Relationships between kinematics of arc-continent collision and kinematics of thrust faults, folds, shear zones, and foliations in the Nevadan orogen, northern Sierra Nevada, California

Tectonophysics, Elsevier

223

191 (1991) 223-236

Science Publishers

B.V., Amsterdam

Relationships between kinematics of arc-continent collision and kinematics of thrust faults, folds, shear zones, and foliations in the Nevadan orogen, northern Sierra Nevada, California Steven H. Edelman Department

of Geological Sciences, University of Tennessee, Knoxville,

(Received

November

30, 1987; revised version accepted

TN 37996-1410,

August

USA

10, 1988)

ABSTRACT Edelman, S.H., 1991. Relationships between kinematics of arc-continent collision and kinematics of thrust zones, and foliations in the Nevadan orogen, northern Sierra Nevada, California. In: A. Perez-Estaun (Editors), Deformation and Plate Tectonics. Tectonophysics, 191: 223-236.

faults, folds, shear and M.P. Coward

The Mesozoic Nevadan orogeny in the northern Sierra Nevada metamorphic belt, California, may be attributed to arc-continent collision. stratigraphic data and macroscopic cross cutting relations suggest successive accretion of two arcs along an active continental margin. The younger accretion event involved the Early Jurassic Slate Creek terrane, which is a 3-5 km thick pseudostratigraphic arc fragment. The Slate Creek thrust, an isoclinally folded fault with a subhorizontal median surface, carries the Slate Creek terrane at least 40 km eastward (continentward) over the pre-existing continental margin terrane amalgam. No rock units can be correlated across the Slate Creek thrust which is thus interpreted as an arc-continent suture. In addition to the Slate Creek thrust, the Nevadan orogen includes a major east-vergent imbricate thrust set east of and beneath the Slate Creek thrust, and steep west-vergent reverse shear zones, macroscopic upright folds, and steep foliations that overprint and cut the east-vergent structures. These data suggest a mode1 for the relationships between the kinematics of arc collision and Nevadan erogenic structures. The Slate Creek arc terrane accreted by westward partial subduction of the continental margin along the east-vergent Slate Creek thrust. The continental margin was imbricated along an east-vergent thrust set. The structurally higher, inactive Slate Creek thrust-suture was deformed by steep west-vergent shear zones, folds, and foliations which may have accommodated shortening of the east-vergent thrust sheet. This deformation occurred within an active, continental margin arc that probably initiated by subduction flip after collision of the Slate Creek arc. This kinematic model is consistent with the structural geometry and chronology of the Nevadan orogen while qualitatively maintaining lithosphere-scale strain compatibility. This model has implications for problems related to emplacement of large crystalline thrust sheets, displacements beneath and at the margins of shortened crustal segments, and interaction of oppositely-verging structures. The Nevadan orogen is a slate belt, and the structural-plate tectonic mode1 presented for the Nevadan orogeny may be testable in slate belts of other orogens.

oceanic rocks. A model for Nevadan arc accretion and deformation is presented in this paper as a

Introduction An understanding of the relationships between plate tectonic processes and the structure of con-

case study of how simatic crust may accrete to a continental nucleus and deform to form crust of continental thickness and structure. The Nevadan

tinental crust has remained obscure despite advances in both fields independently. This paper proposes hypotheses for some of these relationships by developing a structural-plate tectonic model for the Mesozoic Nevadan orogeny in the northern Sierra Nevada, California (Fig. 1). The Nevadan orogen contains structurally complex, Paleozoic and Mesozoic continental margin and 0040-1951/91/$03.50

0 1991 - Elsevier Science Publishers

orogeny is defined by thrust faults, steep shear zones, upright folds, and steep foliations. Although every orogen is unique, certain regional structural associations are observed repeatedly in many erogenic belts, for example foreland fold-thrust belts. An understanding of the common structural associations is certainly a preB.V.

S.H. EDELMAN

ends of the shortened folds of opposite Pfiffner,

vergence

1980; Canadian

-how

without

and what controls

direction

Possible

and

answers

the

orogeny

plate tectonic

model.

and 1986)

interfering

with

the dominant

order

and

Milnes Price,

ver-

of overprinting?

to these questions

the Nevadan

faults and

(e.g. Alps, Cordillera,

do these operate

one another, gence

zones? (3) Thrust

are offered

are integrated

for

into

a

The Nevadan orogen in the northern Sierra Nevada Stratigraphy,

general structure,

The Nevadan

Fig. 1. Generalized geologic map of the northern part of the

batholith

major Nevadan faults. SCT=

1929;

SC = Smartville Complex (160 Ma volcanic and plutonic rocks and 200 Ma basement);

Ju = CalJovian and older Jurassic

volcanogenic rocks. Easr-oergent GM = Grizzly window;

Mountain

LOW=

faulrs: F, = Ft thrust faults;

thrust;

HCW = Higgins

Comer

Lake Oroville window; SC? = Slate Creek

thrust; TT = Taylorsville thrust. Terrunes in suture zone: CT = Calaveras

terrane;

FRT = Feather

Tuolumne River terrane;

River

TRT =

Red Ant terrane of Fig. 2 is not

separable at this scale. Cross section A-B X-Y

terrane;

and COCORP line

are shown in Fig. 2. From Day et al. (1985), Pdelman

and Sharp (1986. 1989), Pdelman et al. (1989) Hietanen (1981); Ricci et al. (1985), and other sources.

has long been recognized

as a Jurassic fold-, fault-, and cleavage-forming event in the wall rocks of the Sierra Nevada

Sierra Nevada metamorphic belt showing tectonic terranes and Slate Creek terrane (200 Ma);

orogeny

and tectonics

in California

Bateman

and

(Blackwelder, Clark,

1974).

1914; Knopf, Because

structures considered to be Nevadan shown to range in age from Middle

many

have been Jurassic to

Cretaceous (Nokleberg and Kistler, 1980; Tobisch and Fiske, 1982; Paterson et al., 1987; Edelman et al., 1989), the term “Nevadan orogeny” will be used loosely here to include all these structures (compare

Schweickert

et al., 1984a).

