The Newport fault: Eocene listric normal faulting, mylonitization, and crustal extension in northeast Washington and northwest Idaho
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The Newport fault: Eocene listric normal faulting,
mylonitization, and crustal extension in
northeast Washington and northwest Idaho
TEKLA A. HARMS Department of Geology, Amherst College, Amherst, Massachusetts 01002
RAYMOND A. PRICE Department of Geological Sciences, Queen's University, Kingston, Ontario K7L 3N6
ABSTRACT morphic rocks and granitic plutons. The fault has a distinctive U-shaped
trace that straddles the state boundary between Washington and Idaho,
The Newport fault is a spoon-shaped, shallowly dipping fault north of Spokane (Fig. 1). Its southern limb extends 10 to 25 km to the
zone, across which a Proterozoic to Tertiary sedimentary suprastruc- east and west of the town of Newport (Fig. 2), whereupon the trace of the
ture is juxtaposed above an infrastructure of basement gneiss and fault turns north along both its western and eastern limbs. With
granitic batholiths as a result of Eocene normal faulting and crustal northward-decreasing stratigraphie separation, each limb dies out within
attenuation. Chloritic microbreccia occurs at the top of the Newport 15 km of the international boundary.
fault zone, below which a zone of mylonitization as much as 500 m The Newport fault lies within the Purcell anticlinorium, a regional-
thick is developed in footwall rocks, including the Eocene Silver Point scale structure that occupies much of the western part of the Cordilleran
Quartz Monzonite. Detailed kinematic analysis of fabric and struc- foreland fold and thrust belt in Montana, Idaho, northeastern Washington,
tures in the mylonite and microbreccia establishes that displacement and southern British Columbia (Fig. 1). The Kootenay arc forms the
was normal in sense on both sides of the U-shaped fault trace. The
direction of extension was 74°-254°, as shown by consistently
oriented mylonitic lineation. Crustal attenuation across the Newport
and adjacent Purcell Trench faults may have reached as much as 68
km (120%), and, in any case, it exceeded 35 km (40%). During faulting,
the footwall moved up and out to the east and west relative to the
hanging wall, forming two flanking footwall culminations or crustal-
scale boudins. Tectonic denudation of the footwall infrastructure is
shown by Eocene K-Ar cooling ages for granitic rocks within it.
Chrontours that increase in age outward from the Newport fault re-
cord abrupt cooling of the infrastructure as it was drawn away from
the hanging wall. The Silver Point Quartz Monzonite was emplaced
into the dilatant zone between the footwall infrastructural culmina-
tions. During displacement, hanging-wall strata were rotated down
the listric-fault surface, the footwall rotated upward, and the fault
surface flattened, probably as a consequence of isostatic adjustment to
mass redistribution caused by normal faulting. A roll-over anticline
developed in the hanging wall as it moved into the zone of attenuation
between the footwall culminations. Syntectonic Eocene extrusive
rocks and coarse clastic deposits accumulated in a growth fault basin
on the west flank of the hanging-wall anticline. Crustal extension and
normal displacement on the Newport fault are compatible with a re-
gional Eocene strain regime that crossed northern Idaho, Washington,
and southern British Columbia and produced dextral transcurrent
faulting, core-complex development, and clockwise rotation of crustal
blocks throughout that area.
INTRODUCTION
Figure 1. Tectonic setting of the Newport fault. Heavy stipple
The Newport fault of northeastern Washington and northwestern indicates core complexes in the Omineca belt. PRC = Priest River
Idaho is a north-plunging, spoon-shaped, shear zone that juxtaposes a complex; KC = Kettle complex; OC = Okanogan complex in both
sedimentary suprastructure of middle Proterozoic, Paleozoic, and Eocene Washington and British Columbia; VC = Valhalla complex; SC =
strata against an underlying crystalline infrastructure of high-grade meta- Shuswap complex.
Geological Society of America Bulletin, v. 104, p. 7 4 5 - 7 6 1 , 1 4 figs., June 1992.
745
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by Amherst College Library userQuaternary alluvium
Columbia River
Group basalt
Tiger Formation
Pend Oreille Andesite
Silver Point
Quartz Monzonite
48°45'
Selkirk Crest
Igneous Complex
Phillips Lake
Granodiorite
Undivided Cretaceous
plutonic rocks
Jurassic plutonic rocks
Undivided Paleozoic
sedimentary sequence
48°30'
Windermere Supergroup
Belt Supergroup above
the Prichard Formation
Prichard Formation
of the Belt Supergroup
metamorphic rocks
Shallowly dipping
normal fault
Thrust fault
Steep fault
Faults are dashed where inferred,
otherwise observed. Decorations
lie on hanging wall.
Axis of Snow
Valley anticline
117°30' 117°00' 116°30'
Figure 2. Generalized geologic map of the region surrounding the Newport fault. Adapted from Aadland and Bennett (1979), Clark (1967,
1973), Harms (1982), Harrison and Jobin (1963, 1965), Harrison and Schmidt (1971), Miller (1974a, 1974b, 1974c, 1974d, 1982a, 1982b,
1982c), Miller and Clark (1975), Miller and Engels (1975), Park and Cannon (1943), Pearson and Obradovich (1977), and Schroeder (1952).
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by Amherst College Library userNEWPORT FAULT, WASHINGTON AND IDAHO 747
western flank of the Purcell anticlinorium in southern British Columbia. It mentary strata; Mesozoic and Cenozoic granitic intrusive rocks, Eocene
coincides with the suture between parautochthonous rocks deposited along volcanic rocks, and Eocene nonmarine clastic deposits (Aadland and Ben-
the ancient continental margin of North America and the tectonic collage nett, 1979; Clark, 1967; Miller, 1974a, 1974b, 1974c, 1974d; 1982a,
of accreted terranes that make up the Intermontane belt of the Cordillera 1982b, 1982c; Pearson and Obradovich, 1977; Schroeder, 1952). In a
to the west (Price, 1981). The Omineca belt straddles this suture. It is a general sense, however, the geologic framework of the area can be consid-
regional tectonic welt characterized by the widespread occurrence of meta- ered in terms of two principal components: (1) a sedimentary
morphic and plutonic igneous rocks (Monger and others, 1982). The Priest suprastructure that is dominated by Proterozoic strata and a number of
River complex (Reynolds and others, 1981), which lies in the footwall of granitic plutons and (2) a mid-crustal crystalline infrastructure, the Priest
the Newport fault, is one of a number of culminations, including the River complex, that consists of batholiths and metamorphic host rocks
Okanogan, Kettle, Valhalla, and Shuswap complexes (see Fig. 1), that (Fig. 4).
make up the Omineca belt across northern Washington and southern The hanging wall of the Newport fault is part of a sedimentary
British Columbia. Because of their similarity to metamorphic core com- suprastructure of middle Proterozoic Belt Supergroup strata. The sequence
plexes of the southwestern United States, there is a growing consensus that there is -9,100 m thick, over half of which is the Prichard Formation
these infrastructural culminations are the result of Eocene crustal extension (Miller, 1974a). A sequence of lower Paleozoic(?) clastic and carbonate
and low-angle normal faulting (Price, 1979,1981,1982; Price and others, strata — 1,200 m thick disconformably overlies the Belt in the northwestern
1981; Coney, 1980; Rehrig and Reynolds, 1981; Armstrong, 1982; quadrant of the hanging wall (Miller, 1974a, 1974b). Belt strata in the
Harms, 1982; Harms and Price, 1983; Parrish and others, 1988; Harms hanging wall have undergone low-grade burial metamorphism, but deli-
and Coney, 1989). cate sedimentary structures are well preserved throughout.
