Andres Aslan

Mantle-driven dynamic uplift of the Rocky Mountains and Colorado Plateau and its surface response: Toward a unified hypothesis

The correspondence between seismic velocity anomalies in the crust and mantle and the differential incision of the continental-scale Colorado River system suggests that significant mantle-to-surface interactions can take place deep within continental interiors. The Colorado Rocky Mountain region exhibits low-seismic-velocity crust and mantle associated with atypically high (and rough) topography, steep normalized river segments, and areas of greatest differential river incision. Thermochronologic and geologic data show that regional exhumation accelerated starting ca. 6–10 Ma, especially in regions underlain by low-velocity mantle. Integration and synthesis of diverse geologic and geophysical data sets support the provocative hypothesis that Neogene mantle convection has driven long-wavelength surface deformation and tilting over the past 10 Ma. Attendant surface uplift on the order of 500–1000 m may account for ∼25%–50% of the current elevation of the region, with the rest achieved during Laramide and mid-Tertiary uplift episodes. This hypothesis highlights the importance of continued multidisciplinary tests of the nature and magnitude of surface responses to mantle dynamics in intraplate settings.

Middle Holocene Sea-Level Rise and Highstand at +2 M, Central Texas Coast

New data suggest a revised picture of middle Holocene sea-level change for the Texas Gulf of Mexico coast, and suggest reevaluation of coastal evolution. First, brackish marsh facies with calibrated radiocarbon ages of 7.7 to 7.8 ka have been recovered from depths of −8.5 to −9 m in a core from the ancestral Colorado River delta, and are interpreted to represent a sea-level pinning point. Second, a series of ridges along the Copano Bay margin farther south consist of shelly mud and fine sand with subtidal foram assemblages, occur at elevations of 1.95 m above the modern intertidal zone, and have produced calibrated radiocarbon ages on foram tests of ca. 6.8 to 4.8 ka. These ridges are interpreted to represent relict shallow subtidal to intertidal spits that provide minimum sea-level positions for the middle Holocene, and are now emergent because of later sea-level fall. In aggregate, these data show rates of sea-level rise during this time period that are very comparable to, or even lower than, published eustatic rates, and suggest a middle Holocene sea-level highstand for this non-uplifting, non-rebounding, and very slowly subsiding part of the North American coastline.

Holocene evolution of the western Orinoco Delta, Venezuela

The pristine nature of the Orinoco Delta of eastern Venezuela provides unique opportunities to study the geologic processes and environments of a major tropical delta. Remote-sensing images, shallow cores, and radiocarbon-dating of organic remains form the basis for describing deltaic environments and interpreting the Holocene history of the delta. The Orinoco Delta can be subdivided into two major sectors. The southeast sector is dominated by the Rio Grande—the principal distributary—and complex networks of anastomosing fluvial and tidal channels. The abundance of siliciclastic deposits suggests that fluvial processes such as overbank flooding strongly influence this part of the delta. In contrast, the northwest sector is represented by few major distributaries, and overbank sedimentation is less widespread relative to the southeast sector. Peat is abundant and occurs in herbaceous and forested swamps that are individually up to 200 km 2 in area. Northwest-directed littoral currents transport large volumes of suspended sediment and produce prominent mudcapes along the northwest coast. Mapping of surface sediments, vegetation, and major landforms identified four principal geomorphic systems within the western delta plain: (1) distributary channels, (2) interdistributary flood basins, (3) fluvial-marine transitional environments, and (4) marine-influenced coastal environments. Coring and radiocarbon dating of deltaic deposits show that the northern delta shoreline has prograded 20–30 km during the late Holocene sea-level highstand. Progradation has been accomplished by a combination of distributary avulsion and mudcape progradation. This style of deltaic progradation differs markedly from other deltas such as the Mississippi where distributary avulsion leads to coastal land loss, rather than shoreline progradation. The key difference is that the Orinoco Delta coastal zone receives prodigious amounts of sediment from northwest-moving littoral currents that transport sediment from as far away as the Amazon system (∼1600 km). Late Holocene progradation of the delta has decreased delta-plain gradients, increased water levels, and minimized overbank flooding and siliciclastic sedimentation in the northwest sector. These conditions, coupled with large amounts of direct precipitation, have led to widespread peat accumulation in interdistributary basins. Because peat-forming environments cover up to 5000 km 2 of the delta plain, the Orinoco may be an excellent analogue for interpreting ancient deltaic peat deposits.