The Nevadan orogeny is best represented in the Paleozoic through Late Jurassic (early Rimmeridgian) oceanic and continental margin metasedimentary and western Sierra Nevada

metaigneous metamorphic

rocks of the belt. A sim-

requisite for elucidating erogenic and continental crust-forming processes. The Nevadan orogen

plified map of the northern part of the metamorphic belt is shown in Fig. 1. In the eastern part of

contains three types of structures that are reported repeatedly from the internal parts of mountain

the metamorphic

belts

but

which

large-scale

strain

ting:

pose

compatibility

(1) Large horizontal

line rock, including son, 1982; Hatcher

fundamental thrust

and

questions tectonic

of set-

sheets of crystal-

belt, continentally-derived

quartz

sandstone and other rocks of the Shoo Fly Complex are unconformably overlain by Upper Devonian

through

sedimentary

Middle

strata

Jurassic

volcanogenic

in the Northern

Sierra

and terrane

ophiolites (Iverson and Smithand Williams, 1986)-how are

(schweickert et al., 1984b; Harwood, 1988) (Fig. 1). The Slate Creek terrane, in the central and

they detached from their original crystalline substrates and transported without opening spaces behind them? (2) Extensive regions of steep slaty cleavage and upright folds (e.g. Cambrian slate belt of North Wales and Carolina slate belt of the southeastern U.S.; Hobbs et al., 1976, pp. 403405)-how is the implied horizontal shortening in these belts accommodated at depth and at the

western parts of the belt, consists of an LowerJurassic (200 Ma) pseudostratigraphic sequence interpreted as an oceanic arc (Edelman et al., 1989; Edelman and Sharp, 1989; Saleeby et al., in press; M.E. Bickford and H.W. Day, unpublished U-Pb zircon data). A structurally complex suture zone which consists of several oceanic terranes (FRT, CT, TRT in

KINEMATICS

OF ARC

COLLISION

AND

NEVADAN

OROGENIC

STRUCTURES

225

E3 5 k”

”

I

HORIZON1 AL SCALE=VtRTICAL SLATE

CREEK

TEARANE

(c)

Fig. 2.(a) Cross section

A-B

(see Fig. 1 for location)

fault;

fault;

DP = Dogwood

D = Dowmeville

in Fig. 1); other abbreviations Symbols

as in (a). (c) Simplified locations

across

Peak fault;

as in Fig. 1. Querries

200 Ma Slate Creek rocks and autochthonous terrane.

SCALE

_,.

kl)

part of the northern

GC = Goodyears

in Smartville

line drawing

of COCORP

Sierra Nevada

Creek fault;

Complex

160 Ma rocks. (b) Schematic

I

(SC)

reflect uncertainty

structural

succession

seismic profile (X-Y

of some faults are shown (K-GM

metamorphic

belt. BB = Big Bend

RAT = Red Ant terrane

of allochthonous

before emplacement

in Fig. 1); from Nelson

are projected

(not distinguished

of the extents

of Slate Creek

et al. (1986). Surface

along strike).

Fig. 1) lies between the continental Northern Sierra terrane and the oceanic Slate Creek terrane. The

Plate models posed collisions

Feather

1972; Schweickert and Cowan, 1975; Edelman, 1985, 1987; Ingersoll and Schweickert, 1986) or imbrication of arcs formed in situ (Burchfiel and

River terrane

Red Ant terrane

is a Paleozoic

ophiolite,

the

(not shown in Fig. 1; see RAT in

Fig. 2) is an early Calaveras terrane is

Mesozoic blueschist, the a late Paleozoic-Triassic

chert-argillite melange, and the Tuolumne River terrane is a Paleozoic ophiolitic melange overlain by early Mesozoic arc volcanic rocks, argillite, and chert. The well known Smartville Complex (Xenophontos and Bond, 1978; Day et al., 1985; Beard and Day, 1987) in the western part of the terrane is a 160 Ma volcanic-plutonic 200 Ma Slate Creek terrane

complex basement

built into (Fig. 1).

Plutonic rocks with ages of about 160 Ma intrude the other Sierran terranes (Snoke et al., 1982) and coeval

volcaniclastic

rocks

occur

east

of

the

Taylorsville thrust (Ju in Fig. 1). These ca. 160 Ma intrusive rocks and coeval (Callovian-Kimmeridgian) volcanogenic rocks postdate amalgamation of the terranes they intrude and overlie, and thus represent the initial stages of a Middle Jurassic-Cretaceous continental margin arc which culminated in emplacement of the Sierra Nevada batholith (Hamilton, 1969).

Davis,

1981;

for the Nevadan orogeny proof oceanic arcs (Moores, 1970,

Saleeby,

1981, 1983;

Sharp,

1985).

The collisional models proposed a Late Jurassic collision between the 160 Ma Smartville Complex (including its basement to the west. However, disproved

rocks) and the rock units this model appears to be

by new evidence

Creek Complex the Smartville

accreted Complex

that the 200 Ma Slate

before is younger

165 Ma, and that than this accre-

tion and is built into Slate Creek basement. the Smartville situ.

Complex

arc apparently

Thus,

formed

in

The Slate Creek terrane may represent an oceanic arc that collided before 165 Ma. Edelman (1987) proposed two successive arc collisions, with the ensimatic Tuolumne River arc terrane (Fig. 1) accreted during the earlier collision. This chronology is indicated by the following: (1) The Slate Creek Complex tectonically over lies both the Tuolumne River and Calaveras terranes 2).

along the “Slate

Creek thrust”

(Figs.

1 and

S.H. EDELMAN

226

(2) The Slate Creek thrust is cut by a 165 Ma pluton (SP in Fig. 3). The Nevadan orogeny is defined by two oppositely-verging younger

sets

of

structures

(Fig.

and more conspicuous

2).

The

set consists

of

steep west-vergent (east-dipping) reverse faults and related folds and cleavages. The other set consists of

shallow

east-vergent

(west-dipping)

thrust

faults, including the Slate Creek thrust, and rare overturned

folds. The east-vergent

systematically

overprinted

by

structures are

the west-vergent

structures (Speed and Moores, 1980; Moores and Day, 1984; Day et al., 1985; Edelman et al., 1989). The summary of Nevadan structure presented below is chiefly from Day et al. (1985) and Edelman et al. (1989) unless otherwise cited. Steep west-vergent

structures

Faults The throughgoing steep faults of the Foothills fault system (Clark, 1960) are the most obvious of the Nevadan structures (“later steep faults” in Fig. 1). In cross section A-B (Fig. 2a) these faults are represented by the Downieville, Goodyears Creek, Dogwood Peak, and Big Bend faults. The steep faults are defined by curvilinear zones of intense foliation within which contrasting rock units are juxtaposed

(Fig. 3). Consistently

steep

stretching and streaking lineations in the foliations indicate that they are dip-slip shear zones (stereograms in Fig. 3). Separations of stratigraphic and structural planar features indicate dip-slip displacements in the range of l-10 km with east sides up. The faults are thus west-vergent reverse faults. Their curvilinear traces indicate that they are folded, with map-scale bends of up to 80 o and more in the northwestern Sierra Nevada (Fig. 1). The steep faults are superposed on an earlier west-vergent thrust system which is largely cryptic and predates the east-vergent structures. This earlier thrust system stacks, in descending structural order, the Northern Sierra terrane, Feather River terrane, Red Ant terrane, Calaveras terrane, and Tuolumne River terrane (Fig. 2b). The open barbs on the later steep faults in Fig. 2a mark segments of these later faults that occupy the structural positions of earlier faults (compare Fig.

