Geology topics

Paul O'Sullivan

Publications and source records attributed to Paul O'Sullivan.

6 recordsLinked to original sources

USGS2022

U-Pb and fission-track data from zircon and apatite resolve latest- and post-Alleghanian thermal histories along the Fall Line of the Atlantic margin of the southeastern United States

Although the Atlantic continental margin of the eastern United States is an archetypal passive margin, episodes of rejuvenation following continental breakup are increasingly well documented. To better constrain this history of rejuvenation along the southern portion of this continental margin, we present zircon U-Pb (ZUPb) age, zircon fission-track (ZFT) age, apatite U-Pb (AUPb) age, and apatite fission-track (AFT) age and length data from six bedrock samples. The samples were collected along the boundary between the exposed Appalachian hinterland (Piedmont province) and the updip limit of passive margin strata (Coastal Plain province). The samples were collected from central Virginia southward to the South Carolina–Georgia border. ZUPb age distributions are generally consistent with geologic mapping in each of the sample areas. The AUPb data are highly discordant owing to high common-Pb abundances, but for two plutons at the northern and southern ends of the sample area, they define a discordia regression line that indicates substantial Permo-Triassic exhumation-driven cooling. ZFT age distributions are highly dispersed but define central values ranging from Permian to Jurassic. AFT data mostly appear to define a singular underlying cooling age, generally approximately Jurassic or Early Cretaceous. Apatite fission tracks are moderately long (mean lengths in the range of ~13.5 µm), however track lengths for one sample in central North Carolina are shorter (~12.5 µm). To interpret the post-breakup thermal history, we present inverse models of time-temperature history for the five plutonic samples. The models show a history of (1) rapid cooling (>10 °C/m.y.) from deep-crustal to near-surface temperatures by the Triassic, (2) hundreds of degrees of Triassic reheating, (3) Jurassic–Early Cretaceous cooling (at rates of 1–10 °C/m.y.), and (4) slow Late Cretaceous–Cenozoic cooling (~1 °C/m.y.). An additional suite of forward models is presented to further evaluate the magnitude of maximum Triassic reheating at one sample site that is particularly well constrained by thermal maturity data. The model results and geologic reasoning suggest that the inverse models may overestimate Triassic paleotemperatures but that other aspects of the inverse modeling are robust. Overall, this thermal history can be reconciled with several aspects of the lithostratigraphy of distal parts of the continental margin, including the lack of Jurassic–earliest Cretaceous strata beneath the southern Atlantic coastal plain and Cretaceous–Cenozoic grain-size trends.

Geosphere↗

USGS2022

The role of preexisting upper plate strike-slip faults during long-lived (ca. 30 Myr) oblique flat slab subduction, southern Alaska

Upper plates of subduction zones commonly respond to flat slab subduction by structural reactivation, magmatic arc disruption, and foreland basin inversion. However, the role of active strike-slip faults in focusing convergent deformation and magmatism in response to oblique flat slab subduction remains less clear. Here, we present new detrital apatite fission-track (dAFT) ages from 12 modern catchments in the eastern Alaska Range, Alaska, USA, to reveal how the dextral Denali fault system has facilitated bedrock exhumation and topographic growth during ca. 30 Ma-to-present oblique flat slab subduction of the Yakutat oceanic plateau. Additionally, a 940 ka ( 40 Ar/ 39 Ar whole rock) basalt flow is spatially associated with Cenozoic structures, locally reset AFT ages and provides the first evidence for Quaternary volcanism along the southern flank of the eastern Alaska Range. We integrate our new data with other thermochronologic, geochronologic, and regional geologic datasets to show that (1) most high topography regions in southern Alaska have undergone rapid bedrock cooling and exhumation since ca. 30 Ma; (2) elevated terrain and young cooling are spatially associated with long-lived active strike-slip fault systems; (3) topographic growth associated with strike-slip fault deformation led to local inversion of basin systems and drainage reorganization; (4) the onset of oblique oceanic plateau subduction is coeval with a southward shift in arc magmatism from one region of active strike-slip faulting to another above the northeastern edge of the flat slab; and (5) Quaternary volcanism marks the revival of magmatism in the eastern Alaska Range above the geophysically imaged northeastern edge of the flat slab. Our analysis of the post-30 Ma geologic evolution of southern Alaska demonstrates that strike-slip fault systems that were active at the time of slab flattening evolved into transpression zones that focused bedrock cooling, rock exhumation, and topographic growth.

