos and Bond, 1978) with sheeted dikes yielding Pb/U 
zircon ages of 162 m.y .. The Smartville sheet was abducted during the Nevadan 
orogeny eastward over ophiolitic melange and arc rocks which have scattered 
ages and strong similarities to wallrocks of the American River complex 
(Moores and Day, 1983). These relations suggest that the hot attentuated 
Smartville arc rode over a rift edge similar to the American River complex. 
An additional example of imbrication along rift edges in the upper 
Jurassic Josephine ophiolite and neighboring arc segments is described in 
Saleeby and others (1982). It is suggested that Jurassic arc terranes of the 
California region underwent multiple rifting and imbrication events, and that 
rift edges and boundary transforms served as nucleation sites for convergence 
and imbrication. It is also suggested that young warm ophiolitic crust is 
preferentially preserved along such zones. The tendency towards imbrication 
inunediately following rifting and transform disruption is interpreted to be a 
result of the opening of small intra-orogenic ophiolite floored basins 
contemporaneous with the accretion of the larger Cordilleran framework to 
North America. (Saleeby, 1983). 
168 
REFERENCES 
Behrman, P.G., 1978. Pre-Callovian rocks west of the Melones Fault Zone, 
central Sierra Nevada foothills. In Howell, D.E., and McDougall, K.A., 
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Behrman, P.G., Parkison, G.A., 1978. Paleogeographic significance of the 
Callovian to Kimmeridgian strata, central Sierra Nevada foothills, 
California. In Howell, D.E., and McDougall, eds., Mesozoic 
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169 


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Recognition and significance of boundary transforms and rift edges within island arc terranes of the western Sierra Nevada

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