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TECTONOPHYSICS
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ELSEVIER
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Tectonophysics 305 (1999) 141-152
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, 16802, USA. Phone:
Karlsruhe, Germany.
Neogene basins of the northern Rio Grande rift: partitioning and
asymmetry inherited from Laramide and older uplifts
\
Karl S. Kellogg *
u.s.
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Geological Survey, Mail Stop 913, Box 25046, Federal Center. Denver, CO 80225, USA
Jlhampton
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Received 3 March 1998; accepted 141uly 1998
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Abstract
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Three asymmetric Neogene basins in the northern Rio Grande rift of New Mexico and Colorado - the San Luis basin.
the upper Arkansas River graben, and the Blue River graben - are tilted toward large flanking normal faults and lie astride
the similarly asymmetric Late Cretaceous-early Tertiary (Laramide) San luan-San Luis. Sawatch. and Front Range-Gore
Range uplifts, respectively. The steep, thrust-faulted side of each uplift is on the same side as the down-rotated side of each
of the Neogene basins. In addition. the direction of stratal tilt changes northward across the Villa Grove accommodation
zone from east in the San Luis basin to west in the upper Arkansas River graben. This accommodation zone coincides
approximately with the northward change from the east-directed San Juan-San Luis uplift to the west-directed Sawatch
uplift. These observations. supported by seismic-reflection studies across the San Luis basin and studies of several other
superimposed pairs of rift basins and Laramide uplifts, suggest that the basin-bounding normal faults are listric and merge
at depth with the older thrusts. which are also listric and root into the crust at about 15-16 km. The Blue River graben
is complicated by lack of basin fill and a thrust history along the west side of the Gore Range that is at least as old as
late Paleozoic. Nonetheless. the Neogene valley is demonstrably tilted west and lies astride an overall west-directed thrust
system. similar to other thrust-and-basin relationships in the northern Rio Grande rift. © 1999 Elsevier Science B.Y. All
rights reserved.
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Keywords: Rio Grande rift; Colorado; Laramide orogeny; Neogene extension; inherited structure
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1. Introduction
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The Rio Grande rift extends northward from New
Mexico into the heart of the southern Rocky Mountains, where the deep basin fill that defines major rifting terminates at the north end of the upper Arkansas River Valley near Leadville. Colorado
(Tweto. 1979; Fig. I), However. Neogene normal
• Tel.: + 1 303 236 1305; Fax:
[email protected]
+ 1 303 2360214; E-mail:
faults and grabens that are approximately coeval with
formation of the Rio Grande rift can be traced along
trend at least as far north as the Wyoming border;
the Blue River graben is the largest of these northernmost rift structures. In Colorado and northern
New Mexico, the Rio Grande rift developed along
the axial zone of the Late Cretaceous-early Tertiary (Laramide) San Juan-San Luis uplift (Tweto,
1975, 1979; Chapin and Seager, 1975; Cordell, 1978;
Eaton, 1979; Chapin and Cather, 1994; Russell and
Snelson, 1994), which, in turn, developed along
0040-1951/99/5 - see front matter © 1999 Elsevier Science B. V. All rights reserved.
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K.s. Ke/logg/Tt'ctO/lOphysics 3U5 (/YYYj N/-/52
~MAP
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AREA
COLORADO
f--;}~-.i :j
BASIN-FILL DEPOSITS
Including Santa Fe. Dry Union. and
.
Troublesome Formations
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NEOGENE VOLCANIC ROCKS
Basalt and subordinate rhyolite
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OLIGOCENE VOLCANIC AND
SEDIMENTARY ROCKS
Mostly volcanic rocks of intermediate
composition
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TERTIARY AND UPPER
CRETACEOUS INTRUSIVE ROCKS
CJ
, "
LOWER TERTIARY TO
CAMBRIAN SEDIMENTARY
ROCKS
D
PROTEROZOIC ROCKS
- . . •. . . Normal fault with Neogene
movement-Bar and ball on
downthrown side; dotted
where concealed
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-,..- ...... Thrust or reverse fault with
inferred Laramide
movement-Teeth on
upthrown side; major late
Paleozoic movement on
west side of Gore Range
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Monocline axis, showing
direction of maximum dip
°CI======S2.5.......50KM
Fig.!. Map showing major tectonic features of the Colorado portion of the Rio Grande rift. simplified and modified from Tweto (1979).