Millerfirstdescribed the Newport fault as a thrust fault (Miller, 1971, Correlative sequences of Belt and Paleozoic strata occur in outward-
1974a, 1974b, 1974c, 1974d), presumably because it is very shallowly facing panels to both the west and east of the Newport fault (Fig. 4). On
dipping, and its U-shaped trace conveys the impression of a klippe. His the west side, just east of the town of Chewelah, this sequence is north-
interpretation has been subsequently followed (Cheney, 1980; Rhodes and striking and moderately west-dipping, so that deeper stratigraphic levels
Cheney, 1981). In this paper, however, we will show that the Newport are exposed toward the Newport fault. Rusty-weathering quartzite and
fault is a normal fault, unrelated to thrust faults of the region. Analysis of fine-grained garnetiferous quartz-muscovite-biotite schist with minor inter-
the fabric of mylonitic and cataclastic fault rocks, which are well devel- layered amphibolite (Miller, 1974b, 1974c; Miller and Clark, 1975) that
oped in an -500-m-thick zone along the Newport fault (Harms, 1982; occur at the base of this panel probably represent metamorphosed lower
Miller, 1971), provides a robust basis for establishing the kinematics of Prichard Formation, as degree of metamorphism decreases upward, and
displacement along the fault. Diagnostic structural relationships between the schist grades into strata identifiable as Prichard Formation. (Locally,
rocks of the footwall and those of the hanging wall are used to outline the the easternmost and structurally lowest exposure of this sequence is a
evolution of the fault and to evaluate the regional tectonic significance of gneiss that may be Belt basement [Harms, 1982; Miller, 1974b].) The total
the displacement on it. Constraints imposed by the unusual geometry of true thickness of this panel, including schistose rocks, is only slightly
the U-shaped Newport fault provide unique insights into processes of greater than that in the hanging wall of the Newport fault. Similarly, a
horizontal extension in continental lithosphere. north-striking, east-dipping panel of Belt Supergroup overlain by early
Paleozoic strata occurs in the vicinity of Pend Oreille Lake, east of the
REGIONAL GEOLOGY Purcell Trench (Harrison and Jobin, 1963,1965; Harrison and Schmidt,
1971). Rehrig and Reynolds (1981; Reynolds and others, 1981; Rehrig
The Newport fault intersects a large and varied suite of rocks along its and others, 1982,1987) have established the occurrence of cataclastic and
140-km-long trace (Figs. 2 and 3). These include Precambrian metamor- mylonitic rocks locally within the Purcell Trench and conclude that an
phic basement rocks; Middle Proterozoic sedimentary rocks of the Belt important, E-dipping normal fault lies in the trench at the base of the Belt
Supergroup and their metamorphosed equivalents; early Paleozoic sedi- sequence. We agree with this interpretation.
Figure 3. Schematic west-southwest-east-northeast cross section through the Newport fault and surrounding footwall. Line of section runs
from Chewelah to the north side of Pend Oreille Lake east of the Purcell Trench and is shown in Figure 2. Horizontal and vertical scales are
equal; scale matches that of Figure 2. See Figure 2 for key.
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by Amherst College Library user748 HARMS AND PRICE
The crystalline infrastructure of the Purcell anticlinorium, exposed in
the Priest River complex (Reynolds and others, 1981), constitutes the
second major geologic component of the region. It includes first, medium-
to high-grade schists and gneisses; second, several Cretaceous granitic in-
trusive complexes; and third, two adjacent, distinctive Eocene plutons
(Fig. 2).
1. A variety of metamorphic rocks occurs in the footwall of the
Newport fault. To the east, isolated and irregular roof pendants and
screens are predominantly quartzo-feldspathic paragneisses and quartz-
mica schists of upper amphibolite grade (Miller, 1982a, 1982b; Harms,
1982; Nevin, 1966). South of the fault, metamorphic rocks are abundant
and more continuous (Clark, 1967,1973; Weissenborn and Weiss, 1976;
Rhodes, 1986). A zircon U-Pb date of 1576 ± 1 3 Ma (Evans and Fischer,
1986) and Rb-Sr dates ranging from 1510 to 2053 Ma (Armstrong and
others, 1987) for augen gneisses from the Priest River complex indicate
that it includes remnants of pre-Belt crystalline basement. Mylonitic augen
gneiss that may be Cretaceous or Paleogene also occurs south of the fault
(Bickford and others, 1985; Armstrong and others, 1987).
2. Abundant granitoid rocks occur as bodies of batholithic dimen-
sions that have broad, migmatitic contacts with the metamorphic country
rocks. They range in composition from granite to granodiorite and are
characteristically both biotite and muscovite bearing. The Phillips Lake
Granodiorite occurs west of the Newport fault (Miller and Clark, 1975).
The footwall to the east of the fault is underlain by the Selkirk Igneous
Complex of Miller (1982a, 1982b). Miller mapped several individual
bodies in the Selkirk Igneous Complex but nevertheless suggested that it is
a single intrusive mass of common age and origin. Miller and Engels
(1975) reported discordant K-Ar mineral pair analyses from these intru-
sive rocks that range to as old as 92 Ma. As discussed below, we interpret
the plutons to be late Cretaceous in age.
Figure 4. Generalized geologic map showing the distribution of The two-mica composition, characteristically diffuse contacts with
two contrasting Proterozoic to Paleozoic sedimentary sequences ver- metamorphic host rocks, and size of these intrusive bodies suggest that they
sus the crystalline infrastructure surrounding the Newport fault. contain a large component of crustal melt and equilibrated relatively deep
Orientation data are from Harms (1982), Miller (1974a, 1982c), Miller in the crust (Miller and Engels, 1975; Miller and Bradfish, 1980). Fur-
and Clark (1975), and Harrison and Jobin (1963). thermore, as some of the metamorphic host rocks are pre-Beltian in age, they
were part of a crystalline basement that lay at mid-crustal depth and
temperature conditions, covered by >10 km of Belt and Paleozoic strata
A contrasting, second Proterozoic and Paleozoic sedimentary se- during pluton emplacement.
quence is carried in the Jumpoff Joe thrust sheet west of Chewelah. The Granitic plutons that intrude the supracrustal sedimentary rocks in
base of this second supracrustal sequence is the Deer Trail Group, which the hanging wall of the Newport fault were emplaced at much shallower
Miller and Whipple (1989) interpreted as a western facies equivalent of levels in the crust. They occur high in the stratigraphic sequence, within
the upper Belt Supergroup. It is unconformably overlain by late Protero- upper Belt and Paleozoic units, are smaller in comparison to the Selkirk
zoic Windermere Supergroup strata, which are in turn overlain by Igneous Complex, and have relatively sharp contacts with thin contact
Paleozoic strata that are a thicker and deeper-water facies than coeval units metamorphic aureoles. Miller and Engels (1975) reported concordant
that directly overlie Belt strata east of Chewelah (Miller and others, 1973; K-Ar biotite-hornblende and biotite-muscovite mineral pair dates ranging
Miller and Clark, 1975). This early Paleozoic off-shelf facies and the between 85 and 101 Ma from plutons within the hanging wall.