New incision rates along the Colorado River system based on cosmogenic burial dating of terraces: Implications for regional controls on Quaternary incision

New cosmogenic burial and published dates of Colorado and Green river terraces are used to infer variable incision rates along the rivers in the past 10 Ma. A knickpoint at Lees Ferry separates the lower and upper Colorado River basins. We obtained an isochron cosmogenic burial date of 1.5 ± 0.13 Ma on a 190-m-high strath terrace near Bullfrog Basin, Utah (upstream of Lees Ferry). This age yields an average incision rate of 126 +12/–10 m/Ma above the knickpoint and is three times older than a cosmogenic surface age on the same terrace, suggesting that surface dates inferred by exposure dating may be minimum ages. Incision rates below Lees Ferry are faster, ∼170 m/Ma–230 m/Ma, suggesting upstream knickpoint migration over the past several million years. A terrace at Hite (above Lees Ferry) yields an isochron burial age of 0.29 ± 0.17 Ma, and a rate of ∼300–900 m/Ma, corroborating incision acceleration in Glen Canyon. Within the upper basin, isochron cosmogenic burial dates of 1.48 ± 0.12 Ma on a 60 m terrace near the Green River in Desolation Canyon, Utah, and 1.2 ± 0.3 Ma on a 120 m terrace upstream of Flaming Gorge, Wyoming, give incision rates of 41± 3 m/Ma and 100 +33/–20 m/Ma, respectively. In contrast, incision rates along the upper Colorado River are 150 m/Ma over 0.64 and 10 Ma time frames. Higher incision rates, gradient, and discharge along the upper Colorado River relative to the Green River are consistent with differential rock uplift of the Colorado Rockies relative to the Colorado Plateau.

Mud volcanoes of the Orinoco Delta, Eastern Venezuela

Abstract Mud volcanoes along the northwest margin of the Orinoco Delta are part of a regional belt of soft sediment deformation and diapirism that formed in response to rapid foredeep sedimentation and subsequent tectonic compression along the Caribbean–South American plate boundary. Field studies of five mud volcanoes show that such structures consist of a central mound covered by active and inactive vents. Inactive vents and mud flows are densely vegetated, whereas active vents are sparsely vegetated. Four out of the five mud volcanoes studied are currently active. Orinoco mud flows consist of mud and clayey silt matrix surrounding lithic clasts of varying composition. Preliminary analysis suggests that the mud volcano sediment is derived from underlying Miocene and Pliocene strata. Hydrocarbon seeps are associated with several of the active mud volcanoes. Orinoco mud volcanoes overlie the crest of a mud-diapir-cored anticline located along the axis of the Eastern Venezuelan Basin. Faulting along the flank of the Pedernales mud volcano suggests that fluidized sediment and hydrocarbons migrate to the surface along faults produced by tensional stresses along the crest of the anticline. Orinoco mud volcanoes highlight the proximity of this major delta to an active plate margin and the importance of tectonic influences on its development. Evaluation of the Orinoco Delta mud volcanoes and those elsewhere indicates that these features are important indicators of compressional tectonism along deformation fronts of plate margins.

Denudation and flexural isostatic response of the Colorado Plateau and southern Rocky Mountains region since 10 Ma