Cross-sections A-A', B-B', and C-C are shown in Fig. 3. AG = upper Arkansas River graben: APF = Antelope Pass normal fault: BRF
= Blue River normal fault: BRG = Blue River graben: CF = Castle Creek fault zone: 'ET = Elk Range thrust zone: GF = Gore fault:
GHM = Grand Hogback monocline: K = Kremmling: KT = Kerber Creek thrust: L = Leadville: MR = Mosquito Range: PP = Poncha
Pass: S = Salida: VGAZ = Villa Grove accommodation zone; WMP = western Middle Park basin: WRT = Williams Range thrust.
.
,
K.S. Kellogg/Tectonophysics 305 (1999) 1-11-152
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structures that were as old as Proterozoic (Tweto,
1979).
Stratal tilt within basins of the Rio Grande rift
alternates from east to west, segmenting the rift system (Chapin and Cather. 1994: Russell and Snelson,
1994) similar to the East African rift (e.g .• Bosworth
et al.. 1986). Each segment of the Rio Grande rift is a
complex north-striking half graben that is separated
from adjacent segments by an accommodation or
transfer zone, across which stratal tilts reverse. The
accommodation zones are crustal flaws (fault zones
or poorly defined structural lineaments) which in
places exploited Laramide and older zones of weakness and locally acted as a conduit for magma (e.g .•
Russell and Snelson. 1994).
This study explores the relationships between the
direction of stratal tilt within each major normalfault-bounded segment of the northern Rio Grande
rift and the steep, thrust-faulted side of the Laramide
uplift within which the segment lies. The evidence
strongly suggests that the geometry of the Tertiary·
basins may be genetically linked to the symmetry
of the older Laramide uplift. Three grabens of the
northern Rio Grande rift, underlying the San Luis
basin, the upper Arkansas River Valley, and the Blue
River Valley (Fig. 1), support a model, based, in part,
on ideas presented by Wise (1963): Scholten (1967);
Royse et al. (1975); Schmidt et al. (1984); Russell and Snelson (1994), and Kellogg et al. (1995),
which explains how Laramide faults and uplifts controlled the placement and orientation of Neogene
faulting and basin formation. This paper tests the
applicability of the model, described in the next
section, to the major structural features along and
adjacent to various parts of the northern Rio Grande
rift.
2. Basement uplifts and Neogene grabens: a
possible genetic link
,
'"
Deep seismic reflection profiles across several
Laramide uplifts, such as the Wind River Mountains
in Wyoming and the Manzano uplift in New Mexico, indicate that on at least one side listric thrusts,
flattening at depth into the middle crust at about
15-16 km, bound the uplifts (Sharry et ai., 1986;
de Voogd et aI., 1988; Russell and Snelson, 1994).
I·B
The bounding thrusts commonly control the position of Tertiary normal faults by offering zones of
weakness that can be exploited during crustal extension (Royse et at.. 1975; Smith and Bruhn, 1984;
Arabasz et al.. 1992; Kellogg et aI.. 1995). Tertiary
basin-bounding normal faults also appear to root
into older thrusts in the southern Albuquerque basin
(Russell and Snelson, 1994). Reiter et al. (1992)
presented a quantitative, isostatic model for the reactivation of Laramide thrusts and the formation
of half grabens by rotation of domino-style crustal
blocks.
Southwestern Montana contains many excellent
examples of Neogene extensional features that reactivate Laramide structures. At least four Neogene
extensional valleys as wide as 20 km formed along
the axial zones of large (as wide as 30 km) Laramide
uplifts (Scholten, 1967). The thrusts define the east
side of each uplifts and produce large basement
overhangs above a footwall syncline in Paleozoic
and Mesozoic rocks (e.g. Kellogg et aI., 1995)
(Fig. 2A). According to a model for structural inversion (Schmidt and Dresser, 1984; Schmidt et aI..
1984; Kellogg et ai., 1995), the unsupported eastern
side of a basement arch in Montana underwent rotational sagging into the underlying or adjacent basin
sediments, thereby producing extension. fracturing,
and possibly normal faulting along the crestal zone
of the arched basement block. Similar crestal normal
faulting ('keystone faulting') was recognized long
ago in the basement uplift of the Owl Creek Mountains. Wyoming (Wise. 1963). The fractures were
later exploited during Tertiary crustal-wide extension. resulting in large-scale collapse and development' of a basin along the axial part of the Laramide
arch (Fig. 2B). 'Perched basement wedges' (Lageson. 1989), bounded on one side by Laramide-age
thrusts and on the other side by Neogene normal
faults, typically result from this process (Fig. 2C).
