New Articles for Geosphere Posted Online in October
Boulder, Colo., USA: GSA’s dynamic online journal, Geosphere,
posts articles online regularly. Locations and topics studied this month
include the Coast Mountains batholith, British Columbia; the Basin and
Range/Rio Grande rift transition; the Ulungarat Basin of Arctic Alaska; the
Surprise Valley landslide complex, Grand Canyon, Arizona; and the Sevier
foreland basin, Utah. You can find these articles at
https://geosphere.geoscienceworld.org/content/early/recent
.
Jurassic–Cenozoic tectonics of the Pequop Mountains, NE Nevada, in the
North American Cordillera hinterland
Andrew V. Zuza; Christopher D. Henry; Seth Dee; Charles H. Thorman; Matthew
T. Heizler
Abstract:
The Ruby Mountains–East Humboldt Range–Wood Hills–Pequop Mountains (REWP)
metamorphic core complex, northeast Nevada, exposes a record of Mesozoic
contraction and Cenozoic extension in the hinterland of the North American
Cordillera. The timing, magnitude, and style of crustal thickening and
succeeding crustal thinning have long been debated. The Pequop Mountains,
comprising Neoproterozoic through Triassic strata, are the least deformed
part of this composite metamorphic core complex, compared to the migmatitic
and mylonitized ranges to the west, and provide the clearest field
relationships for the Mesozoic–Cenozoic tectonic evolution. New field,
structural, geochronologic, and thermochronological observations based on
1:24,000-scale geologic mapping of the northern Pequop Mountains provide
insights into the multi-stage tectonic history of the REWP. Polyphase
cooling and reheating of the middle-upper crust was tracked over the range
of <100 °C to 450 °C via novel 40Ar/39Ar
multi-diffusion domain modeling of muscovite and K-feldspar and apatite
fission-track dating. Important new observations and interpretations
include: (1) crosscutting field relationships show that most of the
contractional deformation in this region occurred just prior to, or during,
the Middle-Late Jurassic Elko orogeny (ca. 170–157 Ma), with negligible
Cretaceous shortening; (2) temperature-depth data rule out deep burial of
Paleozoic stratigraphy, thus refuting models that incorporate large cryptic
overthrust sheets; (3) Jurassic, Cretaceous, and Eocene intrusions and
associated thermal pulses metamorphosed the lower Paleozoic–Proterozoic
rocks, and various thermochronometers record conductive cooling near
original stratigraphic depths; (4) east-draining paleovalleys with ~1–1.5
km relief incised the region before ca. 41 Ma and were filled by 41–39.5 Ma
volcanic rocks; and (5) low-angle normal faulting initiated after the
Eocene, possibly as early as the late Oligocene, although basin-generating
extension from high-angle normal faulting began in the middle Miocene.
Observed Jurassic shortening is coeval with structures in the
Luning-Fencemaker thrust belt to the west, and other strain documented
across central-east Nevada and Utah, suggesting ~100 km Middle-Late
Jurassic shortening across the Sierra Nevada retroarc. This phase of
deformation correlates with terrane accretion in the Sierran forearc,
increased North American–Farallon convergence rates, and enhanced Jurassic
Sierran arc magmatism. Although spatially variable, the Cordilleran
hinterland and the high plateau that developed across it (i.e., the
hypothesized Nevadaplano) involved a dynamic pulsed evolution with
significant phases of both Middle-Late Jurassic and Late Cretaceous
contractional deformation. Collapse long postdated all of this contraction.
This complex geologic history set the stage for the Carlin-type gold
deposit at Long Canyon, located along the eastern flank of the Pequop
Mountains, and may provide important clues for future exploration.
