Rebecca M. Flowers

Unroofing, incision, and uplift history of the southwestern Colorado Plateau from apatite (U-Th)/He thermochronometry

The source of buoyancy for the uplift of cratonic plateaus is a fundamental question in continental dynamics. The ~1.9 km uplift of the Colorado Plateau since the Late Cretaceous is a prime example of this problem. We used apatite (U-Th)/He thermochronometry (230 analyses; 36 samples) to provide the fi rst single-system, regional-scale proxy for the unroofi ng history of the southwestern quadrant of the plateau. The results confi rm overall southwest to northeast unroofi ng, from plateau margin to plateau interior. A single phase of unroofi ng along the plateau margin in Late Cretaceous to Early Tertiary (SevierLaramide) time contrasts with multiphase unroofi ng of the southwestern plateau interior in Early and mid- to Late Tertiary time. The Early Cretaceous was characterized by northeastward tilting and regional erosion, followed by aggradation of ≥1500 m of Upper Cretaceous sediments along the eroded plateau margin. Sevier-Laramide denudation affected the entire southwestern plateau, was concentrated along the plateau margin, and migrated from northwest to southeast. Following a period of relative stability of the landscape from ca. 50‐30 Ma, signifi cant unroofi ng of the southwestern plateau interior occurred between ca. 28 and 16 Ma. Additional denudation north of the Grand Canyon took place in latest Tertiary time. Mid-Tertiary dates from the Grand Canyon basement at the bottom of the Upper Granite Gorge limit signifi cant incision of the modern Grand Canyon below the Kaibab surface to 1500 m difference in vertical structural position. If these models are correct, they indicate that a “proto‐Grand Canyon” of kilometer-scale depth had incised post-Paleozoic strata by the Early Eocene. Evidence for kilometerscale mid-Tertiary relief in northeast-fl owing drainages along the plateau margin, as well as the mid-Tertiary episode of plateau interior unroofi ng, imply that the southwestern plateau interior had attained substantial elevation by at least 25‐20 Ma, if not much earlier. These observations are inconsistent with any model calling for exclusively Late Tertiary uplift of the southwestern plateau. Sevier-Laramide plateau surface uplift and incision thus result from one or more processes that enhanced the buoyancy of the plateau lithosphere, expanding the Cordillera’s orogenic highlands into its lowstanding cratonic foreland. The onset of the Laramide slab’s demise at ca. 40 Ma and the major pulse of extension in the Basin and Range from ca. 16‐10 Ma appear to have had little infl uence on the denudation history of the southwestern plateau. In contrast, the post-Laramide unroofi ng episodes may be explained by drainage adjustments induced by rift-related lowering of regions adjacent to the plateau, without the need to otherwise modify the plateau lithosphere. Our data do not preclude a large component of post‐Early Eocene elevation gain (or the geodynamic mechanisms it may imply), but they do point toward Laramide-age buoyancy sources as the initial cause of signifi cant surface uplift, ending more than 500 m.y. of residence near sea level.

Radiation damage control on apatite (U-Th)/He dates from the Grand Canyon region, Colorado Plateau

Individual detrital apatite grains from the Esplanade, Coconino, and Moenkopi Formations in the Grand Canyon region of the Colorado Plateau yield (U-Th)/He dates from 104 to 5 Ma. The range of dates within each unit far exceeds analytical uncertainty, but correlates with both He concentration [He] and effective U concentration [eU]. These dates are all signifi cantly younger than the sandstone units, indicating partial to complete He loss following deposition. Recently published laboratory diffusion data suggest that He retentivity in apatite increases with radiation damage. Forward models predict that the consequences of this effect will be manifested most clearly as a correlation between (U-Th)/He dates and the [He] and [eU] in suites of apatites that (1) are characterized by a large span of [eU], and (2) had thermal histories in which suffi cient time elapsed for the apatite He diffusion kinetics to diverge prior to reheating and partial resetting. Apatites in the sedimentary units investigated fi t these cri teria. Using geologically reasonable deposition, burial, and unroofi ng histories, simulations that include the effect of radiation damage on apatite He retentivity can reproduce the observed distributions of apatite dates and correlations with parent and daughter concentrations. These results suggest that a span of (U-Th)/He dates positively correlated with [eU] may provide important information regarding a sample’s thermal history.