Fig. 3. Foliation trend map and cross section for area outlined in Fig. 1. Stereograms show stretching and mineral streaking hneations; contoured diagrams show number of measurements, with contours at 0 (dashed), 5, 10, 15, 20 (black) times a random distribution. Diagrams are located in, or have arrows that point to, the general areas represented. Note that strongest foliations occur along mapped later steep faults, indicating that these faults are at least in part shear zones. Steep lineations suggest shear zones are dip-slip. SC?

= Slate Creek thrust

(barbed on map); SP = 165 Ma Scales pluton. From Fxlelman et al. (1989).

2a and b). The locations and orientations of the steep faults are strongly controlled by the earlier faults which were apparently reactivated. Foldr Macroscopic folds are defined mainly by folding of the Slate Creek thrust (Fig. 2a and see below). These folds are tight to isoclinal with steeply east-dipping hinge surfaces and shallow, doubly plunging hinge lines (Fig. 3). Folding out-

KINEMATICS

OF ARC

COLLISION

AND

NEVADAN

OROGENIC

227

STRUCTURES

lasted faulting as indicated by the warped traces

0.5 and 0.1. The intermediate

of the later faults (Fig. 1). In addition, the folding

is unknown

probably

steepened

the later faults by an un-

Mesoscopic

folds are uncommon and difficult

to relate to a specific larger-scale

fold set. The

hinge lines of some mesoscopic folds are subvertical and parallel to stretching lineations

problems

insignificant.

to be addressed

below are

is accommodated

and how the implied displacements

The

at depth

at the margins

of the shortened zone are accommodated

without

creating space problems.

in steep

shear zones.

East-oergent

low-angle structures

Northeastern

Foliations Foliations

large-scale

how this deformation

known amount.

principal strain (Y)

but is probably

are most strongly developed along

Sierra Nevada

East-vergent structures in the northeasternmost

the later steep faults (Fig. 3). Between the faults,

Sierra Nevada have been known for some time

the foliation is weaker and/or occurs in narrow zones separated by zones hundreds of meters wide

(McMath,

with weak foliation.

Grizzly Mountain thrusts (Figs. 1 and 2a) as well

The

foliation

is generally

1966; D’Allura et al., 1977; Speed and

Moores, 1980). They include the Taylorsville as east-vergent

folds (Speed and Moores,

and

1980).

steeply east-dipping. The strong foliations along the faults are interpreted as shear zone foliations;

These

steeply plunging stretching

linea-

folds (Speed and Moores, 1980; Day et al., 1985).

zones.

The Taylorsville and Grizzly Mountain thrusts cut and imbricate the Northern Sierra terrane strati-

tions

indicate

they

are

and streaking dip-slip

shear

Throughout the Sierra Nevada metamorphic belt, steep lineations (Nokleberg and Kistler, 1980, their

structures

are deformed

by west-vergent

graphic succession, and the Taylorsville thrust cuts

Fig. 5) and steep strain X-axes (Tobisch et al., 1977; Paterson et al., 1987) suggest regional verti-

rocks as young as Callovian.

cal extension and dip-slip shear. In the Northern Sierra terrane (Fig. l), kinematic analysis of folds

Slate Creek thrust The Slate Creek thrust is defined in the area of section A-B (Fig. 2a) by the contact between the

suggests steep Nevadan extension axes (Varga, 1985). Foliations between faults mark minor shear zones and/or axial plane foliations associated with folds. Macroscopic strain accommodated vergent structures

by steep west-

cause the faults are folded in map view, they probably initiated at shallower dips and rotated toward steeper dips during folding. The orientations and locations of the faults were controlled in large part by the inherited, west-vergent, terranebounding faults (Fig. 2b). The macroscopic strain accommodated by the steep Nevadan structures is east-west horizontal shortening (perpendicular to foliations and fold hinge surfaces) and vertical extension (crustal thickening). The amount of macroscopic strain is unknown, but judging from the cross section (Fig. the across strike stretch is probably

“Slate

Creek thrust” is extended

to include

all

such contacts in the northern Sierra Nevada (F, LOW, HCW, SCt in Fig. 1). The Slate Creek thrust is overprinted by the later steep structures

The steep Nevadan structures taken together define a steep west-vergent fold-thrust system. Be-

2)

Slate Creek terrane and the earlier thrust pile of Fig. 2b. For the purposes of this paper, the term

between

discussed above; the thrust is tightly folded and is cut by steep west-vergent faults (Fig. 2a). The Slate Creek thrust is an east-vergent thrust fault with a displacement of more than 40 km. The thrust is cut by the 165 Ma Scales pluton (SP in Fig. 3; U-Pb zircon ages, Saleeby et al., 1988; M.E. Bickford and H.W. Day, pers. commun., 1988) and is of course older than the steep westvergent folds and faults that deform and cut it. The Slate Creek thrust carries the Slate Creek terrane pseudostratigraphic sequence of ultramafic-intermediate igneous rocks in its hangingwall. The preserved thickness of this sequence is 3-5 km. Volcanic, volcaniclastic, plutonic, and cumulate ultramafic rocks comprise the sequence,

S.H. EDELMAN

228

and ultramafic

rocks of mantle

or lherzolite) Creek

are rare.

terrane

is an areally

sheet of oceanic original

mantle

supracrustal presence rocks

rocks (TRT, of

along

the Slate ancient

extensive,

and thrust

Creek

terrane

petrologic

blages

(Slate

thin

over

suggests

Moho.