View article:
https://pubs.geoscienceworld.org/gsa/geosphere/article-abstract/doi/10.1130/GES02307.1/609081/Jurassic-Cenozoic-tectonics-of-the-Pequop
Deformation between the highly oblique Yakutat–North American plate
boundary and the Eastern Denali fault
Eva Enkelmann; Sarah Falkowski
Abstract:
This study investigates the spatial and temporal pattern of rock exhumation
inboard of the highly oblique Yakutat–North American plate boundary. We aim
to quantify how far deformation is transferred inboard of the Fairweather
transform plate boundary and across the Eastern Denali fault. We present
new detrital apatite and zircon fission track data from 27 modern drainages
collected on both sides of the Eastern Denali fault and from the Alsek and
Tatshenshini River catchments that drain the mountainous region between the
Fairweather fault and the Eastern Denali fault. By integrating our data
with published bedrock and detrital geochronology and thermochronology, we
show that exhumation reaches much farther inboard (>100 km) of the
Fairweather fault than farther north in the St. Elias syntaxial region
(<30 km). This suggests that the entire corridor between the Fairweather
and Eastern Denali faults exhumed since mid-Miocene time. The Eastern
Denali fault appears to be the backstop, and late Cenozoic exhumation
northeast of the fault is very limited.
View article:
https://pubs.geoscienceworld.org/gsa/geosphere/article-abstract/doi/10.1130/GES02410.1/609080/Deformation-between-the-highly-oblique-Yakutat
Zircon (U-Th)/(He-Pb) double-dating constraints on the interplay
between thrust deformation and foreland basin architecture, Sevier
foreland basin, Utah
E.J. Pujols; D.F. Stockli
Abstract:
The Cretaceous Cordilleran foreland basin strata exposed in the Book Cliffs
of eastern Utah and western Colorado have motivated important concepts
linking thrust belt deformation and foreland basin evolution largely on the
basis of sequence stratigraphy, stratal architecture, and sediment
provenance evolution. However, these methods and approaches generally
cannot provide critical insights into the temporal or causal linkages
between foreland basin architecture and thrust belt deformation. This is in
part due to discrepancies in age resolution and lack of evidence with which
to directly couple sediment supply and basin-fill evolution to thrust belt
unroofing. New detrital zircon (DZ) geothermochronometric data from Upper
Cretaceous proximal to distal foreland basin strata in the Book Cliffs
provide new quantitative insights into sediment origin and dispersal in
relation to thrust belt deformation and exhumation. Detailed DZ U-Pb and
(U-Th)/He double dating reveals that the Book Cliffs foredeep detritus was
mainly delivered by transverse routing systems from two major sources: (1)
Neoproterozoic and Lower Paleozoic strata from the central Utah Sevier
thrust belt, and (2) Permian–Jurassic and synorogenic Cretaceous strata
recycled from the frontal part of the thrust belt. A dramatic increase in
Sierran magmatic arc and Yavapai-Mazatzal DZ U-Pb ages, as well as
Paleozoic DZ He ages, in the deeper marine portions of the foreland basin
points to axial fluvial and littoral sediment input from the Sierran
magmatic arc and Mogollon highland sources. Both transverse and axial
transport systems acted contemporaneously during eastward propagation of
the Late Cretaceous thrust belt. DZ He depositional lag time estimates
reveal three distinct exhumation pulses in the Sevier thrust belt in the
Cenomanian and Campanian. The exhumation pulses correlate with shifts in
sediment provenance, dispersal style, and progradation rates in the
foreland basin. These new data support conceptual models that temporally
and causally link accelerated exhumation and unroofing in the thrust belt
to increases in sediment supply and rapid clastic progradation in the
foreland basin.
View article:
https://pubs.geoscienceworld.org/gsa/geosphere/article-abstract/doi/10.1130/GES02372.1/609079/Zircon-U-Th-He-Pb-double-dating-constraints-on-the
Mantle control on magmatic flare-ups in the southern Coast Mountains
batholith, British Columbia
M. Robinson Cecil; George E. Gehrels; Margaret E. Rusmore; Glenn J.
Woodsworth; Harold H. Stowell ...