(U-Th)/He thermochronometry constraints on unroofing of the eastern Kaapvaal craton and significance for uplift of the southern African Plateau

The timing and causes of the >1.0 km elevation gain of the southern African Plateau since Paleozoic time are widely debated. We report the first apatite and titanite (U-Th)/He thermochronometry data for southern Africa to resolve the unroofing history across a classic portion of the major escarpment that encircles the plateau. The study area encompasses ∼1500 m of relief within Archean basement of the Barberton Greenstone Belt region of the eastern Kaapvaal craton. Titanite dates are Neoproterozoic. Apatite dates are Cretaceous, with most results clustering at ca. 100 Ma. Thermal history simulations confirm Mesozoic heating followed by accelerated cooling in mid- to Late Cretaceous time. The lower temperature sensitivity of the apatite (U-Th)/He method relative to previous thermochronometry in southern Africa allows tighter constraints on the Cenozoic thermal history than past work. The data limit Cenozoic temperatures east of the escarpment to ≤35 °C, and appear best explained by temperatures within a few degrees of the modern surface temperature. These results restrict Cenozoic unroofing to less than ∼850 m, and permit negligible erosion since the Cretaceous. If substantial uplift of the southern African Plateau occurred in the Cenozoic as advocated by some workers, then it was not responsible for the majority of post-Paleozoic unroofing across the eastern escarpment. Significant Mesozoic unroofing is coincident with large igneous province activity, kimberlite magmatism, and continental rifting within and along the margins of southern Africa, compatible with a phase of plateau elevation gain due to mantle buoyancy sources associated with these events.

Apatite 4He/3He and (U-Th)/He Evidence for an Ancient Grand Canyon

A Grand Old Canyon In the southwestern United States, the Grand Canyon is a striking example of the power of erosion over time. Over millions of years, flowing river water carved out the canyon that today measures over 1.6 km deep and 29 km long. Most models posit that the majority of the canyon formed 5 to 6 million years ago. Using thermochronometry, Flowers and Farley (p. 1616, published online 29 November) examined the temperature-dependent diffusion of helium within mineral grains representative of the canyon basement, which cools as erosion brings crustal rocks near the surface. After validating the approach across the younger eastern canyon, the model suggests that the western canyon experienced an ancient cooling event induced by erosional processes, such that the canyon likely reached near modern depths by 70 million years ago—nearly 60 million years earlier than generally believed. The Colorado River carved the Grand Canyon to nearly its modern depth 60 million years earlier than was generally believed. The Grand Canyon is one of the most dramatic features on Earth, yet when and why it was carved have been controversial topics for more than 150 years. Here, we present apatite 4He/3He thermochronometry data from the Grand Canyon basement that tightly constrain the near-surface cooling history associated with canyon incision. 4He/3He spectra for eastern Grand Canyon apatites of differing He date, radiation damage, and U-Th zonation yield a self-consistent cooling history that substantially validates the He diffusion kinetic model applied here. Similar data for the western Grand Canyon provide evidence that it was excavated to within a few hundred meters of modern depths by ~70 million years ago (Ma), in contrast to the conventional model in which the entire canyon was carved since 5 to 6 Ma.

Tempo of burial and exhumation within the deep roots of a magmatic arc, Fiordland, New Zealand

The U-Pb thermochronology of titanite, apatite, and rutile from a crustal profile through a Mesozoic magmatic arc in Fiordland, New Zealand, is used to constrain the timing and duration of significant vertical movements during arc construction and evolution. Titanite data from deep-crustal (12-13 kbar) basement and cover rocks of central Fiordland imply that contractional arc thickening (;25 km) occurred by 111.1-113.4 Ma, within a few million years of a major phase of mid- to deep-crustal magmatism. This finding suggests that this cycle of magmatism, arc thickening, and high-grade metamorphism occurred in #6.2 m.y. In contrast to rapid burial, significant unroofing of the central Fiordland gran- ulites was more protracted, requiring an additional 40-45 m.y. These new data are con- sistent with continued residence of the granulites in thickened arc crust for 15-20 m.y., with subsequent major unroofing recorded by rutile cooling to ,450 8C by ca. 70 Ma. Such temporal constraints are essential for comprehensive models of the growth, modifi- cation, and unroofing of magmatic arc systems.