In most cases. strata in the basins adjacent to the
basement wedge dip toward both the major valleybounding normal faults and older uplift-bounding
thrusts.
The idea that normal faults commonly exploit
older thrust is well established. For example. seismic
studies across the Idaho-Wyoming thrust belt show
that Neogene normal faults commonly sole into underlying thrust ramps (Royse et al.. 1975). Field re-
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144
K.S. KellogglTectonophvsics 305 (J999) J.JJ-J52
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Laramide sedimentary
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LATE CRETACEOUS·
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-15-t=:=~===::::=-_~.:...:.........2P~R~E~S~E~N~T..2..~..J
Fig. 2. Schematic diagrams showing major tectonic elements during evolution of an asymmetrical Laramide uplift and collapse durin
Neogene extension: no vertical exaggeration. PC" = Precambrian rocks: M~~ = Mesozoic and Paleozoic rocks. Adapted, in part, fror
Schmidt et al. (1984) and Kellogg et al. (1995) .
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lations hips also demonstrate that some nonnal faults
adjacent to basins in southwestern Montana sole into
older thrust surfaces (Kellogg et a!., 1995); similar relationships have been mapped in southeastern
Idaho (Kellogg, 1992).
The mechanisms by which Laramide (and earlier) crustal shortening controlled Neogene extension
are complicated and still poorly understood, and the
model outlined here for listric thrusts and nonnal
faults, although favored by the author, is one of
several that have been proposed. Seismic evidence
for listric Laramide thrusts and Neogene nonnal
faults commonly is equivocal and some researchers
suggest that basin-bounding nonnal faults may be
planer to at least 15 Ian depth (Dozer and Smith,
1985) or steepen with depth and root into thrusts
that ,also steepen downward (e.g" Sales. 1983). A
new school of thought propose that north-striking
Laramide faults in New Mexico and southern Colorado have a large component of dextral slip and
a relatively small component of reverse slip (e.g
Bauer and Raiser, 1995; Wawrzyniec and Geissman
1995). For example, movement along one of tht
major faults bounding the east side of the southern·
most San Juan-San Luis uplift (the Picuris-Peco~
fault in the southernmost Sangre de Cristo Mountains near Santa Fe. New Mexico) has an ancestry
that is as old as Proterozoic and an inferred 37 Ian
of mostly Laramide dextral strike-slip motion (Bauer
and RaIser. 1995); the fault is viewed as a positive flower structure that steepens downward to an
essentially vertical attitude. If one accepts that the
Laramide orogeny was marked by significant crustal
shortening, however, then it is difficult to reconcile
. such shortening along downward-steepening faults .
In addition, the inferred. predominantly strike-slip,
non-listric geometry inferred for the Picuris-Pecos
fault precludes exploitation of the fault by Neogene
nonnal faults along the east side of the adjacent rift
basin (Espanola Basin) .
3. Gcologic setting of the northern Rio Grande
rift
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Laramide crustal contraction and resultant uplift along the San Juan-San Luis arch began about
70 Ma (late Campanian) and created a highland
that shed clastic detritus into bounding basins until the Late Eocene (Tweto. 1975), at which time a
widespread erosion surface extended across most of
Colorado (Epis and Chapin. 1975). Rio Grande rifting began shortly after 29 Ma (Tweto, 1979; McIntosh et aI.. 1992) and was marked by a change in volcanism from the intermediate compositions that characterized the widespread Oligocene volcanic fields
(Steven. 1975) to bimodal basalt-rhyolite (mostly
basalt) volcanism that characterizes the Rio Grande
rift (Lipman and Mehnert, 1975). This fundamental
change in volcanic style coincides with a change
throughout the western United States from subduction-driven to extension-driven tectonic regimes in
the Late Oligocene to Early Miocene (Christiansen
and Lipman, 1972). A SUbsequent period of rapid
faulting and extension in New Mexico and the San
Luis basin of Colorado culminated in the Middle
to Late Miocene (Chapin and Cather, 1994) and
was accompanied or closely followed by voluminous
basaltic volcanism between 3 and 8 Ma (Seager and
Morgan. 1979) .
