EARLY MESOZOIC PALEOTECTONIC-PALEOGEOGRAPHIC RECONSTRUCTION OF SOUTHERN SIERRA-NEVADA REGION
Aapg Bulletin(1978)
Abstract
Metamorphic country rocks of the southern Sierra
Nevada occur as isolated roof pendants and as the
western wall of the batholith. Metasedimentary
rocks are primarily Triassic to early Jurassic in
age as shown by fossil dates in conjunction with
field and petrographic data. The lower Mesozoic
strata were deposited across a complex paleo-basement
consisting of two contrasting terranes. The
western terrane is exposed along the foothills and
consists of the Kings-Kaweah ophiolite belt, a latest
Paleozoic to possibly earliest Mesozoic remnant
of disrupted oceanic lithosphere. This belt appears
to represent an oceanic fracture zone complex that
was transported northward into the region by large-scale wrench faulting. The eastern terrane which
can only be inferred from petrochemical studies on
the batholith, was sialic in character. It may
also have been displaced by wrench faulting, but to
a lesser degree. The zone of joining between the
eastern continental terrane and the western oceanic
terrane is termed the foothill suture.
Lower Mesozoic strata deposited on the oceanic
terrane consist of: (1) chert-argillite olistostromes
containing exotic blocks of upper Permian limestone
and occasional blocks of quartzitic sandstone;
(2) quartzitic to sub-arkosic flysch; (3) olistostromes derived from ophiolite basement rocks; and
(4) basalt-andesite volcanic rocks. Strata deposited
on the continental terrane consist of: (1) Quartzose
to sub-arkosic flysch; (2) Quartzlte-arglllite
olistostromes with large slide blocks of shallow
water limestone; (3) massive quartzitic to sub-arkosic
sandstones, limestone and calcareous sandstones;
and (4) silicic tuffs and ash flows. The flysch
sequences of each terrane are apparently correlative.
The eastern sequence contains a more proximal
facies and the western sequence contains a
more distal facies. The western facies flysch
was partly reworked with chert-argillite into
olistostromes.
The early Mesozoic paleogeography of the region
was apparently controlled by a complex plate juncture
that involved both large-scale wrench movements and
oblique subduction. The Triassic was characterized
primarily by tectonic truncation of the continental
margin by wrench faulting. North-northeast structural and stratigraphic trends typical of the Paleozoic
were overprinted by northwest trends. The
major wrench zone had a dextral sense of motion and
extended through the truncated margin and into the
oceanic domain. The southern extension of this zone
was a large fracture zone which extended to the
equatorial and possibly southern proto-Pacific.
During northward transport of the fracture zone complex,
slide blocks of shallow water limestone were
acquired from an equatorial oceanic faunal belt.
As the fracture zone complex moved into the proximity
of the truncated margin, slices of the continental margin were differentially transported
northward. Some fragments were displaced as far as
southeastern Alaska. During Late Triassic to Early
Jurassic time a submarine fan complex was shed off
the truncated continental shelf and dispersed across
the fracture zone complex. The sands of this fan
complex are believed to be in part an extension of
the Navajo- Aztec sands which accumulated on the ancient
shelf. At about the same time a change in
relative plate motions resulted in a convergent
component along the complex plate juncture. The
fracture zone complex was accreted as the hanging
wall of an oblique subduction zone. Eruption of
oceanic arc-type rocks along the western terrane and
ignimbrites along the eastern terrane indicate the
onset of subduction related magmatism. Both volcanic
assemblages were interstratified with Navajo-Aztec
affinity sands. During arc volcanism both
terranes of the southern Sierra underwent differential
northward transport by intra-arc wrench faulting
which dissipated the strike-slip component of
oblique subduction. Volcanic centers and the submarine
fan complex were dismembered as they were
built. Slide blocks of shallow water limestone
and sandstone were shed off the shelf edge and
reworked as olistostromes. Basement uplifts in
the western terrane shed ophiolite assemblage
olistostromes and triggered reworking of the chert-argillite olistostrome complex along with flysch.
MoreTranslated text
Key words
southern sierra nevada region,reconstruction,paleotectonic-paleogeographic
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