Abstract:
The southern Coast Mountain batholith was episodically active from Jurassic
to Eocene time and experienced four distinct high magmatic flux events
during that period. Similar episodicity has been recognized in arcs
worldwide, yet the mechanism(s) driving such punctuated magmatic behavior
are debated. This study uses zircon Hf and O isotopes, with whole-rock and
mineral geochemistry, to track spatiotemporal changes in southern Coast
Mountains batholith melt sources and to evaluate models of flare-up
behavior and crust formation in Cordilleran arc systems. Zircon Hf isotope
analysis yielded consistently primitive values, with all zircon grains
recording initial εHf between +6 and +16. The majority (97%) of zircons
analyzed yielded δ18O values between 4.2‰ and 6.5‰, and only
five grains recorded values of up to 8.3‰. These isotopic results are
interpreted to reflect magmatism dominated by mantle melting during all
time periods and across all areas of the southern batholith, which argues
against the periodic input of more melt-fertile crustal materials as the
driver of episodic arc magmatism. They also indicate that limited crustal
recycling is needed to produce the large volumes of continental crust
generated in the batholith. Although the isotopic character of intrusions
is relatively invariant through time, magmas emplaced during flare-ups
record higher Sr/Y and La/Yb(N) and lower zircon Ti and Yb
concentrations, which is consistent with melting in thickened crust with
garnet present as a fractionating phase. Flare-ups are also temporally
associated with periods when the southern Coast Mountains batholith both
widens and advances inboard. We suggest that the landward shift of the arc
into more fertile lithospheric mantle domains triggers voluminous magmatism
and is accompanied by magmatic and/or tectonic thickening. Overall, these
results demonstrate that the magmatic growth of Cordilleran arcs can be
spatially and temporally complex without requiring variability in the
contributions of crust and/or mantle to the batholith.
View article:
https://pubs.geoscienceworld.org/gsa/geosphere/article-abstract/doi/10.1130/GES02361.1/608294/Mantle-control-on-magmatic-flare-ups-in-the
Thermochronological transect across the Basin and Range/Rio Grande rift
transition: Contrasting cooling histories in contiguous extensional
provinces
Michelle M. Gavel; Jeffrey M. Amato; Jason W. Ricketts; Shari Kelley;
Julian M. Biddle ...
Abstract:
The Basin and Range and Rio Grande rift (RGR) are regions of crustal
extension in southwestern North America that developed after Laramide-age
shortening, but it has not been clear whether onset and duration of
extension in these contiguous extensional provinces were the same. We
conducted a study of exhumation of fault blocks along a transect from the
southeastern Basin and Range to across the RGR in southern New Mexico. A
suite of 128 apatite and 63 zircon (U-Th)/He dates (AHe and ZHe), as well
as 27 apatite fission-track (AFT) dates, was collected to investigate the
cooling and exhumation histories of this region. Collectively, AHe dates
range from 3 to 46 Ma, ZHe dates range from 2 to 288 Ma, and AFT dates
range from 10 to 34 Ma with average track lengths of 10.8–14.1 µm.
First-order spatiotemporal trends in the combined data set suggest that
Basin and Range extension was either contemporaneous with Eocene–Oligocene
Mogollon-Datil volcanism or occurred before volcanism ended ca. 28 Ma, as
shown by trends in ZHe data that suggest reheating to above 240 °C at that
time. AHe and ZHe dates from the southern RGR represent a wider range in
dates that suggest the main phase of cooling occurred after 25 Ma, and
these blocks were not reheated after exhumation. Time-temperature models
created by combining AHe, AFT, and ZHe data in the modeling software HeFTy
were used to interpret patterns in cooling rate across the study area and
further constrain magmatic and/or volcanic versus faulting related cooling.
The Chiricahua Mountains and Burro Mountains have an onset of rapid
extension, defined as cooling rates in excess of >15 °C/m.y., at ca.
29–17 Ma. In the Cookes Range, a period of rapid extension occurred at ca.