Despite paleobotanical evidence that suggests that
the Rocky Mountains have been topographically
high since the Eocene (e.g., Gregory and Chase,
1994), other studies indicate that the rise of the
modem Rocky Mountains was accompanied by a
period of rapid faulting and extension in the Middle
to Late Miocene (Chapin and Cather, 1994; Steven
et aI.. 1997). Such uplift is consistent with apatite
fission-track ages which indicate that the Sawatch
and Sangre de Cristo ranges underwent significant
uplift since about 20 Ma (Bryant and Naeser, 1980;
Lindsey et aI., 1986; Kelley et aI., 1992).
3.1. The San Luis basin and adjacent uplifts
The San Luis basin extends from about 10 km
south of Salida, Colorado, to about 80 km south
of the Colorado-New Mexico border; only the Colorado portion of the basin is shown in Fig. 1. The
basin is bordered on the west by a gently east-dip-
ping surface underlain mostly hv Proterozoic crvstalline rocks and Oligocene volcanic rocks and' is
bounded on the east by the Sangre de Cristo normal
fault system. which also defines the western marO'in
of the rugged Sangre de Cristo Mountains (Figs~ 1
and 3A). The northern San Luis basin unde~ent
8-12% extension (Kluth and Schaftenaar. 1994). increasing southward along the Rio Grande rift to
greater than 28% extension in central New Mexico
(Chapin and Cather, 1994).
Seismic reflection surveys and deep exploratory
drilling indicate that the San Luis basin consists
of two east-tilted half grabens. separated by the
north-trending Alamosa horst (Brister and Gries.
1994). The eastern part of the eastern half graben
(Baca graben) contains as much as 5.6 km of the
Upper Oligocene to middle Pleistocene Santa Fe
Group above a basal sequence as old as Eocene.
Other researchers report that the basin is as much as
6.4 km deep (Kluth and Schaftenaar, 1994). The near
absence of pre-Tertiary sedimentary rocks beneath
the San Luis basin is a relict of erosion over the crest
of the former Laramide uplift.
The 'perched basement wedge' (terminology of
Lageson, 1989) of Proterozoic rock along the west
side of the present Sangre de Cristo Mountains
(Fig. 3A) is a remnant of the eastern flank of the
San Juan-San Luis uplift. The crystalline rocks of
the basement wedge were thrust eastward during
the Laramide orogeny above mostly Pennsylvanian
and Permian sandstone and conglomerate; Neogene
normal faults subsequently exploited several of the
thrust surfaces (Lindsey et aI., 1983). Structural relief across the Sangre de Cristo normal fault system, which bounds the west margin of the basement
wedge, is at least 9200 m (Kluth and Schaftenaar,
1994). The system of Laramide thrusts and Neogene
normal faults extends along or near the east side of
the entire San Luis basin. a style of deformation that
is very similar to that interpreted along the east side
of the southern Albuquerque basin in New Mexico
(Russell and Snelson. 1994).
The northern end of the Laramide thrust system along the west side of the Sangre de Cristo
Mountains disappears under the San Luis basin and
reappears on the west side of the valley as the northdirected Kerber Creek thrust (Lindsey et aI., 1983)
(Fig. 1). This geometry suggests overall northeast
146
K.S. Kellogg/Tectonophysics 305 (1999) 141-152
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C
GORE RANGE
BLUE RIVER WILLIAMS
BLUE RIVER
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Fig. 3. Cross-sections indicated in Fig. I; no vertical exaggeration. Symbols for rock units are the same as for Fig. I; dashed lines
suggest stratal relationships at depth. Scale is 2 times that of Fig. I.
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Laramide movement, with a right-lateral component
along the north-striking part of the thrust system and
a left-lateral component on the Kerber Creek thrust.
Sales (1983) suggested that the Tertiary normal
faults along the Sangre de Cristo fault system merge
at depth with the older thrusts, which generally
steepen downward, a geometry based, in part, on
laboratory modeling experiments. Deep seismic reflection studies resolve neither the orientation nor
the curvature of the Sangre de Cristo fault system,
although modeling suggests that the most likely orientation is about 60°. to a depth of about 6 km (Kluth
and Schaftenaar, 1994). Using this angle, a graphical
method reported by Kluth and Schaftenaar (1994)
indicates that the depth of detachment (flattening)
is at about 16 km, the depth of the brittle-ductile
transition zone estimated from heat-flow studies.