Windermere Supergroup characterize the Kootenay arc in the Metaline 3. The Silver Point Quartz Monzonite, which occurs along the
district of northern Washington (Park and Cannon, 1943) and adjacent southern footwall of the Newport fault, contrasts with other intrusive rocks
parts of southern British Columbia. of the Priest River complex in both composition and age. It contains
Several points emerge from the regional geometry of these supra- hornblende, biotite, and sphene, but not muscovite (Miller, 1974d) and is
crustal sequences. The stratigraphic succession in the hanging wall of the isotopically distinct from the Selkirk Igneous Complex (Whitehouse and
Newport fault correlates with sections in the footwall at Chewelah and others, 1989, and in press). Whitehouse and others (in press) have ob-
Pend Oreille Lake to the west and east. Taken together, they form an tained a date of 52.1 ±1.2 Ma for the Silver Point Quartz Monzonite from
integral part of the Purcell anticlinorium. Hanging-wall strata are continu- U-Pb analysis of zircon; concordant K-Ar mineral pair dates from 47 Ma
ous to the north with supracrustal rocks of the Purcell anticlinorium and, to 51 Ma (Miller and Engels, 1975) are in agreement with this age.
consequently, must still occupy their original position relative to correla- The very shallow dip and spoon shape of the Newport fault divides
tive footwall panels to the east and west. In contrast, the Kootenay arc the area into two distinct domains, a hanging-wall "flap" of supracrustal
assemblage in the hanging wall of the Jumpoff Joe thrust is quite different sedimentary strata that extends into the Purcell anticlinorium to the north,
from the hanging wall of the Newport fault. On this basis, the Newport and a footwall complex of mid-crustal plutonic and metamorphic rocks
fault could not root immediately to the west, into the Jumpoff Joe thrust that is exposed on the three sides of the U-shaped fault and is continuous
sheet (see Miller, 1971). beneath it (Fig. 3). The hanging wall is detached from Priest River com-
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by Amherst College Library userNEWPORT FAULT, WASHINGTON AND IDAHO 749
plex rocks that underlie it and has been separated from what were origi- displacement and not a result of reactivation or two-stage faulting (in
nally contiguous, correlative sedimentary sequences that now flank the contrast to Cheney, 1980; Rhodes and Cheney, 1981).
complex to the east and west at Pend Oreille Lake and at Chewelah (Fig. Consistent with northward-diminishing stratigraphic separation
4). Displacement on the Newport fault juxtaposed rocks from shallower across the Newport fault, mylonitization and brecciation die out north-
crustal levels over a domain of rocks from greater depth, with a significant ward along both limbs. Mylonite and chloritic microbreccia cannot be
stratigraphic gap across the fault. (Northward along the Newport fault the traced farther than lat 48°30' on the west limb and approximately lat
stratigraphic gap decreases.) This distribution of lithologic units reflects the 48°50' on the east limb (Harms, 1982; Miller, 1982b). At the western
nature and magnitude of displacement on the Newport fault. This is nor- terminus, a coarse-grained (fragments range from several centimeters to > 1
mal, not thrust, offset. m in size), well-indurated, monolithologic tectonic breccia occurs between
chloritic microbreccia of the fault zone and Paleozoic strata of the hanging
NEWPORT FAULT ZONE wall, and in steeply dipping, anastomosing seams that cut the hanging-
wall strata. The position of this breccia suggests that it formed high in the
The Newport fault zone is marked by two contrasting types of fault fault zone from fragmentation but comparatively little fault displacement.
rock: (1) 1-5 m of chloritic microbreccia occurs at the top of the fault In its northern extremities, where fault rocks do not occur, the Newport
zone, and (2) a thicker and more diffuse zone of strongly foliated and fault is defined by limited stratigraphic separation between supracrustal
lineated mylonite underlies the microbreccia and grades downward into sedimentary sequences that occur in both the footwall and hanging wall
unmylonitized rocks. The zone of mylonitization varies in thickness from (Fig. 4).
15 m to 500 m and is much better developed along the southern and
eastern limbs of the Newport fault than it is on the western limb. All units Chloritic Microbreccia
of the footwall domain (the Selkirk Igneous Complex, pre-Belt basement
schists and gneisses, Phillips Lake Granodiorite, and, notably, the Eocene The chloritic microbreccia is a cohesive, massive, fine-grained, and
Silver Point Quartz Monzonite) are mylonitic where they abut the New- homogeneous cataclasite in which there is no ordered fabric whatsoever. It
port fault. Brecciation and brittle disruption of fabric overprint mylonitic stands in marked contrast to the underlying, foliated, mylonitic fault rocks.
foliation at the transition between the two fault rock types. Narrow (1- to The chloritic microbreccia is uniform in character over the length of the
3-cm-thick) crosscutting veins of chloritic microbreccia extend down fault. It is characterized by white feldspar porphyroclasts floating in a
into and below the mylonite, but otherwise, chloritic microbreccia green aphanitic matrix (Fig. 7). Porphyroclasts are generally angular,
forms a distinct layer above mylonites in the fault zone. Detachment of reaching as much as 3 mm or 4 mm, but averaging 1 mm in length.
hanging-wall strata occurs at a sharp contact at the top of the chloritic Amphibole porphyroclasts occur locally; no lithic fragments have been
microbreccia. observed. The matrix of the microbreccia consists of extremely fine grains,
The chloritic microbreccia defines a single, continuous, shallow- too fine to distinguish optically, within a mat of pervasive chlorite. The
dipping curviplanar surface. Three-point calculations show this surface is matrix is also siliceous, which makes the microbreccia extremely resistant
essentially planar over short intervals (as much as 5 km) along strike (Fig. to erosion. (Chloritic microbreccia underlies most of the low hills that
5). At the present level of exposure, most of the surface dips less than 30°. confine the southwestern edge of the Pend Oreille River meanderplain to
At two locations, it attains dips of 45° and 55°. Locally, it is essentially the west of the town of Newport.)
horizontal (dip < 1°) and projects at a negligible dip under the hanging Angular porphyroclasts in the chloritic microbreccia indicate brittle
wall. Consequently, as defined by the chloritic microbreccia, the Newport comminution during cataclasis; however, the texture of the matrix demon-
fault has an overall concave-up, Iistric shape in three dimensions, with a strates that recrystallization and new mineral growth occurred as well. The
gentle north plunge. Mylonitic foliation conforms to this shape. It is west- absence of chlorite in rocks cut by the Newport fault shows that it could
dipping in the east limb, east-dipping in the west limb, and north-dipping not be derived directly through cataclasis but instead requires that second-
across the southern trace (Fig. 6). In fact, foliation in the mylonites very ary hydration of the mafic minerals in units of the footwall occurred. A
closely follows the orientation of the chloritic microbreccia surface, on significant quantity of water must have been introduced to the fault zone,
even a local scale (see Fig. 5). presumably through the juxtaposition of sedimentary strata in the hanging
Lineations defined by the preferred orientation of acicular minerals, wall against the warm, comparatively anhydrous footwall. The veins of
elongated quartz ribbons, and trails of crushed feldspar grains are ubiqui- chloritic microbreccia that extend below the fault zone suggest that hy-
tous in Newport fault zone mylonites. They lie within the local mylonitic drated and effectively fluidized,fine-grainedcataclastic material was also
foliation but have a consistent west-southwest-east-northeast (azimuth injected into the footwall along what may have been hydraulically induced
74°) trend throughout the fault zone (Fig. 5). fractures.
