News Release

New articles for Geosphere posted online in October

Peer-Reviewed Publication

Geological Society of America

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 .

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.
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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.
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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.
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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.
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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.
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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.
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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.
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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.
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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.
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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.
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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.
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