3.2. Villa Grove accommodation zone
VanAlstine (1968) suggested that the upper
Arkansas River graben is structurally continuous
with the San Luis basin, but westward stratal dip
of Miocene rocks and the down-to-the-east normal
fault along the west side of the upper Arkansas River
graben dies out just north of Poncha Pass (Fig. 1),
the topographic gap between the San Luis basin and
the upper Arkansas River graben. The area surrounding Poncha Pass is complexly faulted and contains
Proterozoic rocks that are exposed across virtually
....
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K.S. Kellogg I Tectonophysics 305 (19<)<) 1-11-152
:·~~~.~;.~·'fr~h:,+~~;'!I"'·~~the entire rift. South of Poncha Pass, stratal dip is
.
.
"."~ ·?~;';-l':~#~'~.-··~;!east and major normal faulting is on the east side
-:''''':';~':''<~';'~'~~''';!'''.
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the San Luis basin ' :
alona
the Sanare
de Cristo
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. -:fault zone (compare Fig. 3A and 38), The reversal in
.. ",
' ~~;stratal dip across Poncha Pass is about 20 krn north
~':of the Villa Grove accommodation zone (VGAZ in
,1:Fig. 1) of Chapin and Cather (1994), a proposed
. ?":northeast-trending crustal-scale stI1lctural lineament
\ :or zone of weakness across which stratal tilt suppos:~:: edly reverses, Despite the 20-krn relocation in the
,~~ present study, the name 'Villa Grove accommodation
:": zone' is retained,
.~~ A Laramide transition zone between the strongly
,~!; west-directed Sawatch uplift and the east-directed
. ~~ San Luis uplift is approximately coincident with
~~ the Villa Grove accommodation zone. The northern
' .. '.: .;' :"):;\' .~.~ limit of east-directed thrusts adjacent to the San Luis
.' ,"':>'.;:" :'!\.},';/ ..., basin is about ~~ krn south of the accommodation
'. ~. :" ·'2.(.j:' .:' , zone, at the position of the Kerber Creek thrust. The
....:',: )1: ·,:·/~f·::~:'·~tl s?uthern limit of west-di:ec~ed thrusting on the west
r .. ·•• ! \~\ot~~;i~'::'1:~':'.'~ Side of the Sawatch uplIft IS not clear, but appears
. ,:,,~.:.~':,:..;: .. I:-/.~Aj to be at about the same latitude as the Villa Grove
l~ accommodation zone.
.. ' ~ ',,' ".\'" ~ ;>",1
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;:( 3.3. The Sawatch lIplift
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The Laramide Sawatch uplift (Sawatch anticline
of Tweto, 1975) is a large north-plunging arch of
. ~:l Proterozoic rock, the east flank of which, from the
~)t crest of the Mosquito Range eastward into South
'r'
\,J Park, is overlain by relatively undeformed, mostly
J,- ~
flat-lying late Paleozoic and Mesozoic rocks (Tweto,
1979). In contrast, most of the west side of the uplift contains a complex of steep, discontinuous faults
which, near Aspen, is called the Castle Creek fault
zone (Bryant, 1979). Total displacement across the
fault zone is about 3600-4300 m, and beds adjacent to the zone dip steeply west or are overturned.
Faults dip both east and west and are characteristically discontinuous along strike, although many
of the faults are clearly east-dipping thrust or reverse faults (Spurr, 1898; Bryant, 1979). Tweto et al.
(1978) interpreted most of the faults along the west
side of the Sawatch uplift to be normal faults, including those of the Castle Creek fault zone. However,
intrusive relationships with the fault zone indicate
that the faults formed during the Laramide orogeny
(Bryant, 1979), suggesting that the zone is contrac-
;~
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.',
1~7
tional rather than extensional. Recent mapping along
the west side of the Sawatch uplift. north of the
Castle Creek fault zone, has demonstrated the contractional character of the west margin of the uplift
(Wallace and Blaskowski, 1989). In addition, the
steeply dipping to overturned beds adjoining the uplift on the west characterize a large footwall syncline
structure, similar to those commonly mapped adjacent to and beneath Laramide basement-involved
thrusts (e.g., Kellogg et aI., 1995). Inferred northward migration of the Colorado Plateau may have
added a component of dextral strike slip along the
fault zone (Wawrzyniec and Geissman, 1995), although supportive data are inconclusive .