Because

that the

the Slate Creek

exclusive

terrane

along

Kinematics of east-vergent Nevadan structures The east-vergent structures constitute a set of structures

orientation,

distinctly

and relative

different

formed exposed

sheet,

in style,

age from the later steep

Sierra Nevada, by

and folds that accommodated

which

Mountain-Taylorsville

in turn

by west-vergent

may

have

been

de-

folds that accommodated

of a still lower thrust

sheet that is not

in this region.

This proposed two systems tightly

against

the other Sierran terranes), the Slate Creek thrust can be interpreted as a suture (Edelman et al., 1983; Edelman, 1985).

Nevadan

thrust

faults

of the Grizzly

diachronous

of Nevadan

by the fact that

rock assem-

is juxtaposed

shortening

shortening

ultramafic

was detached

mutually

Creek

thrust

orogen in the northern

the Slate Creek thrust may have been deformed west-vergent

thrust from its

eastward

cumulate

the Slate Creek

juxtaposes

the Slate

CT, RAT in Fig. 2). The

serpentinized

thrust

terms,

arc rocks that is detached substrate

the Nevadan

origin (harsburgite

In broad

folded

Taylorsville

structures

the Slate

than thrusts

development

the

Creek

Grizzly

(Fig.

2b);

of the

is supported thrust

is more

Mountain deformation

and of

thrust hangingwalls predicts that structurally higher thrust sheets would be deformed more than lower ones because higher sheets must accumulate all the deformation of the lower thrusts. In addition, the age constraints on the steep faults and the Taylorsville

and Grizzly

mit them to be coeval.

Mountain

Thus,

thrusts

per-

the steep west-ver-

west-vergent structures. The east-vergent structures are consistently cut and folded by the steep

gent faults, folds, and foliations in the Nevadan orogen may reflect shortening of the hangingwall

faults and folds.

of the Taylorsville,

The

east-vergent

thrust

system

constitutes

a

structural succession of, from bottom to top: (1) autochthonous Northern Sierra rocks below (east of) the Taylorsville thrust; (2) allochthonous rocks between the Taylorsville and Slate Creek thrusts; (3) the allochthonous Slate Creek terrane above the Slate Creek thrust. The Slate Creek thrust is pre-165 Ma and the Taylorsville thrust cuts rocks

bly lower east-vergent This interpretation

Grizzly

Mountain,

and possi-

thrusts. cannot

be proven

with exist-

ing data. The critical test of this idea, that is, whether the steep structures that deform the Slate Creek thrust truncate against the Grizzly Mountain and Taylorsville thrusts, lies deep in the subsurface. Deep seismic reflection by COCORP

as young as Callovian (Imlay, time scale of Palmer, 1983),

1961; 1699163 Ma, so the Slate Creek

shows oppositely dipping reflectors at depth that disappear just above their projected intersections (Nelson et al., 1986; see Fig. 1 for location of

thrust faults.

of the east-vergent

seismic

is probably

Relationship structures

the oldest

line

X-Y

and

Fig.

2c

for

simplified

migrated section). Further, the reflectors that could correspond to Nevadan faults have “true” dips of between

the east-

and

west-vergent

32-54” (Nelson et al., 1986) much shallower than the 80-90” dips of these faults observed at the surface.

Because the east-vergent structures are cut and folded by the west-vergent structures, one could propose two sequential deformational events of opposite vergences. However, recent work in thrust belts has shown that much of the deformation in thrust sheets occurs as they override lower thrusts (Suppe, 1983; Hatcher, this volume), and Coward (1983) suggested that earlier faults may steepen due to folding above lower decoupling zones. In

The

surface

dips

are based

on dips

of

shear zone foliations and the straight map traces of the faults across deep canyons. Therefore, it is possible that at least some of the COCORP reflections are not Nevadan faults, as appreciated by Nelson et al. (1986). The seismic data do not address the problem at hand. Figure 4 shows a model for the kinematic interaction of the oppositely verging structures. The first order boundary condition is the initiation of

KINEMATICS

OF ARC

COLLISION

AND

NEVADAN

0R0i35NtC

______Z&&_f______lmbricate

Map of tmbricate

229

STRUCTURfiS

an imbricate

thrust

tain) beneath

a collisional

ally

thrust

higher

negative

thrust

(Taylorsville-Grizzly (Slate

displacement

bricate

thrust

suture Creek gradient

in its transport

Moun-

or other structurthrust),

with

along

direction

the

a im-

(Fig. 4a).

Material below the thrust sheet is considered to be rigid during its deformation; if it were not rigid, the thrust would probably An increment the imbricate

thrust imposes

upon

the thrust

strain

field is characterized

to the transport

-

1

I

diagram

shortening.

and

structures.

illustrating

the thrust

(a) Initiation

in which subduction

adjacent

material thrust.

an inactive

points

After an increment (steep

thrust, tions

foliations formed

zones)

strain

between

ellipses (XZ

sections,

to material

arrows

imbricate

outside

thrust

deformation

and associated

deformation

axes relative

direction

(“spin”

mulate

in increased

resulting

no longer

(d) Backthmsts efficiently

ther shortening tightening, favorably ents during

oriented

accommodate and folding

formed in coaxial

fabrics.

in faults,

fold

structural

style to Fig. 3. Foliation

and

sections

finite

Small rotation

of principal

strain

rotation”);

with respect

rotation”).

to accu-

and steepening

as ideal “end

of pure shear are

increment of deformation. mechanisms are similar to

steepening

of all planes,

by a “spin” 4b)

imbricate

or “external of all structures with

which

is mani-

rigid rotation” respect to the

thrust. Thrust sheet deformation

two mechanisms is continued mation increment in Fig. 4c. The backthrusts eventually they can no longer efficiently tening of the synthetic thrust ternal rotation of inter-shear

by these

for another steepen

defor-

such

that

accommodate shorsheet (Fig. 4d). Exzone material de-

creases commensurate

with decreasing of intra-shear

internal

ro-

shortening.