19–7 Ma. In the San Andres Mountains, Franklin Mountains, Caballo
Mountains, and Fra Cristobal range, rapid extension occurred from ca. 23 to
9 Ma. Measured average track lengths are longer in Rio Grande rift samples,
and ZHe dates of >40 Ma are mostly present east of the Cookes Range,
suggesting different levels of exhumation for the zircon partial retention
zone and the AFT partial annealing zone. The main phase of fault-block
uplift in the southern RGR occurred ca. 25–7 Ma, similar to what has been
documented in the northern and central sections of the rift. Although rapid
cooling occurred throughout southern New Mexico, thermochronological data
from this study with magmatic and volcanic ages suggest rapid cooling was
coeval with magmatism in the Basin and Range, whereas in the Rio Grande
rift cooling occurred during an amagmatic gap. These observations support a
model where an early phase of extension was facilitated by widespread
ignimbrite magmatism in the southeastern Basin and Range, whereas in the
southern Rio Grande rift, extension started later and continues today and
may have occurred between local episodes of basaltic magmatism. These
differences in cooling history make the Rio Grande rift tectonically
distinct from the Basin and Range. We infer based on geologic and
thermochronological evidence that the onset of extension in the southern
Rio Grande rift occurred at ca. 27–25 Ma, significantly later than earlier
estimates of ca. 35 Ma.
View article:
https://pubs.geoscienceworld.org/gsa/geosphere/article-abstract/doi/10.1130/GES02381.1/608293/Thermochronological-transect-across-the-Basin-and
Metamorphism of the Sierra de Maz and implications for the tectonic
evolution of the MARA terrane
Andrew Tholt; Sean R. Mulcahy; William C. McClelland; Sarah M. Roeske;
Vinícius T. Meira ...
Abstract:
The Mesoproterozoic MARA terrane of western South America is a composite
igneous-metamorphic complex that is important for Paleozoic paleogeographic
reconstructions and the relative positions of Laurentia and Gondwana. The
magmatic and detrital records of the MARA terrane are consistent with a
Laurentian origin; however, the metamorphic and deformation records lack
sufficient detail to constrain the correlation of units within the MARA
terrane and the timing and mechanisms of accretion to the Gondwana margin.
Combined regional mapping, metamorphic petrology, and garnet and monazite
geochronology from the Sierra de Maz of northwest Argentina sug- gest that
the region preserves four distinct litho-tectonic units of varying age and
metamorphic conditions that are separated by middle- to lower-crustal
ductile shear zones. The Zaino and Maz Complexes preserve Barrovian
metamorphism and ages that are distinct from other units within the region.
The Zaino and Maz Complexes both record metamorphism ca. 430–410 Ma and
show no evidence of the regional Famatinian orogeny (ca. 490–455 Ma). In
addition, the Maz Complex records an earlier granulite facies event at ca.
1.2 Ga. The Taco and Ramaditas Complexes, in contrast, experienced medium-
and low-pressure upper amphibolite to granulite facies metamorphism,
respectively, between ca. 470–460 Ma and were later deformed at ca. 440–420
Ma. The Maz shear zone that bounds the Zaino and Maz Complexes records
sinistral oblique to sinistral deformation between ca. 430–410 Ma. The data
suggest that at least some units in the MARA terrane were accreted by
translation, and the Gondwana margin of northwest Argentina transitioned
from a dominantly convergent margin to a highly oblique margin in the
Silurian.
View article:
https://pubs.geoscienceworld.org/gsa/geosphere/article-abstract/doi/10.1130/GES02268.1/608292/Metamorphism-of-the-Sierra-de-Maz-and-implications
Cenozoic structural evolution of the Catalina metamorphic core complex
and reassembly of Laramide reverse faults, southeastern Arizona, USA
Daniel A. Favorito; Eric Seedorff
Abstract:
This study investigates the Late Cretaceous through mid-Cenozoic
structural evolution of the Catalina core complex and adjacent areas by
integrating new geologic mapping, structural analysis, and geochronologic
data. Multiple generations of normal faults associated with mid-Cenozoic
extensional deformation cut across older reverse faults that formed during
the Laramide orogeny. A proposed stepwise, cross-sectional structural
reconstruction of mid-Cenozoic extension satisfies surface geologic and
reflection seismologic constraints, balances, and indicates that detachment
faults played no role in the formation of the core complex and Laramide
reverse faults represent major thick-skinned structures. The orientations
of the oldest synextensional strata, pre-shortening normal faults, and
pre-Cenozoic strata unaffected by Laramide compression indicate that rocks
across most of the study area were steeply tilted east since the
mid-Cenozoic. Crosscutting relations between faults and synextensional
strata reveal that sequential generations of primarily down-to-the-west,
mid- Cenozoic normal faults produced the net eastward tilting of ~60°.