Numerous Laramide and younger, mostly felsic
plutons that lie along the northeast-trending Colorado mineral belt intrude the Sawatch uplift. A
large gravity low coincides with the belt and indicates that the plutons form the apophoses of a large,
mostly hidden plutonic complex (Tweto, 1975).
The northwest-striking, southwest-directed Elk
Range thrust crops out west of the Castle Creek
fault zone (Fig. 1). The thrust, largely a beddingplane structure in Permian and Pennsylvanian rocks,
cuts rocks as young as Cretaceous (Bryant, 1979);
total transport distance is at least 5-6 krn. Bryant
(1979) suggested that the Late Cretaceous or Early
Paleocene Elk Range thrust formed by gravity sliding off the Sawatch uplift. Alternatively, although the
Elk Range thrust is oblique to the west margin of the
Sawatch uplift, it is geometrically permissible that
the thrust roots beneath the large west-directed basement block that forms the Sawatch uplift (Fig. 3B).
To the northwest, contraction along the Elk Range is
transferred to a west- to southwest-directed blind
thrust system that underlies the Grand Hogback
monocline. The blind thrust is likely an out-of-basin
thrust involving a basement wedge (Perry and Grout,
1988).
3.4. The IIpper Arkansas River graben
The north-striking upper Arkansas River Valley
occupies the northernmost major graben in the Rio
Grande rift system and splits the east-central portion of the Sawatch uplift. Although complicated in
detail, the graben contains a west-dipping basin-fill
sequence (Upper Miocene and Lower Pliocene Dry
-----------~
148
K.S. Kellogg / Tectonophysics 305 (1999) 141-152
Union Formation) that is truncated on the west side
by a large 'master' normal fault with at least 3000 m
vertical offset (Knepper, 1976) (Fig. 3B). Resistivity
and gravity data (Tweto and Case. 1972) also indicate that the bedrock surface beneath the Dry Union
Formation near Buena Vista slopes steeply westward
and is truncated by the large east-dipping normal
fault (Tweto. 1978) (Fig. 3B).
The east side of the graben contains a number of
down-to-the-west normal faults that step up almost
to the crest of the Mosquito Range. Many of these
faults have a Laramide ancestry, as shown by their relationship to dated Laramide intrusive rocks (Tweto,
1979). "These west-dipping faults originated as antithetic faults in the flank of the Laramide Sawatch
anticline, and they were reactivated in Neogene time
to serve as elements of the upper Arkansas Valley
graben structure." (Tweto, 1979, p. 44.)
3.5. The Gore and Front Range uplifts
The Gore Range is a narrow, rugged, uplifted
block of Proterozoic rock bounded on the west by the
Gore fault and on the east by the Blue River normal
fault (Fig. 1). The Blue River Valley follows a 5 to 9
km wide graben that is floored mostly by Upper Cretaceous shale (Tweto et aI., 1978; Kellogg, 1997) and
separates the Gore Range from the western side of
the Front Range. The Williams Range thrust bounds
the graben on the east and is a low-angle to nearly
horizontal structure that probably steepens farther
to the east under the Front Range block, where it
presumably overlies basement rock (Fig. 3C). The
Williams Range thrust defines the western structural
boundary of the Front Range, which, on the east, is
bounded by high-angle Laramide contractional faults
(Fig. 1). Erslev (1993) suggested that the Williams
Range thrust is a back thrust in an overall eastdirected Laramide thrust system, which implies that
that Front Range is completely detached.
The Gore fault has had a long geologic history,
the details of which are somewhat controversial. It
was active during Proterozoic, late Paleozoic, and
Late Cretaceous-early Tertiary time (Tweto, 1979)
and approximately defines the eastern structural margin of the late Paleozoic central Colorado trough
(Fig. 3C). This trough collected a thick, eastwardthinning clastic assemblage, banked against the east-
em margin of the trough and shed westward from the
Ancestral Front Range uplift (DeVoto. 1980). The
uplift began during early Pennsylvanian time and
created a region that remained topographically high
until Late Jurassic time; sedimentary rocks older
than Jurassic are missing from an area that includes
most of the present Front Range. Gore Range. Park
Range. Middle Park and the northern Blue River
graben (Tweto. 1975). Tweto et al. (1970) interpreted the Gore fault. on the west side of the Gore
Range. as a normal fault. but a more recent interpretation suggests that it represents major Pennsylvanian and Early Permian west-directed contractional
faulting that accompanied uplift of the Ancestral
Front Range (Ye et al.. 1996). Evidence is lacking
for major Laramide movement along the Gore fault
(Tweto and Lovering, 1977), although Snyder (1980)
documented major west-directed Laramide thrusting
along the western margin of the Park Range (northern continuation of the Gore Range). The complex
pattern of faulting and the steep to overturned eastern
margin of the central Colorado trough mirrors that on
the west side of the Sawatch uplift, all of which supports a model for both late Paleozoic and Laramide
contractional movement along the Gore fault.