Because the transition from brittle to ductile deformation is depend-
ent on temperature, which increases with depth, both mylonites and cata- Mylonitic Fault Rocks
clasites can form simultaneously at different depths in one fault zone
(Sibson, 1977; Sibson and others, 1981). As Sibson (1977) has pointed The mineral composition of Newport fault mylonites is remarkably
out, they become juxtaposed where dip-slip displacement is of sufficient constant over the length of the fault zone and reflects the quartzo-
magnitude to carry the former up into the brittle regime, with the resulting feldspathic character of crystalline rocks in the footwall domain from
relative distribution of the two contrasting fault rock types controlled by which the mylonites were derived. Quartz, plagioclase, and potassium
whether slip on the fault is normal or reverse in sense. The fact that feldspar are the major constituents of the mylonite; biotite is a common
chloritic microbreccia overlies mylonites in the Newport fault zone, and accessory. Amphibole and sphene are volumetrically minor, but conspicu-
that the mylonites are derived from, and gradational into, footwall rocks, ous, components of mylonites developed from the Silver Point Quartz
confirms that the Newport fault is a normal fault. Mid-crustal fault rocks Monzonite, in which both minerals occur. Similarly, large porphyroclasts
were carried up to, and structurally under, fault rocks from more near- of muscovite and, less commonly, sillimanite needles occur in mylonites
surface levels. Cataclastic overprinting of mylonitic fabric at the top of the that cut high-grade metamorphic rocks. Despite recrystallization and the
Newport fault zone is an inherent consequence of crustal-scale, dip-slip development of the mylonitic fabric, there has been little change in mineral
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by Amherst College Library user750 HARMS AND PRICE
NEWPORT FAULT ZONE FAULT ROCK FABRIC DATA
KEY: — r r ^ — MEAN MYLONITIC FOLIATION
POLE TO MYLONITIC FOLIATION
MYLONITIC LINEATION
MEAN POLE TO MYLONITIC FOLIATION
POLE TO CHLORITIC MICROBRECIA
SURFACE
CHLORITIC MICROBRECCIA SURFACE
STRIKE AND DIP OF CHLORITIC
MICROBRECCIA SURFACE
3 — hanging wall east
O — no asymmetry MYLONITE SAMPLE SITES,
KINEMATIC ANALYSIS
C — hanging wall west
NEWPORT FAULT ZONE
MYLONITIC LINEATIONS
SILVER POINT QUARTZ
MONZONITE LINEATIONS
n = 11 10 km
Figure 5. Newport fault zone, fault-rock data shown along the trace of the fault. The attitudes of short, planar segments of the chloritic
microbreccia surface have been calculated from three-point solutions of outcrop distribution for the length of the fault. Synoptic, lower-
hemisphere, equal area projections show orientations of mylonitic lineations, and compare the average mylonite foliation to the chloritic
microbreccia surface for representative 3-km-long segments of the fault centered on the location indicated by the stippled arrows. Open and
half-filled circles along the Newport fault trace indicate the sample location and sense of shear determined in mylonites from the fault zone.
constituents from the protoliths. Some samples show alteration of biotite principally, are reduced in grain size and concentrated along feldspar grain
to chlorite, but unaltered biotite is equally common; aggregates of needle- boundaries. Quartz grains are flattened and mold around feldspar porphy-
like white mica occur in some feldspar porphyroclasts. roclasts (Fig. 8a). Higher in the fault zone, where feldspar porphyroclasts
Across the mylonitic zone in the Newport fault there is progressive become progressively smaller, strongly elongate quartz grains and aligned
development of mylonitic fabric upward from original igneous or meta- platy minerals define a more regularly planar-flattening foliation. Conspic-
morphic textures of the footwall protoliths. Greatest grain-size reduction uously elongate quartz "ribbon" grains, several centimeters long and only a
and most penetrative foliation are reached adjacent to the chloritic micro- few millimeters thick (Fig. 8c), are common. Internally, quartz ribbons
breccia. Because of the crystalline texture and general similarity of compo- consist of small, serrate, second-order subgrains, which are themselves
sition in footwall units, this range of mylonite fabrics can be interpreted as flattened and define a within-grain foliation that is commonly oblique to
expressing a consistent gradient in shear strain across the fault zone (Bell the ribbon boundary (Fig. 8d). (In mylonitized Silver Point Quartz Mon-
and Etheridge, 1973). The first expression of mylonitization of the footwall zonite, which has a low quartz content, development of through-going
is the development of "mortar" texture, in which biotite and chlorite, flattening fabric is inhibited, and mortar-textured mylonites with irregular
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foliation are widespread.) S-C mylonites (Berth6 and others, 1979), in
which a second, microscopically spaced shear foliation occurs in conjunc-
tion with the mylonitic flattening fabric, are common in the upper part of SOUTH
LIMB
the Newport mylonite zone (Fig. 8b). The shear (C-) fabric in them is 11 = 87
defined by crenulations or sigmoidal bends in the quartz ribbon-flattening
fabric and is accompanied by concentrations of comminuted biotite and
chlorite. Parallelogram or retort-shaped phyllosilicate grains are common
in these mylonites. They occur where mica basal cleavage lies parallel to WEST LIMB
n = 27
the flattening fabric and the two remaining grain boundaries have been
sheared along C-surfaces (Fig. 8e). In S-C mylonites of the Newport fault
zone, either one of the two planar fabrics can be significantly better devel-
oped than the other. Ultramylonite occurs sporadically and only adjacent
to the chlorite microbreccia. It is characterized by the complete transfor-
mation of all original grains into lenticular aggregates of small subgrains mean
within a single foliation.
Outcrop scale heterogeneity is superimposed on this overall strain
gradient. Seams of more intense mylonitization anastomose within the
fault zone, isolating lozenge-shaped areas in which there has been rela-
1
0 • 2 % [7" .] 4 - 6 % a io% 12- 14%
tively less shear strain. [ | 2 -4 % M 6 • 8 % 110 • 12 % 114 - 10%
Kinematics Figure 6. Synoptic lower-hemisphere, spherical Gaussian density
plots of poles to mylonitic foliation in the west, south, and east limbs of
The fabric in fault rocks of the Newport fault zone provides the most the Newport fault. Density is shown in percent of total number of
direct basis for establishing the nature of offset along the fault. The con- data. The mean for each domain is shown.
spicuous west-southwest-east-northeast-trending lineation in the mylo-
nites is interpreted as the direction of shear. It indicates dip-slip
displacement with a small oblique component along the north- reflect the nature of strain within the chloritic microbreccia, although data
south-striking east and west limbs of the fault zone and oblique slip along are admittedly unevenly distributed along the fault as preservation of
the southern limb. The sense of shear is reflected in the asymmetry in slickenside striations is limited to fresh road and railroad cut exposures.