Furfold

and the macroscopic deformation approaches coaxial irrotational strain. All material planes and lines steepen as they rotate toward the XY-plane and the X-axis, respectively, of the macroscopic strain ellipsoid. In particular, folds tighten, faults steepen, and foliations and lineations acquire

Along-strike produce

for clarity.

are envisioned

and produces

of faults, and crenulation

of

strain

gradi-

map-view

warps

Compare

traces

of

that they can

accommodated The two defor-

tation (noncoaxiality)

horizontal

foliations.

to

(c) Differen-

continue

to the point

this phase of deformation hinges,

(Fig.

or “internal

coaxial

requires fested

finite strains

steepen

inactive

thrust

the “backthrusting” and “coupling” modes of thrust sheet shortening defined by Dunne and Ferrill (1988). The shortening of the thrust sheet

shear

Schematic

mechanisms

imposed during each These two deformation

linea-

sheet shortening

is macroscopically

steepening

of the imbricate

This thrust sheet deformainternally by two deforma-

deformations in which some combination shear (coaxial plane strain) and simple

and streaking

rotation

or “external

and thrust

parallel

members,” with progressive, superimposed coaxial and noncoaxial deformations, or with composite

in backthrust

rotation

indicate

that back-

at the imbricate

foliations

(noncoaxiahty

sheet. (b)

structures

Y = 1) are shown.

circles

the im-

including

the backthrusts.

indicates

tial displacement structures.

terminate

a

mark

at this stage is thrust

initiate,

with down dip stretching

arrow near strain ellipsoid larger

that

in noncoaxial

zones, and backfolds spinning

thrust

displacement,

sheet shortening

shear

during

along

in the imbricate

of differential

thrust

perhaps

that will be displaced

residing

produces

left. Black triangles

The suture or other higher

structure

accommodate thrusts

deformation

thrust,

dicp%

of dif-

thrust by thrust sheet

sheet

of imbricate

collision b&ate

accusation

along an imbricate

how

of strain

by shortening

material between the shear zones by folding and cleavage formation.

.4

mation

displacement

an increment

along

tion mechanisms: (1) backthrusts that are shear zones with noncoaxial internal deformation, and (2) macroscopically coaxial deformation of

(d)

ferential

and lock.

displacement

sheet (Fig. 4b). The macroscopic

and vertical extension. tion is accommodated

Fig. 4. Conceptual

deform

of differential

omitted

this

model

from cross

zone material,

S.H. EDELMAN

230

steeper

dips

gradients

surfaces,

in fold

tions pare

foliations, lines

the warped

Fig. 3. The model

thinning,

variable

4d) (Ramsay,

shear zones, folds, and foliathe steep west-vergent

structures

faults

local vertical

in faults,

and

(Fig.

as described

the map view structural

the later

strain

Wood, 1974, pp. 383). This model

that characterize

of Nevadan

Along-strike

map view warps and

hinge

1967, fig. 7-105; produces

plunges.

may produce

fold hinge plunges

and

uplifts,

would

without

in their present

Com-

style in Fig. 4d to

also explains

which

above.

set

the steep dips of accommodate

crustal

for the northern drawn

to

thicknesses, mated Nevada

and plate

do “plate

tectonic

displacements dimensional

the

cross sections inferred collisions

tinct

arc

vergent

Fig.

are

and

Slate

River

arc collided

(Fig.

em

cross

Sierra

SCT

terrane

Tuolumne

a west-

the interpreted

of the northern

Sierra

Note that the sections

Jurassic

(NST), Feather

the The

west-vergent

showing

evolution

convergent River

of Red Ant terrane

River

terrane

(c) Arc-continent

set

continental in the North-

terrane

(RAT).

ophiolite

and melange

Mountain Slate

collision

of

’

170-150

+ active arc magnatism

Ma----0

(e)

B

subduction

omitted by

steepening

150~(?)120 MS“------.__ + active arc magnetism

above

of

beneath

and of

(d) Imbri-

in front

the

sheet is accommodated

previous

arc magmatism

for clarity.

above A-B

crustal

of the

Taylorsvilleby west-ver-

above an east-dishortening,

structures

Mountain

hypothetical

tive arc magmatism omitted

con-

along the Taylorsville-Grizzly

(e) Continued

and Grizzly

a deeper,

Age

zone west of and below the cross section

for clarity.

Taylorsville -100 I

thrust

(SCr).

in (b), but cross-cut-

Shortening

gent faults and folds. Active pping

A

surface

thrust

that the Slate Creek arc accre-

margin

thrust-suture.

Mountain

thrust. along the

within those constraints.

set (TT-GM)

thrust

Creek

Grizzly

Inactive fault

Creek

(Fig. 2) indicate

cation of the continental

Active fault

an oceanward-directed

of the Slate Creek terrane

Slate

tion is the younger d

along

collision

are the same as for the collision

ting relations

of ktbsphere

first along

of active arc and its basement

terrane

Calaveras

straints

Base

by

terranes.

the earlier

and Cretaceous.

continentward-directed

Mdla

in

(CT).Active oceanic arcs represented by the Tuolumne River terrane (TRT) and the pseudostratigraphic Slate Creek terrane (SCT).(b) Arc-continent collision of the

_.

(b)

--1 Petrdogic

Creek

sections

to scale. (a) Early

composed

(MT), blueschist

OT

is no

as a funda-

represented

5b);

and structural

in the Jurassic

margin

Pab-topogaphic

there

displacements

is interpreted

segments

tectonic

are drawn

-bathymetric

history

et al., 1985).

because

strike-slip

River suture

Nevada

a.-..,-

structural

is not critical

5. Paleotectonic

plate

H=V

two-

mental consequence of Jurassic collisional accretion of a fringing arc system (Fig. 5a). Two dis-

Tuolumne W

strike-slip

this simple

1984; Harper

Nevadan structures. The Nevadan orogeny

they than

they may have been

or

Tuolumne

0

although

major

but

Possible

and Moores, for

estiSierra

processes

from

(Edelman

In Fig. 5 are shown paleotectonic arc-continent

balanced,

cartoons.”

model

and

are not tightly

structural

are omitted

This omission of

into

in the post-collisional

nevadan orogen

of the Jurassic

insight

important

thickening

evolution

sections

only

orientations.

tectonic

These

are

crustal

of northern

or quantitatively

more

oceanic

thicknesses,

dimensions

rock units.

The sections

average

lithospheric

constrained yield

Nevada.

using

unstrained

evidence Structural

Sierra

scale,

and

thrusts,

detachment corresponds

folding

of

the

may have occurred (querried

an east dipping

is

reflected

fault). Ac-

subduction

zone is

to section in Fig. 2a.