Restorations of the balanced cross section demonstrate that Cenozoic normal
faults were originally steeply dipping and resulted in an estimated 59 km
or 120% extension across the study area. Representative segments of those
gently dipping faults are exposed at shallow, intermediate (~5–10 km), and
deep structural levels (~10–20 km), as distinguished by the nature of
deformation in the exhumed footwall, and these segments all restore to high
angles, which indicates that they were not listric. Offset on major normal
faults does not exceed 11 km, as opposed to tens of kilometers of offset
commonly ascribed to “detachment” faults in most interpretations of this
and other Cordilleran metamorphic core complexes. Once mid-Cenozoic
extension is restored, reverse faults with moderate to steep original dips
bound basement-cored uplifts that exhibit significant involvement of
basement rocks. Net vertical uplift from all reverse faults is estimated to
be 9.4 km, and estimated total shortening was 12 km or 20%. This magnitude
of uplift is consistent with the vast exposure of metamorphosed and
foliated cover strata in the northeastern and eastern Santa Catalina and
Rincon Mountains and with the distribution of subsequently dismembered
mid-Cenozoic erosion surfaces along the San Pedro Valley. New and existing
geochronologic data constrain the timing of offset on local reverse faults
to ca. 75–54 Ma. The thick-skinned style of Laramide shortening in the area
is consistent with the structure of surrounding locales. Because detachment
faults do not appear to have resulted in the formation of the Catalina core
complex, other extensional systems that have been interpreted within the
context of detachments may require further structural analyses including
identification of crosscutting relations between generations of normal
faults and palinspastic reconstructions.
View article:
https://pubs.geoscienceworld.org/gsa/geosphere/article-abstract/doi/10.1130/GES02313.1/608291/Cenozoic-structural-evolution-of-the-Catalina
Ulungarat Basin: Record of a major Middle Devonian to Mississippian
syn-rift to post-rift tectonic transition, eastern Brooks Range, Arctic
Alaska
Arlene V. Anderson; Kristian E. Meisling
Abstract:
The Ulungarat Basin of Arctic Alaska is a unique exposed stratigraphic
record of the mid-Paleozoic transition from the Romanzof orogeny to
post-orogenic rifting and Ellesmerian passive margin subsidence. The
Ulungarat Basin succession is composed of both syn-rift and post-rift
deposits recording this mid-Paleozoic transition. The syn-rift deposits
unconformably overlie highly deformed Romanzof orogenic basement on the
mid-Paleozoic regional angular unconformity and are unconformably overlain
by post-rift Endicott Group deposits of the Ellesmerian passive margin.
Shallow marine strata of Eifelian age at the base of the Ulungarat
Formation record onset of rifting and limit age of the Romanzof orogeny to
late Early Devonian. Abrupt thickness and facies changes within the
Ulungarat Formation and disconformably overlying syn-rift Mangaqtaaq
Formation suggest active normal faulting during deposition. The Mangaqtaaq
Formation records lacustrine deposition in a restricted down-faulted
structural low. The unconformity between syn-rift deposits and overlying
post-rift Endicott Group is interpreted to be the result of sediment bypass
during deposition of the outboard allochthonous Endicott Group. Within
Ulungarat Basin, transgressive post-rift Lower Mississippian Kekiktuk
Conglomerate and Kayak Shale (Endicott Group) are older and thicker than
equivalents to the north. North of Ulungarat Basin, deformed pre-Middle
Devonian rocks were exposed to erosion at the mid-Paleozoic regional
unconformity for ~50 m.y., supplying sediments to the rift basin and
broader Arctic Alaska rifted margin beyond. Although Middle Devonian to
Lower Mississippian chert- and quartz-pebble conglomerates and sandstones
across Arctic Alaska share a common provenance from the eroding ancestral
Romanzof highlands, they were deposited in different tectonic settings.