elements of the mylonitic fabric that can be correlated with components of The overwhelming majority of these surfaces record oblique normal dis-
an ideal, simple-shear strain ellipse (Fig. 8f). Our kinematic analysis was placement (Fig. 9).
based mainly on the relative orientation of S- and C-fabrics (Ramsay and Three mutually consistent lines of evidence (asymmetry in mylonitic
Graham, 1970; Burg and Laurent, 1978; Berthe and others, 1979; Ponce fabric, slickensides within the microbreccia zone, and the relative distribu-
de Leon and Choukroune, 1980), but also on the relative orientation of tion of mylonite and microbreccia) demonstrate that the Newport fault is a
ribbon quartz grains and the fabric of recrystallized subgrains within them normal fault. Mylonites in the east and west segments of the Newport fault
(Vauchez, 1980; Lister and Snoke, 1984) and on the shape asymmetry of zone record opposing directions of oblique, normal dip-slip displacement
porphyroclastic feldspar and phyllosilicate grains (Simpson and Schmid, along azimuth 74°. The hanging wall moved down to the east on the
1983; Watts and Williams, 1979; Eisbacher, 1970). The analysis was western side and down to the west on the eastern side. The parallelism of
conducted with oriented thin sections cut parallel to the mylonitic linea-
tion and perpendicular to the mylonitic foliation from 17 samples collected
along the length of the Newport fault (Fig. 5).
The sense of slip recorded in mylonite from the eastern limb of the
Newport fault is consistently and unequivocally hanging wall down to the
west-southwest (Fig. 5). Exposure of the fault zone is more restricted, and
mylonites are less well developed along the western limb; consequently,
only one sample was obtained there. In it, the sense of shear is hanging
wall down and to the east-northeast. Shear along the southern, east-
west-trending segment of the fault appears to have been linked to that in
the adjacent east and west limbs. Along the western third of the southern
limb, sense of shear, where determinable, is hanging wall to the east. Some
mylonite samples there have flattening foliations only and no fabric
asymmetry. We interpret these as symptomatic of pure shear attenuation.
Conversely, along the eastern two-thirds of the southern fault trace, mylo-
nites yield hanging-wall-to-the-west sense of shear. A reversal in sense of
shear between top-to-the-east and top-to-the-west domains of the fault
occurs ~13 km west of the town of Newport.
Polished and slickensided faults are common within the chloritic
microbreccia. These are minor slip surfaces that can strike and/or dip
opposite to the Newport fault zone and were not, therefore, directly linked Figure 7. Photomicrograph of typical chloritic microbreccia in
to the detachment surface below the hanging wall. Nevertheless, they plane light White scale bar = 0.5 mm.
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NEWPORT FAULT, WASHINGTON AND IDAHO 753
wall "flap" presumably were transported from an original position beneath
Figure 8. Photomicrographs of Newport fault mylonites. The it. The rocks within these metamorphic-plutonic culminations, beyond the
white scale bar in all photomicrographs = 1 mm. (a) Mortar texture Newport fault zone itself, were not internally deformed during their dis-
developed in Silver Point Quartz Monzonite protolith. Plane light, placement. Attenuation in the middle and lower crust must have been
(b) Typical, well-developed S-C mylonite. Protolith is a quartz- concentrated in a linear zone below the hanging-wall "flap," allowing
muscovite schist in the eastern footwall. Plane light, (c) Quartz ribbon separation between the two north-south-trending footwall culminations
in Silver Point Quartz Monzonite mylonite. Plane light, (d) View and creating an intervening structural depression. Over the culminations of
identical to 8c, here shown under crossed polars, illustrating the fabric the Priest River complex, it was the sedimentary suprastructure that was
of recrystallized and elongated quartz subgrains inclined to the ribbon pulled apart, resulting in tectonic denudation of the footwall infrastructure.
grain boundary, (e) Sigmoidal biotite porphyroclast in incipient Price (1979, 1982; Price and others, 1981) has suggested that these rela-
Silver Point Quartz Monzonite mylonite. Cleavage in the biotite lies tionships fit the model presented by Davis (Davis and Coney, 1979) for
parallel to the flattening fabric. Plane light, (f) Components of an development of core complexes in Arizona by crustal-scale "boudinage."
ideal simple-shear strain ellipse as applied to interpretation of New- Extension in the suprastructure is expressed as a discrete normal fault, the
port fault zone mylonites. All photomicrographs show top-to-the-right Newport fault, along which the hanging wall was separated from correla-
displacement. tive sections and moved into a structural depression. The Newport fault is
the surface manifestation of the inhomogeneous attenuation that occurred
in its basement. It ends at the two northern terminations much as does
closure at the tip of a tension gash. The zone of thinning in the middle and
lower crust beneath the Newport fault may extend north into southern
the chlorite microbrectia and mylonite zones, and the compatibility of British Columbia where discontinuous, inward-dipping normal faults
kinematic indicators in each, suggests that the Newport fault rocks are the occur along strike from the Newport fault (see Fig. 14 below), including
result of a single episode of crustal-scale extensional faulting. the east-dipping fault on the east side of Valhalla complex (Parrish, 1984).
The mechanism of middle and lower crust extension beneath the
CRUSTAL ATTENUATION ASSOCIATED WITH Newport fault can be constrained on the basis of several observations from
NEWPORT FAULT DISPLACEMENT footwall rocks south of the fault, between the footwall culminations, where
deeper levels of the crustal-scale structure are exposed due to the north
Because of the very shallow dip of the Newport fault and the pro- plunge of the spoon-shaped fault. The Eocene Silver Point Quartz Monzo-
nounced, vertical, stratigraphic gap observed across it, the horizontal com- nite underlies much of this area. In addition to the mylonitization and
ponent of extension on the fault must be significant. Metamorphic and cataclasis that occur in the fault zone, a weak foliation and distinct primary
plutonic rocks of the footwall that now lie on either side of the hanging- lineation paralleling the east-northeast mylonitic lineation occurs through-
oblique reverse
slip fault
Figure 9. Lower hemisphere, equal-area projections of pol-
ished fault surfaces within the chloritic microbreccia and the
orientation of slickenside striae that occur on each. Half-filled
circles show the orientation of slickenside striae. The black half
of each indicates the down-thrown side. Data are grouped in
three domains: along the southern fault limb from east and west of the reversal in sense of shear indicated by mylonite fabric, and along the east
limb. No data are available for the west fault limb. Of 48 data points in the southeast set, 35 show oblique normal slip, 7 show oblique reverse
slip, and 6 are vertical or strike-slip faults.
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by Amherst College Library user754 HARMS AND PRICE
out the pluton (Fig. 5). Evidently, intrusion and solidification of the Silver
Point Quartz Monzonite occurred during regional stretching. Similar fab-
rics are not found in surrounding footwall rocks. Highly strained or
penetratively stretched rocks do not occur south of the southern fault limb,
and there is no evidence that, in this case, penetrative flow accommodated
outward displacement of the footwall culminations, as suggested by the
models of Block and Royden (1990) and Gans (1987). The Silver Point
Quartz Monzonite appears to have been localized by, and to have accom-
modated some part of the dilatancy in, the extended zone of the footwall.