KINEMATICS

thrust

OF AKC

COLLISION

AND

set (Fig. 2b) formed

(Fig. 5b). This collision Creek

arc collision

was detached

favored

Sierran

substrate

along

as a tectonic

terranes.

and the mechanical Slate

by the Slate

(Fig. 5~). The Slate Creek

This process

weakness

Creek

stratigraphy

thrust (Fig.

in the

Slate

northern

thrust

Creek

thrust-suture

be

margin

Creek

of the pseudo-

the geometry

and is testable.

vergent

of the Moho (White

2a) requires

data

arc

might

1985). In any case, the position

large body of existing

the

flake over

of the continental

231

STRUCTURES

at this time or earlier

from its mantle

by the buoyancy

and Bretan,

OROGENIC

was followed

Moho and thrust eastward the other

NEVADAN

de-

Sierra Nevada

Mountain

thrust

subduction

and

(Roeder,

of the continental

margin

(Fig. and

senses of shear. The later steep structures

have the

opposite

are thus

dips

and

senses

of shear

Reactivation

sumably formed near the subarc Moho, are the deepest level of the arc preserved in the Slate Creek thrust sheet. The collisional subduction dipped

thrust-suture, evidence

west, synthetic

but because

for subduction

from the present

thrust

there is no independent dip direction

it is omitted

model.

In Fig. 5d is shown tinental margin beneath Creek

to the Slate Creek

along

imbrication of the conand in front of the Slate the

Taylorsville-Grizzly

Mountain thrust set. The later steep structures that deform the Slate Creek thrust are depicted as initiating at this time as a west-vergent set of

of

earlier

supand

orientations

pre-

are controlled

scribed kinematic

by an externally

field, presumably

the Slate Creek

arc collision depicted in Fig. 5c. The crustal thickening depicted in Fig. 5e is supported by the present 50 km depth to the Moho in the northern Sierra Nevada (Speed and Moores, 1980). Considering that at least 10 km of overburden Nevadan

has

been

deformation,

eroded

during

as indicated

thrusts

rived from the Sierra Nevada

by the

the crust

Smartville Complex, Callovian volcaniclastic rocks in the northeasternmost Sierra Nevada (JV in Fig.

l),

and

occurred

widespread

during

coeval

this deformation.

intrusive This

rocks, magma-

was probably

Nevadan time. Collision mechanism for producing Three

general

mentioned

Creek arc. Continued shortening reflected by folding and steepening of the west-vergent faults and folds, and by folding of the Taylorsville and Grizzly Mountain thrusts, is depicted as a kine-

tures. The implications

matic continuation of the same deformation above a deeper east-vergent thrust shown hypothetically in Fig. 5e (querried fault). Arc magmatism continued through the Cretaceous to form the Sierra Nevada batholith (Hamilton, 1969). Discussion

The above,

structural-plate tectonic model though speculative, is consistent

outlined with a

of

Valley,

60 km thick

in

is the best-documented such crustal thicknesses.

problems

line and

after

and by huge sediment de-

in the Great

at least

of

in the Introduction:

tism probably reflects eastward, subcontinental subduction initiated after collision of the Slate

and

by exposure

activating

represented

structures

thrusts do not reactivate earlier structures ports the proposition that their locations

plutons and metamorphic rocks thicknesses of Late Jurassic-Recent

many of the earlier west-vergent

and

vergence of the steep that the east-vergent

backthrusts and backfolds. Note that the faults above TT-GM in Fig. 5d are depicted as refrom Fig. 5b. Arc magmatism

to the colli-

SC), in that they have the same dip directions

to the westward The observation

probably

an east-

1973) or at least to the

contributed backthrusts.

ultramafic

in the

Taylorsville-Grizzly

set are “synthetic”

“antithetic.”

in Fig. 5c. Cumulate

orogen

is fundamentally

rocks, pre-

picted

and stratigraphic

system (Day et al., 1985). The Slate

sional subduction partial

structural

The Nevadan

orogenesis

were

(1) large crystal-

thrust sheets, (2) steep foliations and folds, (3) interaction of oppositely-verging strucof the model

(Figs. 4 and

5) for these problems are pointed out here. (1) The Slate Creek thrust displays a minimum displacement of 40 km. The thrust sheet, i.e. the Slate Creek terrane, is about 3-5 km thick. Because the igneous arc rocks of the Slate Creek terrane rest directly on the thrust, the thrust cannot be a subduction zone fault. The original subarc mantle is missing along the Slate Creek thrust. The present model explains this omission by having the suture cut horizontally westward along the subarc Moho and partially subduct the subarc mantle

(Fig.

5~). The

problem

of removing

the

S.H. EDELMAN

232

original

substrate

solved

of the crystalline

in this way without

lems.

This

crust”

model

explains

solution

the features

(Oxburgh, fig. lib)

attributed

geometry

sheet is

space

prob-

(1982) and

tening

strains

may be accommodated

solved in the model

by allowing

shor-

compatibility.

The model adequately

are

not

displacement

the Taylorsville-Grizzly

strain

termination

of

east-vergent

nents are predictions

to

by future mapping,

gradient, the

thrusts.

to and

west-vergent These

of the hypothesis strain

thrust

compared

by the displacement

against

gradient

Mountain

of regional

down-dip

faults

of the

maintaining

that as the

is

at depth

the shortening

such

features

qualitatively

strain

set, the amount the

of how large horizontal

while

components

that predicted

of continen-

tal crust. (2) The problem

and stratigraphic

orogen

contains along

the lithosphere.

is similar to Butler’s (1986,

model for Moho detachment

structural

documented

to “flake tectonics”

splitting

major

Nevadan

lithosphere-scale

to the “vanishing

and Smithson

1972) without

The suggested

creating

is similar

of Iverson

thrust

compo-

to be tested

measurement,

radiomet-

occur above a subhorizontal fault (imbricate thrust in Fig. 4; TT-GM in Fig. 5d). Shortening of

tic dating, seismic reflection and other geophysical profiling, and scientific drilling.

thrust sheets during thrust propagation tion may be a common phenomenon

classical

and mo(Williams

and Chapman, 1983). The displacements at the margins of the shortened zone are accommodated by differential slip along the fault. (3) The problem of the interaction

between

oppositely verging structures is addressed model by restricting the steep west-vergent tures, at any given instant the material gent thrust. nisms

by

during

in the struc-

deformation,

to

above the currently active east-verIn the Nevadan orogen, the mechawhich

hangingwall

deformation

is

accommodated is strongly influenced by pre-existing structure in the deforming rocks, whereas the east-vergent structures cut across the earlier structure and by inference are fundamentally controlled by the vergence of collision. Shortening of a thrust sheet by faults with vergence opposite to that of the main thrust fault is a form of “tectonic wedging” (Price, 1986). The Taylorsville and Grizzly Mountain thrusts are folded, as strongly as the Slate Creek thrust

though not Folding of

the Taylorsville

and Grizzly

thrusts

and

the later

faults,

the last increments

of

folding curred

steep

and

Mountain

of the Slate Creek thrust, during eastward transport

may have ocof the entire

east-vergent thrust stack along a younger, more easterly, and structurally lower thrust east of the Sierra Nevada (Fig. 5e). The significance of the model presented in Figs. 4 and 5 lies in the fact that it relates the kinematics and orientations of structures observed on scale orders from plate tectonic sutures to cleavages and lineations. The model accounts for the

The second thetic

and higher

foreland

thrust

imbricate

fans

order

structures

systems and

in the

are mainly

duplexes

(Boyer

synand

Elliott, 1982) whereas the second and higher order Nevadan structures are antithetic shear zones and associated

structures.