View article:
https://pubs.geoscienceworld.org/gsa/geosphere/article-abstract/doi/10.1130/GES02272.1/608290/Ulungarat-Basin-Record-of-a-major-Middle-Devonian
A constraint on post–6 Ma timing of western Grand Canyon (Arizona, USA)
incision removed: Local derivation indicated by ca. 5.4 Ma fluvial
deposits below Shivwits Plateau basalts north of Grand Canyon
A.T. Steelquist; G.E. Hilley; I. Lucchitta; R.A. Young
Abstract:
The timing of integration of the Colorado River system is central to
understanding the landscape evolution of much of the southwestern United
States. However, the time at which the Colorado River started incising the
westernmost Grand Canyon (Arizona) is still an unsettled question, with
conflicting interpretations of both geologic and thermochronologic data
from western Grand Canyon. Fluvial gravels on the Shivwits Plateau, north
of the canyon, have been reported to contain clasts derived from south of
the modern canyon, suggesting the absence of western Grand Canyon at the
time of their deposition. In this study, we reassess these deposits using
modern geochronologic measurements to determine the age of the deposits and
the presence or absence of clasts from south of the Grand Canyon. We could
not identify southerly derived clasts, so cannot rule out the existence of
a major topographic barrier such as Grand Canyon prior to the age of
deposition of the gravels. 40Ar/39Ar analysis of a
basalt clast entrained in the upper deposit (in combination with prior
data) supports a maximum age of deposition of ca. 5.4 Ma, limiting
deposition to post-Miocene, a period from which very few diagnostic and
dated fluvial deposits remain in the western Colorado Plateau. Analysis of
detrital zircon composition of the sand matrix supports interpretation of
the deposit as being locally derived and not part of a major throughgoing
river. We suggest that the published constraint of <6 Ma timing of Grand
Canyon incision may be removed, given that no clasts that must be sourced
from south of Grand Canyon were found in the only known outcrop of gravels
under the Shivwits Plateau basalts at Grassy Mountain north of Grand
Canyon.
View article:
https://pubs.geoscienceworld.org/gsa/geosphere/article-abstract/doi/10.1130/GES02353.1/608099/A-constraint-on-post-6-Ma-timing-of-western-Grand
Magnetostratigraphy and magnetic properties of the Jurassic to Lower
Cretaceous Girón Group (northern Andes, Colombia)
Giovanny Jiménez; Helbert García-Delgado; John W. Geissman
Abstract:
We report paleomagnetic results from the Jurassic to Lower Cretaceous
continental sedimentary succession exposed in the eastern limb of the Los
Yariguíes anticlinorium, Eastern Cordillera, Colombia. About 820 m of a
stratigraphic section of the upper part of the Girón Group (Angostura del
Río Lebrija and Los Santos Formations) was sampled to construct a magnetic
polarity stratigraphy. A total of 199 independent samples that yield
interpretable and acceptable data have a characteristic remanent
magnetization component (ChRM) isolated between 400 °C and 680 °C in
progressive thermal demagnetization. Demagnetization behavior and rock
magnetic properties are interpreted to indicate that hematite is the
principal magnetization carrier with a possible contribution by magnetite
in some parts of the section. After tilt correction, 123 samples are of
normal polarity (declination [D] = 44.9°, inclination [I]
= +9.7°, R = 110.64, k = 9.87, and α95 =
4.3°), and the other 76 accepted samples are of reverse polarity ( D = 216.4°, I = −6.1°, R = 68.29, k =
9.72, and α95 = 5.5°). The statistical reversal test conducted
on virtual geomagnetic poles is positive (class B). Based on paleontologic
age estimates for the Cumbre and Rosablanca Formations, we assume a
Berriasian age for the Los Santos Formation. The magnetostratigraphic data
from the Girón Group strata are interpreted to suggest an age for the
sampled part of the section between early Kimmeridgian and early
Valanginian (ca. 157–139 Ma). The age of the Angostura del Río Lebrija
Formation is estimated as between early Kimmeridgian and early Tithonian
(ca. 157–146.5 Ma). The age of the Los Santos Formation is estimated
between early Tithonian and early Valanginian (146.5–139.3 Ma). With our
proposed, but nonunique, correlation with the Geomagnetic Polarity Time
Scale, the Jurassic-Cretaceous boundary is interpreted to be located within
the Los Santos Formation. The Girón Group is characterized by two periods
of high (>8 cm/k.y.) and two periods of low (< 2 cm/k.y.)
sedimentation rates. An inferred clockwise rotation of ~44°, based on
paleomagnetic declination data from the Girón Group, is similar to rotation
estimates reported in some previous studies in the general area, and this
facet of deformation could be related to local and regional response to
displacement along regional-scale strike-slip faults.