Over the past 10 Ma, the high-relief landscapes of the Colorado Plateau–southern Rocky Mountains region have been shaped by erosional processes. Incision rates have increased in the southern Rocky Mountains, the Colorado River system has been superimposed across buried Laramide structures as it was integrated from the Rocky Mountains to the Gulf of California, the modern Grand Canyon formed, and there has been widespread denudation of the Canyonlands region of the Colorado Plateau. We examine the spatial and temporal distribution of erosion and its associated isostatic rebound since 10 Ma. Erosion estimates come from apatite fission track (AFT) and apatite (U-Th)/He (AHe) thermochronometric studies at 14 sites across the region, including recent AHe data with ages younger than 12 Ma, and from ca. 10 Ma 40 Ar/ 39 Ar dated basalt paleosurfaces at 55 locations on the perimeter of the Colorado Plateau and in the southern Rocky Mountains. Estimated eroded thickness is added to modern topography above numerous control points to reconstruct a 10 Ma paleosurface across the region (referenced to modern elevations); this also yields an eroded thickness volume. Erosion has been spatially variable since 10 Ma: we find widespread denudation with as much as 2 km of incision along rivers in the Canyonlands region of Utah, 1–1.5 km of incision along rivers exiting the Rocky Mountains onto the eastern piedmont since 6 Ma, ∼1 km removed across the high peaks of the southern Rocky Mountains since 10 Ma, and little net erosion in the Basin and Range. Post–10 Ma flexural isostatic response to the eroded volume is calculated using known variable elastic thickness. This rebound caused much of the Colorado Plateau region to undergo more than 800 m of rock uplift, exceeding 1 km in local areas in the Canyonlands and southwestern Colorado. The Lees Ferry and Glen Canyon areas have been isostatically uplifted >500 m relative to the eastern Grand Canyon and the Tavaputs Plateau has been isostatically uplifted 400 m relative to Browns Park. This differential rock uplift driven by erosional isostasy has created or accentuated many of the features of the modern landscape. This component of rock uplift is “removed” by adding the eroded thickness onto modern topography, then subtracting the calculated rebound. The resulting (pre-erosion and pre-rebound) map provides a model of the 10 Ma landscape, neglecting any tectonic uplift contribution to regional elevations. This model suggests the presence of internal drainages on the Colorado Plateau, that the elevation of the Green River Basin and the Tavaputs Plateau were subequivalent, allowing the Green River to flow southward, and shows high topography in the Rocky Mountains that mimicked modern topography, but with potentially lower relief. Future refinements of both the timing and magnitude of differential erosion and rebound models provide an avenue for improved models for Cenozoic landscape evolution of the region. This paper is an advance over previous studies that focused just on the Colorado Plateau. Here we evaluate isostatic response to erosion in an extended region that includes parts of the Basin and Range, Colorado Plateau, southern Rocky Mountains, and eastern piedmont of the Rocky Mountains. We find that erosion of the southern Rocky Mountains and eastern piedmont is comparable to that of the Colorado Plateau and that the flexural isostatic rebounds of all these regions are coupled and cannot be considered in isolation. Furthermore, we focus on the 10 Ma time frame, rather than the 30 or 70 Ma period of previous researchers, as the key time frame during which the modern landscape rapidly evolved. In addition, the use of AFT and AHe thermochronometric constraints on thicknesses and ages of now-eroded sediments has solved key problems that hampered previous erosion studies. Data and analyses of regional post–10 Ma differential erosion and its resulting differential isostatic rebound provide essential constraints for any viable models for landscape evolution in this classic region.

Detrital zircon U-Pb provenance of the Colorado River: A 5 m.y. record of incision into cover strata overlying the Colorado Plateau and adjacent regions

New detrital zircon U-Pb age distributions from 49 late Cenozoic sandstones and Holocene sands (49 samples, n = 3922) record the arrival of extra-regional early Pliocene Colorado River sediment at Grand Wash (western USA) and downstream locations ca. 5.3 Ma and the subsequent evolution of the river’s provenance signature. We define reference age distributions for the early Pliocene Colorado River (n = 559) and Holocene Colorado River (n = 601). The early Pliocene river is distinguished from the Holocene river by (1) a higher proportion of Yavapai-Mazatzal zircon derived from Rocky Mountain basement uplifts relative to Grenville zircon from Mesozoic supra crustal rocks, and (2) distinctive (∼6%) late Eocene–Oligocene (40–23 Ma) zircon reworked from Cenozoic basins and volcanic fields in the southern Rocky Mountains and/or the eastern Green River catchment. Geologic relationships and interpretation of 135 published detrital zircon age distributions throughout the Colorado River catchment provide the interpretative basis for modeling evolution of the provenance signature. Mixture modeling based upon a modified formulation of the Kolmogorov-Smirnov statistic indicate a subtle yet robust change in Colorado River provenance signature over the past 5 m.y. During this interval the contribution from Cenozoic strata decreased from ∼75% to 50% while pre-Cretaceous strata increased from ∼25% to 50%. We interpret this change to reflect progressive erosional incision into plateau cover strata. Our finding is consistent with geologic and thermochronologic studies that indicate that maximum post–10 Ma erosion of the Colorado River catchment was concentrated across the eastern Utah–western Colorado region.