The map pattern of the area (see Fig. 2) suggests a down-plunge view in
which the Silver Point Quartz Monzonite occurs as a west-dipping body
(Fig. 3). Although no north-south-trending fault bifurcates from the New-
port fault anywhere along its southern trace today, a west-dipping, lower-
crust normal fault, linked to offset on the east limb of the Newport fault,
may have existed and subsequently been followed and engulfed by the
o
Silver Point Quartz Monzonite (Fig. 10). A combination of through-the-
crust faulting (Wernicke, 1981) and inflation by igneous intrusion (Gans,
1987; Thompson and McCarthy, 1986) appears to be the process by
which extension in the footwall of the Newport fault was accomplished.
Emplacement of the Silver Point Quartz Monzonite may also have con-
tributed to maintenance of the relatively flat Moho observed below the
Newport fault structure today (Potter and others, 1986),
Isostatic adjustment to thinning of the suprastructure over the uplifted geneous crustal extension and infrastructural culminations of the
footwall culminations is to be expected (see Buck, 1988; Wernicke and Newport structure. Stipple = sedimentary crustal suprastructure;
Axen, 1988). This would cause the footwall to rotate up as it was dis- dashes = metamorphic and plutonic infrastructure; crosses = Silver
placed outward and thereby would induce flattening of the Newport fault Point Quartz Monzonite. Dashed line in the suprastructure represents
surface. The outward tilt of Belt Supergroup strata that cap the infrastruc- a marker horizon at the top of the Prichard Formation. Heavy lines
ture to the east and west of Newport, and the outward increase in K-Ar represent faults. Stippled lines in the infrastructure serve as footwall
chrontours in the infrastructure discussed below, are symptoms of isostatic marker horizons. Although they do not represent any material hori-
flexure in the footwall. The general parallelism of the east-dipping Purcell zons, they illustrate, in general, the distribution of isothermal surfaces
Trench normal fault with Belt strata in its hanging wall suggests that immediately following extension, (b) Prefaulting configuration, with
bedding-parallel slip developed along that fault just after unroofing and a hypothetical through-the-crust fault in the location of the Silver
tilting of the eastern Newport-fault footwall. Point Quartz Monzonite. Large arrows show sense of displacement
that would occur for crustal domains relative to the western footwall;
RADIOMETRIC DATING AND TECTONIC DENUDATION the length of the arrow schematically represents the relative magni-
tude of offset.
An extensive set of K-Ar biotite, muscovite, and hornblende mineral
dates covering a large region surrounding the Newport fault (Fig. 11) has Miller and Engels (1975) interpreted these data as recording two
been assembled by Miller and Engels (1975), and another set for the dominant periods of intrusion and cooling, one at 100-90 Ma and another
adjacent area in southern British Columbia, by Archibald and others at 52-45 Ma, with full or partial resetting of the K-Ar system in older
(1983,1984). Three broad domains are apparent. (1) Concordant mineral phase plutons surrounding the younger bodies. This model fails to account
pair dates of 85-108 Ma occur in plutons that intrude unmetamorphosed for the distinctive parallelism of footwall chrontours with the trace of the
supracrustal Belt and Paleozoic strata in the hanging wall of the Newport Newport fault, or their lack of parallelism with Eocene intrusive contacts.
fault, east of and above the Purcell Trench fault, and west of the Jumpoff We propose a reinterpretation of the pattern of K-Ar ages that recognizes
Joe fault. (2) Concordant mineral pair dates from 45-52 Ma occur in the the effects of rapid tectonic unroofing of the footwall during lower-crust
Selkirk Igneous Complex and eastern parts of the Phillips Lake Granodio- extension and displacement on the Newport fault (Price and others, 1981;
rite in the footwall domain. (3) Transitional discordant dates from 50 to Harms and Price, 1983), with which Miller and Engels concur (F. K.
100 Ma, with hornblende or muscovite greater than biotite, lie in the Miller, 1991, written commun.). Concordant Cretaceous dates record the
intervening area of the footwall. true time of granite emplacement. The discordant-date chrontour pattern
"Chrontours" of the age data in the footwall of the Newport fault indicates slow secular cooling through the upper part of the crustal infra-
cross plutons and do not outline intrusive contacts. They show a nearly structure and records the time elapsed between cooling to each
symmetric distribution paralleling the trace of the fault. In a broad zone successively lower, characteristic-mineral blocking temperature. We sug-
immediately adjacent to the Newport fault, dates are relatively young and gest that concordant 45-52 Ma dates, excluding the Silver Point Quartz
concordant. With increasing distance from the fault, over a distance of 15 Monzonite, occur as a result of abrupt quenching of Cretaceous plutons in
km to 20 km, mineral-pair dates become discordant and progressively the lower levels of the footwall infrastructure. The wide distribution of
older. Chrontours in the hanging wall of the Newport fault are discordant 45-52 Ma concordant dates implies that the footwall domain passed
in trend and value with respect to those in the footwall and are truncated through the range of muscovite and biotite blocking temperatures virtually
by the fault. Chrontours within the supracrustal panel above the Purcell instantaneously. This requires rapid tectonic uplift in excess of simple
Trench fault also appear to be truncated by that fault. erosional unroofing.
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by Amherst College Library userNEWPORT FAULT, WASHINGTON AND IDAHO 755
Figure II. K-Ar isoiopic-dating data, adapted from Miller and Engels (1975). Light stipple shows outcrop area of granitoid intrusive rocks.
Heavy stipple is the Silver Point Quartz Monzonite. b = biotite, h = hornblende, m = muscovite mineral date. Note that data are based on the old
decay constants kp = 4.72 x 10" 10 /yr, kt = 0.584 * 10~10/yr; and K ^ / K = 1.19 x 10"4, and have not been recalculated. Data are contoured on biotite
ages. Heavy lines indicate fault traces.
The K-Ar date chrontours serve as markers for outlining, in both LISTRIC NORMAL FAULTING OF THE HANGING WALL
space and time, the large-scale structures produced by crustal extension
(see Fig. 10). Chrontours of discordant dates reflect the same outward tilt Normal displacement on the Newport fault caused rotation of
of the footwall infrastructure east and west of the Newport fault as is hanging-wall strata down each inward-facing, concave-up, listric fault
demonstrated by the distribution of supracrustal sequences. Denudation of limb. As a consequence, the hanging wall developed (1) a "roll-over"
sequentially deeper levels of the crust from under the hanging wall is anticline (Dahlstrom, 1970; Hamblin, 1965) in Belt Supergroup and Pa-
reflected in the youngest quenching dates, which occur immediately leozoic strata and (2) a tilted, Eocene, growth-fault basin on the western
beyond the Newport fault trace. The northward closure of footwall chron- flank of that fold.
tours correlates with the northward termination of the Newport fault and Supracrustal strata in the hanging wall lie in the broad, open,
the decrease in infrastructure exposure. north-trending Snow Valley anticline of Schroeder (1952) (Figs. 2 and 4).
