Edmund Stump

Uplift and denudation of the central Alaska Range: A case study in the use of apatite fission track thermochronology to determine absolute uplift parameters

Apatite fission track thermochronology (AFTT) on granitic samples collected in the central Alaska Range in conjunction with geologic constraints from basins to the north (Nenana Basin) and south (Cook Inlet) of the range is used to constrain the timing, amount, rate, and pattern of surface uplift, rock uplift, and denudation since the late Miocene. The conversion from a thermal frame of reference (apatite fission track data) to an absolute frame of reference (with respect to mean sea level), which requires constraining the paleoland surface elevation, the paleomean annual temperature, and the paleogeothermal gradient, is evaluated and shown to be viable in the context of an exhumed apatite partial annealing zone (PAZ). Apatite ages at Denali (Mount McKinley) range from 16 Ma near the summit (∼6 km elevation) to 4 Ma at ∼2 km elevation. A distinctive break in slope in the apatite age profile at an elevation of 4.5 km, also marked by a change in confined track length distributions, marks the base of an exhumed apatite PAZ. Rock uplift and denudation are greatest at Denali, decreasing southward away from the McKinley strand of the Denali fault system as shown by progressively older apatite ages (7–35 Ma) from a suite of samples along the Kahiltna Glacier. A correlative decrease in topography occurs southward from the fault. The central Alaska Range lies within an arc defined by the Denali fault, with the highest peaks (including Denali) concentrated at the arc apex. Patterns of rock uplift and denudation within the central Alaska Range mimic topography. Between early and late Miocene, and possibly earlier, the central Alaska Range was most likely an area of relative tectonic and thermal stability. Rock uplift, denudation, and mean surface uplift of the Denali region began by the Late Miocene (∼5–6 Ma), being ∼8.5 km, ∼5.7 km, and ∼2.8 km, respectively, at average rates of ∼1.5 km/m.y., ∼1 km/m.y., and ∼0.5 km/m.y. The amount of rock uplift, denudation, and surface uplift decreases to ∼3 km, ∼2 km, and ∼1 km at Little Switzerland, some 45 km south of the Denali fault. We conclude that the topographic and rock uplift patterns of the central Alaska Range, the shape and proximity of the McKinley strand of the Denali Fault to these patterns, the timing of the onset of rock uplift and denudation at ∼5–6 Ma, and a significant change in relative plate motion between North America and the Pacific plates circa 5.6 Ma are all inherently related.

Cretaceous and Cenozoic episodic denudation of the Transantarctic Mountains, Antarctica : New constraints from apatite fission track thermochronology in the Scott Glacier region

Apatite fission track thermochronology utilizing vertical sampling profiles, with results interpreted using the concept of exhumed partial annealing zones, is applied in the Scott Glacier area (86°S) of the Transantarctic Mountains (TAM). Patterns in age profiles indicate that episodes of denudation in the Early Cretaceous, Late Cretaceous, and Cenozoic were separated by periods of relative tectonic stability. Thermal modeling of time-temperature histories compared to observed data indicates that denudation episodes commenced at ∼125 Ma, ∼95 Ma, and 50–45 Ma. Magnitude of denudation is constrained only as >700 m for the Early Cretaceous and from barely detectable to 1.5 km for the Late Cretaceous. Since the early Cenozoic, denudation within the TAM Front was similar in magnitude to other localities along the TAM (∼4–6 km), decreasing inland. Rock uplift was also a maximum at the coast, decreasing inland. Patterns of rock uplift and denudation are complicated by Cenozoic faulting, mostly by faults oriented ∼45° to the TAM Front. Along the length of the TAM there is an apparent systematic variation in the angle of these Cenozoic faults to the TAM Front, possibly reflecting greater components of dextral transtension southward along the TAM. The three denudation episodes correspond to regional tectonic events: Early Cretaceous southward translation of the Ellsworth-Whitmore Mountains block of West Antarctica relative to East Antarctica; Late Cretaceous extension in the Ross Embayment between East and West Antarctica; and Cenozoic rejuvenated faulting, magmatism, and deformation within the Victoria Land Basin and its presumed southward extension under the Ross Ice Sheet.

Late Cenozoic uplift of Denali and its relation to relative plate motion and fault morphology

Apatite fission-track analysis of samples that cover a 4-kilometer vertical section from the western flank of Denali (Mount McKinley), North Americas highest mountain, suggests that the mountain massif was formed by rapid uplift (> 1 kilometer per million years) beginning ∼6 million years ago (Ma). Uplift was a result of the morphology of the Denali fault and a change in motion of the Pacific plate with respect to North America at ∼5 Ma, which created opposing tangential vectors of relative movement along the fault and forced the intervening crustal blocks upward.

Episodic uplift of the Transantarctic Mountains

An apatite fission-track age profile from the Scott Glacier region provides evidence of uplift and denudation of the Transantarctic Mountains in the Early and Late Cretaceous. Samples for fission-track analysis were collected over a vertical range of ∼2 km from the Mt. Griffith massif. Apatite ages from the upper 700 m of Mt. Griffith vary little with elevation, indicative of rapid cooling accompanying Early Cretaceous uplift and denudation. Ages from the northeast buttress of Mt. Griffith (the Fission Wall) define a steep gradient and are indicative of rapid cooling accompanying Late Cretaceous uplift and denudation. The two parts of the profile are separated by a fault. Subsequent uplift and denudation of the Mt. Griffith massif in the Cenozoic were required to elevate the massif (and the apatite age profile) to its present position. This younger uplift was most likely initiated in the early Cenozoic, penecontemporaneously with welldocumented early Cenozoic uplift in the Victoria Land region of the Transantarctic Mountains. These three periods of uplift coincide with periods of major plate reorganization in the southwest Pacific region: (1) initial rifting of Australia from Antarctica and impingement of the Pacific-Phoenix ridge with the subduction zone marginal to New Zealand in the Early Cretaceous, (2) separation of Australia, New Zealand, and Antarctica in the Late Cretaceous, and (3) cessation of spreading in the Tasman Sea ∼10 m.y. prior to accelerated spreading between Australia and Antarctica in the early Cenozoic.