Low long-term erosion rates and extreme continental stability documented by ancient (U-Th)/He dates

Zircon and apatite crystals from the western Canadian shield yield (U-Th)/He dates that are the oldest yet reported for terrestrial rocks. Zircon dates from 1.73 to 1.58 Ga are consistent with independent geological and thermochronological constraints, and indicate that the rocks were at temperatures ≤180 °C and crustal depths ≤7–10 km since ca. 1.7 Ga. Apatite dates from 0.95 to 0.55 Ga suggest residence of rocks at temperatures ≤40–50 °C and crustal depths ≤1.5–2 km for ∼1.0–0.6 b.y., when interpreted using conventionally accepted apatite He diffusion kinetics and considering the proposed effect of radiation damage on apatite He retentivity. Our analysis implies long-term integrated unroofing rates of ≤2.5 μm/yr since ca. 1.7 Ga. These rates are significantly lower than the long-term rates suggested by previous thermochronological data sets in continental interiors, but are within the range of short-term erosional estimates. The results are consistent with the extreme stability of this region since the Proterozoic.

Kimberlite (U-Th)/He dating links surface erosion with lithospheric heating, thinning, and metasomatism in the southern African Plateau

Documenting the surface response to processes at depth is a central challenge in continental tectonics. Kimberlites bear an unusual record of both surface and deep processes through their downrafted crustal clasts, pipe erosion levels, and mantle xenolith suites. Here we combine new apatite (U-Th)/He (AHe) data from four South Africa kimberlites ranging in emplacement age from ca. 143 Ma to ca. 74 Ma with a wealth of other geologic information from the pipes to resolve the timing, patterns, and causes of erosion across ∼200 km of the southern African Plateau. Comparison of the AHe dates with the kimberlite eruption ages constrains a strong regional unroofing pulse ca. 117–90 Ma, which was most intense from ca. 100 to 90 Ma. This unroofing phase is synchronous with warming, thinning, and metasomatism of the mantle lithosphere recorded by xenocrysts and peridotite xenoliths within these same kimberlites. These thermochemical changes could be related to still deeper dynamic mantle processes. Our AHe data appear to directly document the erosional response to surface uplift induced by these various events. The AHe results also detect as much as 1.5 km of Cenozoic unroofing that is spatially centered on a proposed mid-Tertiary paleotributary to the Orange River, suggesting drainage patterns as the cause. Our results not only tightly refine the surface history of the interior of the southern African Plateau, but show that kimberlite AHe dating can tie cratonic erosion phases with contemporaneous thermochemical changes in the mantle and thereby point to causative links.

Evolution of the Amphibolite‐Granulite Facies Transition Exposed by the Vredefort Impact Structure, Kaapvaal Craton, South Africa

Geological mapping is integrated with high‐precision U‐Pb zircon and monazite geochronology to document in detail the nature and temporal evolution of the amphibolite‐granulite facies boundary exposed in the Vredefort impact structure of the central Kaapvaal Craton. Our study has revealed that the preimpact configuration of the amphibolite‐granulite facies boundary is best preserved in panels between impact‐related brittle faults and that past concepts of the boundary must be revised. A regional lithologic transition exists between amphibolite facies Outer Granite Gneiss (3.084–3.09 Ga) and a heterogeneous assemblage of granulite facies gneisses (ca. 3.1–3.3 Ga). Tabular deformed quartz syenite bodies, dated at \documentclass{aastex} \usepackage{amsbsy} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{bm} \usepackage{mathrsfs} \usepackage{pifont} \usepackage{stmaryrd} \usepackage{textcomp} \usepackage{portland,xspace} \usepackage{amsmath,amsxtra} \usepackage[OT2,OT1]{fontenc} \newcommand\cyr{ \renewcommand\rmdefault{wncyr} \renewcommand\sfdefault{wncyss} \renewcommand\encodingdefault{OT2} \normalfont \selectfont} \DeclareTextFontCommand{\textcyr}{\cyr} \pagestyle{empty} \DeclareMathSizes{10}{9}{7}{6} \begin{document} \landscape

(U-Th)/He thermochronologic constraints on the evolution of the northern Rio Grande Rift, Gore Range, Colorado, and implications for rift propagation models

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