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by Amherst College Library user756 HARMS AND PRICE
Bedding in both limbs of this fold generally intersects the underlying fault older of the two units, the Pend Oreille Andesite (Schroeder, 1952),
surface at about 60°. Below the western limb of the fold, where Belt consists of at least 150 m of massive rhyodacitic flows and flow breccias
Supergroup and Paleozoic strata dip uniformly 55°-70° west (Miller, (Miller, 1974a; Pearson and Obradovich, 1977). (The Pend Oreille Ande-
1974a), the Newport fault is nearly horizontal (see Fig. 3). In the eastern site has been correlated with the Sanpoil Volcanics [Pearson and
limb of the fold, dips of both Prichard strata, wherever measurable (Miller, Obradovich, 1977], a regionally extensive but discontinuous volcanic unit
1982c; Harms, 1982), and the fault surface locally vary over a range of of north-central Washington; however, the name Pend Oreille Andesite
50°; however, the angle between them remains constant at - 6 0 ° (Fig. will be retained here to refer specifically to those rocks in the hanging wall
12b). These relationships suggest that the Snow Valley anticline formed of the Newport fault.) Pearson and Obradovich (1977) reported K-Ar
during fault displacement. Initially, the Newport fault probably cut radiometric dates from the Pend Oreille Andesite of 50.4 and 51.0 Ma for
through subhorizontal Belt and Paleozoic strata near the ideal 60° dip for biotite and hornblende, respectively. The Pend Oreille Andesite has few
normal faults (see Fig. 10), which is consistent with observations from observable flow boundaries from which to determine its present attitude;
areas of active crustal extension (Jackson, 1987). Below the supracrustal however, it overlies west-dipping Belt strata across an angular unconform-
sequence, the east and west segments of the fault joined to detach the ity and is in turn overlain by the second Eocene unit, the Tiger Formation.
hanging-wall flap from the extending infrastructure. Conforming to the The Tiger Formation consists of a laterally and vertically variable se-
listric shape of the fault, the hanging wall rotated as it was displaced, but quence of coarse, well-indurated, alluvial fan to braided, fluvial, clastic
the angle of intersection between hanging-wall bedding and the fault was deposits (Gager, 1984). Samples collected from the Tiger Formation by
preserved (Fig. 12c). R. A. Price and R. D. McMechan were studied by A. P. Audretch of Shell
Two Eocene units occur in a restricted basin on the west limb of the Canada Resources, who reported recovering early to middle Eocene paly-
Snow Valley anticline (Figs. 2 and 12d). The basin is confined on its nomorphs, consistent with the age of the underlying Pend Oreille Ande-
western and southern sides by the Newport fault, and to the east and north site. The Tiger Formation has more or less the same trend as, but a
by west-facing, dip-slope hills of Belt Supergroup and Paleozoic strata. The shallower (5°-30°) west dip than, the Belt strata over which it lies.
Figure 12. Structures in the hanging wall of the Newport fault, (a) Location of Figures 12b and 12d. Stipple shows areas of bedrock,
(b) Consistent angular relationship between hanging-wall bedding and fault dip along the east trace of the Newport fault, (c) Schematic
structure sections showing the development of a roll-over anticline in hanging-wall strata; consistent angular separation of bedding and fault;
and decreasing rotation of younger, syntectonic, basin-fill strata associated with listric normal faulting. After Hamblin (1965). (d) Tiger
Formation basin. Filled circles indicate drill sites in the basin; accompanying numbers give the elevation of the base of the Tiger Formation in the
drill hole (in feet). Drill locations with stars designate occurrences of conglomeratic fades. Drill data from an unpublished report by C. S. Ferris
and E. Huskinson (1978). Bedding orientations taken from Miller (1974a) and Gager (1984). Patterns are defined in Figure 2.
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by Amherst College Library userNEWPORT FAULT, WASHINGTON AND IDAHO 757
The pattern of tilting and unconformable onlap in the Tiger Forma- others, 1982; Reynolds and others, 1981). This interpretation fails to rec-
tion and Pend Oreille Andesite strongly suggests they are syntectonic ognize the genetic link between listric fault shape and hanging-wall bed-
basin-fill deposits (Harms, 1982; Gager, 1984). Downward rotation of ding tilt. "Unfolding" of the fault, furthermore, would produce tighter
Belt and Paleozoic hanging-wall strata during Newport fault displacement folding and greater tilting of hanging-wall strata. Eocene units in the
produced the basin in which Eocene deposits accumulated. Accordingly, hanging wall form an eastward-tapering wedge, the existence, shape, and
the Eocene units have no offset counterparts in the footwall of the New- areal extent of which can best be interpreted as having been controlled by
port fault. Basin deposits themselves were tilted during and subsequent to the Newport fault. On this basis, we consider it unlikely that the Pend
deposition, but through less net rotation than the Proterozoic and Paleo- Oreille Andesite was ever part of a regional volcanic blanket as suggested
zoic strata beneath them (Fig. 12c). Abrupt and significant changes in the by Pearson and Obradovich (1977) and Cheney (1980). Occurrences of
depth to the base of the Tiger Formation, as demonstrated by drilling the coeval Sanpoil volcanics to the west probably originated in similarly
(C. S. Ferris and E. Huskinson, 1978, unpub. report), are accompanied by localized extensional settings (McCarley Holder and others, 1990). This
localized occurrences of conglomeratic fades on the eastern, relatively follows the interpretation of Ewing (1981) regarding the distribution of the
lower side (Fig. 12d). The distribution of these thickness and facies Kamloops Group, a middle Eocene extrusive suite in the Omineca Belt of
changes indicates at least two north-south-trending, syndepositional faults southern British Columbia.
(Harms, 1982; Gager, 1984), which can be interpreted as synthetic normal The fundamental link between normal displacement on the west limb
faults that merge with the Newport fault at depth. A high-standing block of of the Newport fault and the attitude, distribution, and character of strata
west-dipping upper Belt Supergroup and Paleozoic strata forms the north- in the west flank of the hanging wall provides confirmation of the limited
ern margin of the Tiger basin (Fig. 12d). Immediately south of this contact, mylonite kinematic data from that limb.
the Tiger is thick and conglomeratic as well (C. S. Ferris and E. Huskin-
son, 1978, unpub. report). This margin probably reflects the presence of an AMOUNT OF EXTENSION
east-west-trending, syndepositional, transverse tear fault that accommo-
dated northward decrease in displacement on the Newport fault toward
Reasonable limits can be placed on the amount of crustal extension
the northwest terminus.
across the Newport fault structure. To do so, we assume that extension in
The distinctive spoon shape of the Newport fault, the folding of the infrastructure balances that in the suprastructure, which can be meas-
Proterozoic and Paleozoic strata above the fault, the growth and filling of ured by offset between the Belt-Paleozoic sequence in the hanging wall
the Pend Oreille Andesite-Tiger Formation basin, and the tilting of basin and the same strata east of Chewelah, and in the area of Pend Oreille Lake
fill units, can all be integrated as characteristic features of listric extensional (Fig. 13). This estimate includes extension due to offset along the Purcell
faulting. The curvature of the Newport fault has previously been inter- Trench fault and applies only in the vicinity of the line of section (see Fig.
preted as the product of post-displacement folding of an initially horizontal 2) as extension of the suprastructure dies out northward.
and more regionally extensive fault surface (Cheney, 1980; Cheney and The top of the Prichard Formation is the only stratigraphic horizon
0 MINIMUM ESTIMATE
lf = 125; Al =35; l0 =90
e = 0.39; -40% extension
30 km 38 km
Q MAXIMUM ESTIMATE
lf = 125; Al = 68; l0 =57
e = 1.19; « 120% extension
Figure 13. Minimum (a) and maximum (b) estimates of crustal extension based on retrodeformable structure sections. Figure 3 is used as a
template for these analyses.