Alaska↗

USGS2018

Late Cretaceous-Cenozoic exhumation of the western Brooks Range, Alaska, revealed from apatite and zircon fission track data

We report data for 112 apatite and 31 zircon fission track (AFT and ZFT) outcrop sandstone samples along a transect that spans the western Brooks Range. Sampling targeted structures that modify the Middle Jurassic‐Early Cretaceous early Brookian orogen. The AFT samples record latest Cretaceous to Eocene in situ exhumational cooling and resolve two kinematic phases. The first phase was focused at 65–60 Ma. To the north, cooling age patterns at this time are attributable to wide‐spaced fault‐related folding. Farther south, within the allochthon belt, exhumation was related to uplift of a broad region, likely in the hanging wall of deep‐seated faults that extend into basement rocks. The second kinematic phase occurred around ~45 Ma. It was characterized by north and east directed thrusting to the north, and coeval extension in the allochthon belt to the south. The ZFT cooling ages are all Early Cretaceous or older and put an upper limit on the magnitude of Cenozoic exhumation across the western Brooks Range. Synthesis of exhumation patterns and structural styles show that Paleocene rejuvenation of contraction was roughly contemporaneous along the entire ~1,000‐km orogen. Later, around ~45 Ma in the Eocene, contraction in the frontal parts of the orogen was contemporaneous with extension interior to the orogen. Following previous authors, we suggest that the Paleocene rejuvenation was a far‐field response to subduction of a mid‐ocean ridge in southern Alaska. However, by the Eocene, strain patterns in the western Brooks Range changed, possibly to accommodate rotations of fault blocks in southwestern Alaska.

Alaska↗

USGS2016

Changing exhumation patterns during Cenozoic growth and glaciation of the Alaska Range: Insights from detrital thermochronology and geochronology

Cenozoic growth of the Alaska Range created the highest topography in North America, but the space-time pattern and drivers of exhumation are poorly constrained. We analyzed U/Pb and fission-track double dates of detrital zircon and apatite grains from 12 catchments that span a 450 km length of the Alaska Range to illuminate the timing and extent of exhumation during different periods. U/Pb ages indicate a dominant Late Cretaceous to Oligocene plutonic provenance for the detrital grains, with only a small percentage of grains recycled from the Mesozoic and Paleozoic sedimentary cover. Fission-track ages record exhumation during Alaska Range growth and incision and reveal three distinctive patterns. First, initial Oligocene exhumation was focused in the central Alaska Range at ~30 Ma and expanded outward along the entire length of the range until 18 Ma. Oligocene exhumation, coeval with initial Yakutat microplate collision >600 km to the southeast, suggests a far-field response to collision that was localized by the Denali Fault within a weak Mesozoic suture zone. Second, the variable timing of middle to late Miocene exhumation suggests independently evolving histories influenced by local structures. Time-transgressive cooling ages suggest successive rock uplift and erosion of Mounts Foraker (12 Ma) through Denali (6 Ma) as crust was advected through a restraining bend in the Denali Fault and indicate a long-term slip rate ~4 mm/yr. Third, Pliocene exhumation is synchronous (3.7–2.7 Ma) along the length of the Alaska Range but only occurs in high-relief, glacier-covered catchments. Pliocene exhumation may record an acceleration in glacial incision that was coincident with the onset of Northern Hemisphere glaciation.

Alaska↗

USGS2011

Constraining the age and magnitude of uplift in the northern National Petroleum Reserve in Alaska (NPRA): Apatite fission-track analysis of samples from three wells

A broad, post-mid-Cretaceous uplift is defined in the northern National Petroleum Reserve in Alaska (NPRA) by regional truncation of Cretaceous strata, thermal maturity patterns, and amounts of exhumation estimated from sonic logs. Apatite fission-track (AFT) analysis of samples from three wells (South Meade No. 1, Topagoruk No. 1, and Ikpikpuk No. 1) across the eastern flank of the uplift indicates Tertiary cooling followed by Quaternary heating. Results from all three wells indicate that cooling, presumably caused by uplift and erosion, started about 75–65 Ma (latest Cretaceous–earliest Tertiary) and continued through the Tertiary Period. Data from South Meade indicate more rapid cooling after about 35–15 Ma (latest Eocene–middle Miocene) followed by a significant increase in subsurface temperature during the Quaternary, probably the result of increased heat flow. Data from Topagoruk and Ikpikpuk include subtle evidence of accelerated cooling starting in the latest Eocene–middle Miocene and possible evidence of increased temperature during the Quaternary. Subsurface temperature perturbations related to the insulating effect of permafrost may have been responsible for the Quaternary temperature increase at Topagoruk and Ikpikpuk and may have been a contributing factor at South Meade. Multiple lines of geologic evidence suggest that the magnitude of exhumation resulting from uplift and erosion is 5,000–6,500 ft at South Meade, 4,000–5,500 ft at Topagoruk, and 2,500–4,000 ft at Ikpikpuk. The results from these wells help to define the broad geometry of the uplift, which increases in magnitude from less than 1,000 ft at the Colville River delta to perhaps more than 7,000 ft along the northwestern coast of NPRA, between Point Barrow and Peard Bay. Neither the origin nor the offshore extent of the uplift, west and north of the NPRA coast, have been determined.