Incision history of the Black Canyon of Gunnison, Colorado, over the past ∼1 Ma inferred from dating of fluvial gravel deposits

Spatio-temporal variability in fluvial incision rates in bedrock channels provides data regarding uplift and denudation histories of landscapes. The longitudinal profile of the Gunnison River (Colorado), tributary to the Colorado River, contains a prominent knickzone with 800 m of relief across it within the Black Canyon of the Gunnison. Average bedrock incision rates over the last 0.64 Ma surrounding the knickpoint vary from 150 m/Ma (downstream) to 400–550 m/Ma (within) to 90–95 m/Ma (upstream), suggesting it is a transient feature. Lava Creek B ash constrains strath terraces along a paleoprofile of the river. An isochron cosmogenic burial date in the paleo–Bostwick River of 870 ± 220 ka is consistent with the presence of 0.64 Ma Lava Creek B ash in locally derived, stratigraphically younger sediment. With 350 m of incision since deposition, we determine an incision rate of 400–550 m/Ma, reflecting incision through resistant basement rock at 2–3 times regional incision rates. Such contrast is attributed to a wave of transient incision, potentially initiated by downstream base-level fall during abandonment of Unaweep Canyon at ca. 1 Ma. Rate extrapolation indicates that the ∼700 m depth of Black Canyon has been eroded since 1.3–1.75 Ma. The Black Canyon knickpoint overlies a strong gradient between low-velocity mantle under the Colorado Rockies and higher-velocity mantle of the Colorado Plateau. We interpret recent reorganization and transient incision of both the Gunnison River and upper Colorado River systems to be a response to mantle-driven epeirogenic uplift of the southern Rockies in the last 10 Ma.

Late Miocene erosion and evolution of topography along the western slope of the Colorado Rockies

In the Colorado Rocky Mountains, the association of high topography and low seismic velocity in the underlying mantle suggests that recent changes in lithospheric buoyancy may have been associated with surface uplift of the range. This paper examines the relationships among late Cenozoic fluvial incision, channel steepness, and mantle velocity domains along the western slope of the northern Colorado Rockies. New 40 Ar/ 39 Ar ages on basalts capping the Tertiary Browns Park Formation range from ca. 11 to 6 Ma and provide markers from which we reconstruct incision along the White, Yampa, and Little Snake rivers. The magnitude of post–10 Ma incision varies systematically from north to south, increasing from ∼500 m along the Little Snake River to ∼1500 m along the Colorado River. Spatial variations in the amount of late Cenozoic incision are matched by metrics of channel steepness; the upper Colorado River and its tributaries (e.g., Gunnison and Dolores rivers) are two to three times steeper than the Yampa and White rivers, and these variations are independent of both discharge and lithologic substrate. The coincidence of steep river profiles with deep incision suggests that the fluvial systems are dynamically adjusting to an external forcing but is not readily explained by a putative increase in erosivity associated with late Cenozoic climate change. Rather, channel steepness correlates with the position of the channels relative to low-velocity mantle. We suggest that the history of late Miocene–present incision and channel adjustment reflects long-wavelength tilting across the western slope of the Rocky Mountains.

Introduction: CRevolution 2: Origin and Evolution of the Colorado River System II

A 2010 Colorado River symposium held in Flagstaff, Arizona, in May 2010, had 70 participants who engaged in intense debate about the origin and evolution of the Colorado River system. This symposium, built on two previous decadal scientific meetings, focused on forging scientific consensus where possible, while also articulating continued controversies regarding the Cenozoic evolution of the Colorado River System and the landscapes of the Colorado Plateau–Rocky Mountain region that it drains. New developments involved hypotheses that Neogene mantle flow is driving plateau tilting and differential uplift, with consensus that multidisciplinary studies involving differential incision studies and additional geochronology and thermochronology are needed to test the relative importance of tectonic and geomorphic forcings in shaping the spectacular landscapes of the Colorado Plateau region. In addition to the scientific goals, the meeting participants emphasized the iconic status of Grand Canyon for geosciences, and the importance of good communication between the research community, the geoscience education/interpretation community, the public, and the media. Building on a century-long tradition, this region still provides a globally important natural laboratory for studies of the interactions of erosion and tectonism in the shaping landscape of elevated plateaus.