3.6. The Blue River graben and western Middle Park
Although deep sediment-filled basins of the Rio
Grande rift end at the northern end of the upper
Arkansas River graben, major Neogene faults related
to Rio Grande rifting extend for a considerable distance farther north. The Neogene Blue River normal
fault, along the east side of the Gore Range (Fig. 3C).
is a major element of the Rio Grande rift system that
underwent several thousand meters of down-to-theeast movement, probably no later than the Pliocene
(West, 1978). In addition, numerous north-striking
normal faults in the Blue River graben are almost
entirely east-dipping (Kellogg. 1997), indicating that
the Blue River graben is a west-tilted structure. The
valley, however, contains little Neogene basin-fill deposits, indicating that the drainage basin formed by
the graben was neither structurally deep nor closed
to the north, where the Blue River empties into the
Colorado River near Kremmling (Fig. 1).
The western Middle Park basin (Fig. I) also
forms a west-tilted Neogene half graben, filled by
K.S. KelluggI Tectonuphvsics 305 (1999) 1·J/-152
......
"
- l
~I
. ,",
.
.',
:
~ ~ I.
: ~~. ,..
'.
.,
-
~
... ,
".'
.. ,'-'
,
";,1iocene basin-fill deposits of the Troublesome Fora sequence of tuffaceous siltstones, sand~tones, and conglomerates (lzett, 1975). On the west,
o1he down-to-the-east Antelope Pass nonnal fault
:)ounds the Middle Park basin and approximately
';'ollows the trace of the Williams Range thrust.
~pation,
•. Discussion
•. ",
~"
'l ,.
":'./
OJ
~.
At several localities along the northern Rio
'Grande rift there appeoars to be an association between Neogene nonnal faults and Laramide thrust
.'faults, strongly suggesting that the two types of
faults are genetically linked. The model presented
. here for the development of Tertiary basins along the
, 'crests of earlier thrust-bounded uplifts explains this
:.'association; the San Luis basin and its relationship
_H:.~with the San Juan-San Luis uplift is an excellent ex...'iample (Fig. 3A). In addition, the near coincidence of
:,;the Villa Grove accommodation zone, where stratal
::1 tilt reverses direction, and the boundary between the
.i)0" Sawatch uplift and the San Juan-San Luis uplift,
,~J where thrust-transport direction also switches direc.,:, tion, does not seem merely fortuitous. The Villa
: Grove accommodation zone appears to exploit a pre; existing zone of crustal weakness that is at least as
;' old as Laramide. It is worth noting, although not
,i discussed in this paper, that similar structural precursors may have also existed for other accommodation
': zones (the Embudo, Santa Ana, Tijeras, and Socorro
:< accommodation zones of Chapin and Cather, 1994)
., farther south along the Rio Grande rift (Russell
" and Snelson, 1994; S.M. Cather, written commun.,
1998).
As applied to the Sawatch uplift, the model
0'
r does not as clearly define the relationship between
Laramide and Neogene structures as it does for the
San Juan-San Luis uplift. About 30 km separates
the west margin of the uplift and the upper Arkansas
River graben, a distance considerably wider than
other recognized perched basement wedges. The
western margin of the Sawatch uplift is not universally interpreted as a contractional margin (e.g.
Tweto et aI., 1978), although evidence presented
here strongly suggests that it is contractional. If true,
faults along the west margin of the Sawatch uplift,
possibly including the Elk Range thrust, represent
.Y
0
.
.-
~
,.' ... '
',:' ,,'
O· .
1.+9
the surface traces of a listric west-directed fault system that underlies the Sawatch uplift and accommodated Laramide crustal shortening, eastward tilting.
and uplift. The Dry Union Fonnation in the upper
Arkansas River graben dips toward both the master
nonnal fault along the west side of the valley and
the steep, thrust-faulted side of the Sawatch uplift
(Fig.3B).