Alaska↗

USGS2009

Detrital zircon geochronology of Cretaceous and Paleogene strata across the south-central Alaskan convergent margin

Ages of detrital zircons are reported from ten samples of Lower Cretaceous to Paleogene metasandstones and sandstones from the Chugach Mountains, Talkeetna Mountains, and western Alaska Range of south-central Alaska. Zircon ages are also reported from three igneous clasts from two conglomerates. The results bear on the regional geology, stratigraphy, tectonics, and mineral resource potential of the southern Alaska convergent margin. Chugach Mountains - The first detrital zircon data are reported here from the two main components of the Chugach accretionary complex - the inboard McHugh Complex and the outboard Valdez Group. Detrital zircons from sandstone and two conglomerate clasts of diorite were dated from the McHugh Complex near Anchorage. This now stands as the youngest known part of the McHugh Complex, with an inferred Turonian (Late Cretaceous) depositional age no older than 91-93 Ma. The zircon population has probability density peaks at 93 and 104 Ma and a smattering of Early Cretaceous and Jurassic grains, with nothing older than 191 Ma. The two diorite clasts yielded Jurassic U-Pb zircon ages of 179 and 181 Ma. Together, these findings suggest a Mesozoic arc as primary zircon source, the closest and most likely candidate being the Wrangellia composite terrane. The detrital zircon sample from the Valdez Group contains zircons as young as 69 and 77 Ma, consistent with the previously assigned Maastrichtian to Campanian (Late Cretaceous) depositional age. The zircon population has peaks at 78, 91, 148, and 163 Ma, minor peaks at 129, 177, 330, and 352 Ma, and no concordant zircons older than Devonian. A granite clast from a Valdez Group conglomerate yielded a Triassic U-Pb zircon age of 221 Ma. Like the McHugh Complex, the Valdez Group appears to have been derived almost entirely from Mesozoic arc sources, but a few Precambrian zircons are also present. Talkeetna Mountains - Detrital zircons ages were obtained from southernmost metasedimentary rocks of the Talkeetna Mountains (schist of Hatcher Pass) and, immediately to the south, the northernmost sedimentary sequence of the Matanuska forearc basin (Arkose Ridge Formation). Detrital zircons from the Paleogene Arkose Ridge Formation are as young as 61 and 70 Ma; the population is dominated by a single Late Cretaceous peak at 76 Ma; the oldest zircon is 181 Ma. Sedimentological evidence clearly shows that the conglomeratic Arkose Ridge Formation was derived from the Talkeetna Mountains; our detrital zircon data support this inference. Zircons dated at ca. 90 Ma in the Arkose Ridge sample suggest that buried or unmapped plutons of this age may exist in the Talkeetnas. This is a particularly interesting age as it corresponds to the age of the supergiant Pebble gold-molybdenum-copper porphyry prospect near Iliamna and suggests a new area of prospectivity for Pebble-type deposits. The schist of Hatcher Pass, which was previously assigned a Jurassic depositional age, yielded surprisingly young Late Cretaceous detrital zircons, the youngest at 75 Ma. The probability density curve has four Cretaceous peaks from 76 to 102 Ma, a pair of Late Jurassic peaks at 155 and 166 Ma, three Early Jurassic to Late Triassic peaks at 186, 197, and 213 Ma, minor Carboniferous peaks at 303 and 346 Ma, and a minor Paleoproterozoic peak at 1828 Ma. The schist of Hatcher Pass was largely derived from Mesozoic arc sources, most likely the Wrangellia composite terrane, with some contribution from one or more older, inboard sources, probably including the Yukon-Tanana terrane. We postulate that the schist of Hatcher Pass represents metamorphosed rocks of the Valdez Group that were subducted and then exhumed along the Chugach terrane's 'backstop' during Paleogene transtension. Western Alaska Range - Six detrital zircon samples were collected from a little studied belt of turbidites in Tyonek quadrangle on strike with the Kahiltna assemblage of the central Alaska Range.

Alaska↗