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by Amherst College Library user758 HARMS AND PRICE
exposed in all three supracrustal sections. It serves as a reference line in high in the Tiger Formation indicates that the footwall was exposed by the
measuring the horizontal component of normal fault offset, although its later stages of basin development (Gager, 1984).
absence on the east side of the hanging-wall anticline introduces uncer- 3. Discordant mineral-pair dates as young as 63 Ma have been ob-
tainty into our results. We base our analysis of extension in the suprastruc- tained from the flanks of the infrastructure in the footwall of the Newport
ture on the consistent 60° angle of separation between the Newport fault fault (Miller and Engels, 1975). They demonstrate that slow cooling in
and dipping hanging-wall strata that it cuts. Both the fault surface and the thick, undisturbed crust continued until at least that time. Concordant
displaced domains above and below the fault have rotated to their present K-Ar mineral-pair quenching dates in Cretaceous footwall plutons range
attitudes during offset. Consequently, extension is measured in lineation- from 52 Ma to 46 Ma nearest the trace of the Newport fault, indicating
parallel structure sections that are drawn to be retrodeformable to an that the deepest levels of the footwall were denuded then. Uplift of the
original configuration wherein the eastern and western limbs of the New- easternmost side of the Selkirk Crest by movement on the Purcell Trench
port fault cut subhorizontal strata and have a 60° angle of dip (Fig. 13). fault appears to be slightly younger (Parrish and others, 1988), at least to
Minimum and maximum limits on the amount of extension across the the south of the Hope fault where K-Ar dates range from 45 Ma to 42 Ma.
Newport and Purcell Trench faults result from adopting two end-member
assumptions: that the present exposure of the footwall metamorphic and INTEGRATING NEWPORT FAULT EXTENSION WITH
plutonic infrastructure results either entirely from post-extension erosion REGIONAL EOCENE TECTONICS
(Fig. 13a), or entirely from tectonic denudation (Fig. 13b).
The upper limit of horizontal extension across the southern part of the Crustal attenuation associated with the Newport fault occurred dur-
Newport and Purcell Trench faults is 68 km, or -120%. The minimum ing a well-constrained episode in middle Eocene time. Core complexes
extension is 35 km, or 40%. For comparison, based on its outcrop width, throughout the Omineca belt in southern British Columbia and northern
emplacement of the Silver Point Quartz Monzonite can account for as Washington (see Fig. 1) underwent extension at about the same or a
much as 25-30 km of extension in the southern footwall. Within the slightly later time period (Parrish and others, 1988, and references therein;
bounds of uncertainty, this could compensate only the minimum estimated Harms and Coney, 1989). The distribution of these complexes, and the
suprastructure extension. Figure 13b incorporates flexural rotation and orientations of extension across them, bear a regular relationship to other
flattening of the footwall, which is consistent with regional geologic rela- structures in the region and can be integrated into a Pacific-northwestern,
tionships, whereas Figure 13a incorporates none. This suggests that as- Eocene strain field that provides a tectonic context for the origin and
sumptions used in calculating the maximum extension across the Newport evolution of the Newport fault.
fault are more realistic, and that extension is most likely in excess of 40%. Core complexes in southern British Columbia and northern Washing-
ton, and the normal faults that flank them (Fig. 14), lie in a domain
TIME OF EXTENSION bounded by en echelon strike-slip faults that experienced significant dex-
tral displacement in Eocene time (Price, 1979, 1981, 1982; Price and
The time of displacement on the Newport fault is tightly constrained others, 1981; Parrish and Coleman, 1990). The Northern Rocky Moun-
by a number of independent chronometers to the period between 52 and tain Trench fault, which had hundreds of kilometers of dextral offset in
45 Ma. Significant crustal-scale normal faulting and tectonic denudation total and was active in the Eocene epoch (Price and Carmichael, 1986;
were completed in a brief span of time. Faulting is bracketed by the Gabrielse, 1985), skirts the Omineca belt northeast of the Shuswap com-
following relationships. plex. The Eocene Yalakom and Ross Lake faults (Davis and others, 1978;
1. Intrusion and crystallization of the Silver Point Quartz Monzonite Haugerud, 1985,1991; Parrish and Coleman, 1990) lie to the west of the
at 52.1 ±1.2 Ma (Whitehouse and others, in press) occurred under the Omineca belt, from the Shuswap complex south to the Okanogan com-
influence of the strain regime recorded in Newport fault zone mylonite plex. Southeast of the Newport fault and the Priest River complex, at the
lineations. The Silver Point Quartz Monzonite appears to have intruded an periphery of the Omineca belt, the Lewis and Clark fault system extends
active, dilatant, attenuation zone in the footwall, perhaps localized along a from the south end of the Purcell Trench to north-central Idaho. Harrison
lower crustal continuation of the eastern limb of the Newport fault. In part, and others (1972,1974) suggested that it has had a long history of activity,
intrusion of the pluton enabled lower-crustal extension. The two events, extending back into the Proterozoic and including right-lateral offset of
therefore, appear coeval, although some displacement on the Newport Eocene age (also, Sears and others, 1986; Wallace and others, 1990).
fault outlived intrusion. Cataclastic fault rocks developed from the Silver At least two Eocene, dextral strike-slip faults occur along the Wash-
Point Quartz Monzonite after it cooled below the ductile-brittle transition ington and Oregon Coast Ranges (Fig. 14). One lies along the northern
(near the K-Ar blocking temperature for biotite) at 47 Ma (Miller and Oregon inner continental shelf (Snavely and others, 1980). Johnson
Engels, 1975; see Fig. 11). Differences in whole-rock (Miller and Engels, (1984) and Cheney (1987) proposed that one or more are buried below
1975) and isotope (Whitehouse and others, 1989, and in press) composi- the Cascade foothills and Puget lowland. Numerous paleomagnetic studies
tion between mid-Cretaceous plutons and the Silver Point Quartz Monzo- demonstrate anomalous paleopole positions requiring as much as 15° of
nite suggest that emplacement of the Silver Point Quartz Monzonite, in Eocene and post-Eocene clockwise rotation of the Coast Ranges about
fact, heralded a fundamental change in the tectonic regime in the area. vertical axes, of which ~20°-30° can be attributed specifically to early and
2. Differences in the dip of bedding suggest that basal deposits in the middle Eocene displacement (Simpson and Cox, 1977; Magill and Cox,
Tiger Formation-Pend Oreille Andesite basin accumulated after ~10°- 1981; Magill and others, 1981; Globerman and others, 1982; Beck and
20° of rotation in the underlying hanging wall. Basin development, Engebretson, 1982). Geologic studies suggest that both extension inboard
hanging-wall rotation, and normal faulting proceeded during deposition of of the Coast Ranges and dextral shear along closely spaced faults within
the Pend Oreille Andesite and Tiger Formation, or from 51 Ma (Pearson them are responsible for the rotation (Heller and Ryberg, 1983; Heller and
and Obradovich, 1977) through early to middle Eocene time. The influx others, 1985; Wells and Coe, 1985; Wells and HeUer, 1988). The restored,
of clasts derived from footwall lithologies that occurred locally at horizons early Eocene position of the Coast Ranges is dependent on how rotation is
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