The classical

single-vergence

thrust systems are restricted to well-stratified land rocks. ‘The internal parts of mountain commonly

display

fold nappes. belts

more complex

In particular,

shear zones and

well known

display

late

antithetic

(“ backthrusts”

and

“backfolds”)

forebelts

faults

mountain and

folds

(e.g. the Alps;

Milnes and Pfiffner, 1980). The geometric and chronologic analogy of the Nevadan steep structures to backthrust-backfold structures in collisional orogens was pointed out by Moores and Day (1984). The present paper offers a mechanism for producing these structures. In addition, recent studies have pointed out the importance of second order

antithetic

(Coward

faults

even

in foreland

and Butler, 1985, their fig. 2; Price, 1986).

Coward (1983, p. 121) made the general tion that “sometimes the early thrust steepened direction

settings observamay be

and folded by thrusts with a movement opposite to that of the main thrust

movement.” In the northern Sierra Nevada, the locations and orientations of at least some of the backthrusts are controlled by pre-existing terranebounding faults. The Nevadan orogen is a slate belt similar to slate belts in other mountian chains (Edelman, 1985). Hobbs et al. (1976) gave a detailed account of slate belt structures worldwide and identified the following common attributes of these belts: (1)

KINEMATICS

OF ARC

COLLISION

The most obvious pping tains

fabric

slaty cleavage. a down-dip

scopic

folds

tight

The

cleavage

plane,

and

element

OROGENlt

lineation.

di-

lines

and

are parallel

is parallel

con-

(3) Macrowith

shallow

steep

hinge

to cleavage.

to the flattening

the stretching

lineation

(5)

(XY)

is parallel

to

the long (X) axis, of the finite strain ellipsoid. steep

Nevadan

those

in other

may reflect

structures slate

similar

The dominance

belts.

closely

and

slate

underlying

tectonic

of low grade

that

arc

collisions

processes.

volcanic,

may

to

belts

plutonic,

and ultramafic rocks in many slate belts Carolina slate belt in the U.S. Appalachians) gests

The

correspond many

be an

(e.g. sug-

important

process for producing slate belts. The net effect of the model for Nevadan deformation is to repartition crustal displacements, in the up-dip direction of the east-vergent imbricate thrusts, from the east-vergent structures to the west-vergent structures. It is thus possible that the main east-vergent imbricate thrust (Taylorsville.Grizzly

Mountain

thrusts

set) was “blind”,

its displacement

decreased

ing the Jurassic

topographic

Fe&l,

that is,

to zero before intersectsurface

1988). The total east-vergent

(Dunne

and

displacement

would then be equal to the amount of west-vergent shortening of the hangingwall (Williams and Chapman, 1983). The model presented here suggests that deformation related to arc-continent collision continued after subduction reversal. Arc magmatism related to the reversed, sub~ontinental occurred within the actively defor~ng

subduction collisional

orogen (Fig. 5d,e). The deformation, occurred in a subduction hangingwall

which thus as a “cordil-

leran-type” orogen (Dewey and Bird, 1970), is related to an earlier collision rather than to the active plate-kinematic

233

STRUCTURES

is a steeply

to isoclinal

hinge

(4) Faults

NEVADAN

(2) The slaty cleavage

stretching

are

doubly-plunging surfaces.

AND

regime.

The model scale

is constrained

structure,

and plate kinematic as a working or rejection

by rock

paleogeographic theory.

hypothesis, as further

data

kinematic

simatic

has

material

large

is offered

to modification

are collected,

for the

processes

by

and

deformed

accreted

form crust of continental in the Nevadan

The model

subject

lithosphere-scale

fabrics,

interpretations,

structure

which to

and thickness

orogen.

Conclusions The Mesozoic

Nevadan

ern Sierra Nevada

orogeny

in the north-

may be fundamentally

two arc-continent

collisions.

The

linked

younger

to

colli-

sion involved partial previously assembled

westward subduction of the terrane amalgam which con-

stituted the continental duced an east-vergent

margin. This collision prosuture (Slate Creek thrust)

and east-vergent imbricate faults (TaylorsvilleGrizzly Mountain thrust set) within the continental margin beneath the suture. The material in the Taylorsville-Grizzly Mountain thrust sheet shortened by motion by tightening cluding

folding

mountain deeper ments

on west-vergent

shortening,

of the Taylorsville

thrusts,

may

east-vergent along

shear zones and

of folds. Continued have

Grizzly

occurred

above

thrust.

Displace-

imbricate

the east-vergent

and

in-

imbricate

faults

a are

predicted to decrease in their transport directions. The model addresses several general problems of orogenesis. (1) The problem sheet of crystalline Creek arc” terrane,

of emplacing a large thrust rock, in this case the Slate is solved by underthrusting,

or

subducting, the subarc mantle. The suture (Slate Creek thrust) cut the subarc Moho and rooted somewhere behind the arc, thus subducting the subarc mantle wedge to make room for the colliding and underthrusting presently

underlies

continental

margin

which

the Slate Creek arc flake.

The analysis presented in this paper is not a report of strain measurement in the Nevadan

(2) The problem Nevadan horizontal

orogen7 a~thou~ constraints on some components of the macroscopic strain field were presented. Rather, it is a model for part of the displacement field history (Sanderson, 1982). It represents a practical method of understanding structural evolution at the scale attempted here.

the steep fold hinge planes, steep faults, and steep foliations, is solved by restricting the shortening to the hangingwalls of thrusts beneath the Slate Creek thrust. The displacements at the margins of the shortened zone are accommodated by differential displacement along the thrust faults.

of accommodating at depth shortening1 as reflected by

S.H. EDELMAN

234

(3) The related

problem

positely-verging way,

that

truncate

structures

is, the

against

gent thrust

arc-continent subduction

reversal

large-scale

structural

Nevada

solution

was

followed

evolution here

by

arc. The

of the

northern

qualitatively

Clark,

In: of

1986. Thrust

subduction

The

Nevada,

1960.

ble in slate belts in other orogens.