The upper Arkansas River graben lies astride the
central part of the Sawatch uplift. The relatively large
distance between the valley and the west side of the
uplift, where the model would predict that the basement overhang would sag into the adjacent sedimentary basin, is difficult to reconcile. Possibly, arching
and gravitational spreading (extension) concurrent
with Laramide intrusive activity under the Sawatch
uplift may have contributed to extensional strain as
well as facilitated movement along thrusts, thereby
pennitting the wide 'perched basement wedge:
A complicated relationship also exists between
the Gore Range-Front Range uplift and the Blue
River graben, but nonetheless indicates that the
graben inherited its geometry from older contractional structures. The Gore fault, along the west
side of the Gore Range, is not entirely a Laramide
structure; indeed, most of the movement along the
fault may be late Paleozoic in age. Nonetheless, the
Blue River graben tilts strongly west and sits above
a west-directed thrust system, suggesting that the
nonnal faults sole just as easily into thrusts that
are partly Paleozoic in age as they do into thrusts
that are exclusively Laramide. Irrespective of the age
of the Gore fault, westward tilting of the southern
Blue River graben and western Middle Park basin
was accommodated along nonnal faults, interpreted
as listric, that sole into the thrust (Fig. 3C). The
Gore Range represents a 'perched basement wedge'
bounded on the west by the Gore thrust or reverse
fault and on the east by the Blue River nonnal fault.
Similarly, westward rotation of the western Middle
Park basin resulted by movement along the Antelope
Pass nonnal fault, which may merge at depth with
the Gore thrust.
East-directed, commonly blind Laramide thrusts
that probably also have a late Paleozoic ancestry also
bound the east margin of the Front Range uplift,
although a Neogene rift valley associated with this
margin did not fonn. Apparently, crustal extension
150
K.S. Kellogg/Tectonophysics 305 (1999) 141-152
coeval with Rio Grande rifting did not extend to the
e:lst margin of the Front Range uplift.
5. Conclusions
.' ;
.,
.,
This study documents a style of deformation that
is characteristic, not only of the northern Rio Grande
rift. but also of large parts of the Rocky Mountain
region of North America. Future studies may judge
whether there are analogues throughout the world.
The major conclusions of this study are summarized
as follows.
(1) An empirical association between the sense of
asymmetry of several basins of the northern Rio
Grande rift and the sense of asymmetry of the
Laramide uplift on which the rift basin sits suggests a genetic link. The down-rotated side of each
basin appears on the same side as the thrust-faulted
side of the uplift. The thrust faults that bound the
uplifts are interpreted to be listric and flatten in the
middle crust at about 15-16 km.
(2) The Villa Grove accommodation zone bounds
the down-to-the-east San Luis basin portion of the
Rio Grande rift and the down-to-the-west upper
Arkansas River graben, and also appears to define
a zone that separates the east-vergent San Juan-San
Luis arch from the west-vergent Sawatch uplift.
(3) Fault relationships on the west side of the
Sawatch uplift suggest that a west-directed Laramide
thrust, defined, in part, by the trace of the Castle
Creek fault zone, underlies the uplift. The upper
Arkansas River graben lies astride the Sawatch uplift
and tilts west into a large 'master' normal fault.
(4) The Blue River graben and western Middle
Park basin lie astride the Front Range-Gore Range
uplift and tilt to the west against large east-facing
normal faults. Major late Paleozoic movement along
the thrust on the west side of the Gore Range, associated with uplift of the ancestral Rocky Mountains,
reactivated during the Laramide.
(5) The above relationships suggest a model, similar to one first summarized by Schmidt et a\. (1984)
and expanded upon by Kellogg et al. (1995), in
which rotational sag of a thrust-bounded basement
overhang into an adjacent sedimentary basin produced zones of extension in the arched basement
block. During subsequent crustal extension, the al-
ready-extended zones in the arched central part of
the fault blocks underwent gravitational collapse,
producing an asymmetrical graben along the axial
part of the arch. The master normal fault controlling the down thrown side of the graben roots into the
older thrust. The resultant 'perched basement wedge'
is bounded on one side by a thrust and on the other
side by a younger normal fault.
Acknowledgements
I am indebted to Bruce Bryant, S.M. Cather, E.A.
Erslev, S. Haldeway, D.H. Knepper, D.R. Lageson,
S.A Minor, R.B. Scott, and AR. Wallace for very
helpful reviews. Most of these reviewers also provided invaluable discussions, both in the field and
office.
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