View article:
https://pubs.geoscienceworld.org/gsa/geosphere/article-abstract/doi/10.1130/GES02186.1/608098/Magnetostratigraphy-and-magnetic-properties-of-the
Realignments of the Colorado River by ~2 m.y. of rotational bedrock
landsliding:
The Surprise Valley landslide complex, Grand Canyon, Arizona
Jesse E. Robertson; Karl E. Karlstrom; Matthew T. Heizler; Laura J. Crossey
Abstract:
The Surprise Valley landslide complex is the name used here for a group of
prominent river-damming landslides in Grand Canyon (Arizona, USA) that has
shifted the path of the Colorado River several times in the past 2 m.y. We
document a sequence of eight landslides. Three are Toreva-block landslides
containing back-rotated but only mildly disrupted bedrock stratigraphy. The
largest of these landslides, Surprise Valley landslide, is hypothesized to
have dammed the Colorado River, cut off a meander loop through Surprise
Valley, and rerouted the river 2.5 km south to near its present course at
the Granite Narrows. Another bedrock landslide, Poncho’s runup, involved a
mass detachment from the north side of the river that drove a
kilometer-scale bedrock slab across the river and up the south canyon wall
to a height of 823 m above the river. A lake behind this landslide is
inferred from the presence of mainstem gravels atop the slide that
represent the approximate spillway elevation. We postulate that this
landslide lake facilitated the upriver 133 Mile slide detachment and Toreva
block formation. The other five landslides are subsequent slides that
consist of debris from the primary slides; these also partially blocked and
diverted the Colorado River as well as the Deer Creek and Tapeats Creek
tributaries into new bedrock gorges over the past 1 m.y. The sequence of
landslides is reconstructed from inset relationships revealed by geologic
mapping and restored cross-sections. Relative ages are estimated by
measuring landslide base height above the modern river level in locations
where landslides filled paleochannels of the Colorado River and its
tributaries. We calculate an average bedrock incision rate of 138 m/m.y. as
determined by a 0.674 ± 0.022 Ma detrital sanidine maximum depositional age
of the paleoriver channel fill of the Piano slide, which has its base 70 m
above the river level and ~93 m above bedrock level beneath the modern
river channel. This date is within error of, and significantly refines, the
prior cosmogenic burial date of 0.88 ± 0.44 Ma on paleochannel cobbles.
Assuming steady incision at 138 m/m.y., the age of Surprise Valley
landslide is estimated to be ca. 2.1 Ma; Poncho’s runup is estimated to be
ca. 610 ka; and diversion of Deer Creek to form modern Deer Creek Falls is
estimated to be ca. 400 ka. The age of the most recent slide, Backeddy
slide, is estimated to be ca. 170 ka based on its near-river-level
position. Our proposed triggering mechanism for Surprise Valley landslides
involves groundwater saturation of a failure plane in the weak Bright Angel
Formation resulting from large volumes of Grand Canyon north-rim
groundwater recharge prior to establishment of the modern Deer, Thunder,
and Tapeats springs. Poncho’s and Piano landslides may have been triggered
by shale saturation caused by 600–650 ka lava dams that formed 45 river
miles (73 river km; river miles are measured along the Colorado River
downstream from Lees Ferry, with 1 river mile = 1.62 river kms) downstream
near Lava Falls. We cannot rule out effects from seismic triggering along
the nearby Sinyala fault. Each of the inferred landslide dams was quickly
overtopped (tens of years), filled with sediment (hundreds of years), and
removed (thousands of years) by the Colorado River, as is also the
potential fate of modern dams.
View article:
https://pubs.geoscienceworld.org/gsa/geosphere/article-abstract/doi/10.1130/GES02280.1/608097/Realignments-of-the-Colorado-River-by-2-m-y-of
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