M.P.,

1983. Thrust

Geol., 5: 113-123.

Lawrence,

University

bia, University tional Science to Robert EM.

Carolina,

Colum-

of Tennessee, Knoxville, and NaFoundation grant EAR 84-17894

D. Hatcher,

Moores,

referees

of South

of Kansas,

R.J.

Jr. Reviews Twiss,

and

by H.W. Day,

M.P. and Butler,

D’Allura,

(Editors),

tural

setting

of the Sierra

Nevada

and struc-

batholith,

California.

global

Sierra Nevada,

California:

complex,

the core of a rifted volcanic

arc.

Geol. Sot. Am. Bull., 99: ‘779-791. Blackwelder,

E., 1914. A summary

the geologic

history

of North

Dunne,

epochs

J. Geol.,

in

22: 633-

654. Boyer.

D., 1982. Thrust

systems.

Am. Assoc.

Pet. Geol. Bull., 66: 1196-1230. Burchfiel, tectonic

B.C. and evolution

Davis, of

G.A., the

W.M.

Edelman, nent

1981. Triassic

and Jurassic

Mountains-Sierra

13:

L., 1977. Paleo-

Nevada:

their structural

In: J.H.

Stewart

of the Western

Mineral.

EM.

and Tuminas,

of the northern

and

Pac.

et al. United

Coast

Paleo-

A.C., 1985. Structure

Sierra

Nevada.

J. Geophys.

Geol.

Sot.

Ferrill,

belts and the new

Res., 75: 2625-2647.

D.A.,

1988. Blind

S.H., 198.5. Slate belt structures collision

Nevada, Edelman,

and

vergence

California.

thrust

in the western

California:

systems.

plate kinematic

sion orogen Progr.,

Nevada

Sierra

Progr.,

17: 353.

Progr..

arc-continent

metamorphic

interpretation

S.H. and Moores,

to arc-conti-

northern

for two Mesozoic

Sierra

ysis. Geol. See. Am., Abstr. Edelman,

related

reversal,

Geoi. Sot. Am., Abstr.

S.H., 1987. Evidence

collisions

belt,

of a terrane

anal-

19: 651-652.

E.M., 1984. Late Mesozoic

in the western

USA. Geol.

colli-

Sot. Am.,

Abstr.

16: 498. S.H. and Sharp, boundaries

W.D., 1986. Geometries

in the western

Geol. See. Am., Abstr. Jurassic

W.D.,

Progr.,

belt, California.

metamor-

of ocean-continent 18: 592.

1989. Terranes,

amalgamation

metamorphic

and ages of

Sierra Nevada

on mechanisms

S.H. and Sharp,

Nevada

Nevadan Jurassic Edelman, Mushy, Nevada, Hamilton,

Klamath

and

Geology,

Sierra

Paleontol.

S.H.,

Day,

suture,

H.W.

and

northern

early

faults,

of the western

Sierra

Geol. Sot. Am. Bull.,

arc-continent

Moores,

Sierra Nevada, collision.

Geol.

E.M.,

1983.

The

California:

a Late

Sot.

Abstr.

Am.

15: 565. S.H.,

Day,

T.P. and

H.W., Hacker,

Mesozic ocean-continent SE. and Elliott,

tectonics

Himalaya.

Geol., 16: 33-36.

Progr.,

of the erogenic America.

1985. Thrust

1: 395-408.

tectonics.

Edelman, intrusive

or thick

101: 1420-1433.

Pac. Geol., 8: 79-89, Beard. J.S. and Day, H.W., 1987. Smartville

skinned

deep in the crust.

Dewey, J.F. and Bird, J.M., 1970. Mountain

and pre-Late L.D., 1974. Stratigraphic

Sierra

Am. Bull., 96: 436-450.

Edelman,

P.C. and Clark,

R.W.H.,

Paleogeography

Econ.

tectonics

suturing.

Bateman,

thin

of thrusts

implications.

phic belt: constraints

References

western

E.M. and Robinson,

Paleozoic Sot.

terrane

were very helpful.

and

J. Geol. Sot.

system,

of the Pakistan

J.A., Moores,

Edelman,

two anonymous

structure

417-420.

and

sity of California, Davis, Sigma Xi, and the Geological Society of America. Manuscript prepara-

deep

tectonics,

J. Struct.

Day, H.W., Moores,

by the University

fault

and the continuation

States.

Helpful discussions were had with J.S. Beard, H.W. Day. R.D. Hatcher, Jr., F. Koenemann, E.M. Moores, J.B. Saleeby, R.A. Schweickert, and W.D. Sharp. Field work was partially supported by National Science Foundation Grant EAR 8019697 to H.W. Day and E.M. Moores, the Univer-

The Hall,

Geol. Sot. Am. Bull., 71: 483-496.

skinned,

geogr. Symp.,

tion was supported

tectonics,

Foothills

California.

and paleogeographic

here may be testa-

(Editor), Prentice

in the Alps and Himalayas.

zoic rocks of the northern

is a slate belt, and the structural-

Ernst

California.

143: 857-873.

L.D.,

Coward,

W.G.

Cliffs, N. J., pp. 50-70.

the deep structure

strain

maintained.

model presented

crustal

Coward,

is a permissible

in that lithosphere-scale

is

orogen

from an earlier

to form the marginal

proposed

compatibility plate tectonic

which

an active

terrane.

Development

R.W.H.,

London,

within

arc and resulted

collision

geologic

Geotectonic Butler,

wedges.

occurred

Nevada

Englewood

structures

or merge with the active east-ver-

margin

structural

of op-

in the same

west-vergent

deformation

continental

Nevadan

is solved

steep

to form tectonic

Nevadan

Sierra

of interaction

California.

Moores,

E.M.,

Zigan,

B.R., 1989. Structure

suture

zone in the northern

S.M., across

a

Sierra

Geol. Sot. Am., Spec. Pap., 224: 56 pp.

W., 1969. Mesozoic California and the underflow of

Pacific mantle. Geol. Sot. Am. Bull., 80: 2409-2430. Harper, G.D., Saleeby, J.B. and Norman, E.A.S., 1985. Geom-

KINEMATICS

etry

OF ARC

and

COLLISION

tectonic

Josephine

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