Reconstruction of Australia and Antarctica: evidence from granites and recent mapping

Abstract Recent mapping in both Australia and Antarctica has refined the local stratigraphies, in a number of cases resulting in revisions of age ranges with the discovery of new fossils. Granite studies have provided an expanded data base of petrological, geochemical and isotopic parameters. Using all currently available geological data, we offer a reconstruction of the two continents. The key element in the reconstruction is the alignment of three terranes in northern Victoria Land, Antarctica (Wilson, Bowers and Robertson Bay) with three terranes in western Victoria, Australia (Delamerian, Grampians-Stavely and Stawell). Numerous similarities permit correlation of the terranes across continental boundaries. The alignment produces a fit essentially like that of Sproll and Dietz [1]. In detail, mismatches of the coastlines are improved by postulating movements of two microplates. In the reconstruction, Tasmania is shifted back along the Colac-Rosedale fault to a pre-breakup position approximately 160 km east of its present location. The resultant gap is filled by moving northern Victoria Land 225 km northward along a postulated strike-slip fault at the boundary between the western scarp of the Transantarctic Mountains and the Wilkes Subglacial Basin. A model is proposed in which, at the initial stage of breakup, opening of the Tasman Sea was accompanied by westward movement of Tasmania and southward movement of northern Victoria Land, with a left-lateral, strike-slip fault at their mutual boundary. Once these two microplate fragments had reached their current positions on their parent plates, movement proceeded with the separation of Australia and Antarctica and continued opening of the Tasman Sea.

Fission-track and 40Ar39Ar evidence for episodic denudation of the Gangotri granites in the Garhwal Higher Himalaya, India

Abstract In order to document quantitatively the cooling and denudation history of the Higher Himalayan granites bordering the Tethyan sedimentary zone, fission-track (FT) apatite and 40 Ar 39 Ar mica ages have been determined on the Gangotri leucogranites and biotite granites in the Garhwal region of India. Gangotri is the source area of the Ganges River and lies in the midst of the highest Himalayan peaks in India. A total of 15 apatite ages from a vertical profile (2580–4370 m) on the Gangotri granites yields FT ages in the range of 1.5 ± 0.6 to 2.4 ± 0.5 Ma, indicating that the rock column with a relief of ∼1800 m cooled through 130 ± 10°C within only ∼1 million years during the Late Pliocene. An average denudation rate of ∼2 mm/yr is estimated for the past 2.4 million years. From the Gangotri granites, we also report a muscovite 40 Ar 39 Ar age of 17.9 ± 0.1 Ma and a biotite age of 18.0 ± 0.1 Ma. These reflect cooling of the rocks through 300–350°C, probably related to an Early Miocene pulse of denudation caused by a basement-cover detachment (the Martoli Normal Fault) above the leucogranites. Time-temperature pathways indicate that the cooling of the rocks in the Late Pliocene-Quaternary was five to six times the magnitude of cooling between 18 and 2 Ma, indicating a distinct pulse of rapid denudation in the Late Pliocene-Quaternary. We interpret these young apatite ages and fast denudation as a geomorphic response (increased erosion and cooling) of the rocks to a major pulse of tectonic uplift in the Higher Himalaya shortly before 2.4 Ma. The effect of climatic cooling on this denudation is considered secondary to the role of tectonic forcing, and indeed produced a positive feedback to the primary cause. Although our study is confined to the Garhwal region, it is probable that other granitic bodies of the Higher Himalaya bordering the Tethyan sedimentary rocks, and forming the loftiest summits in the Himalaya, have also experienced episodic denudation — one major pulse in the Early Miocene, which was mainly tectonic denudation, and another in the Late Pliocene-Quaternary, which was mainly erosional. The latter is well recorded by apatite FT data, and is consistent with the hypothesis that rapid uplift and denudation of the Himalayan rocks may have influenced the initiation of the ice ages in the northern hemisphere.

Early Miocene Subglacial Basalts, the East Antarctic Ice Sheet, and Uplift of the Transantarctic Mountains

Subglacially erupted volcanic rocks from Mount Early and Sheridan Bluff, Antarctica, yield whole-rock potassium-argon dates and argon-40/argon-39 release spectra of Early Miocene age. Field associations suggest the existence of the East Antarctic ice sheet and significant uplift of the Transantarctic Mountains by that time.

Fission track analysis of the footwall of the Catalina detachment fault, Arizona: Tectonic denudation, magmatism, and erosion

New apatite and zircon fission track ages obtained from the footwall of the Catalina metamorphic core complex record a complicated cooling history associated with mid-Tertiary extension. Zircon fission track ages record the progressive unroofing of the Catalina metamorphic core complex along the Catalina detachment fault. Zircon fission track ages range from 31.9 to 19.4 Ma, generally decrease in the hanging-wall slip direction, and yield slip rates along the Catalina detachment fault ranging from 1.2 to 12 km Myr−1. In contrast, apatite fission track ages increase in the hanging wall slip direction. Samples from the main range of the Santa Catalina Mountains yield apatite fission track ages of 20.5 to 14.6 Ma; samples from the Santa Catalina Mountain forerange, located closer to the detachment fault, yield apatite fission track ages of 21.4 to 18.8 Ma. Rapid cooling (40° to 60 °C Myr−1) related to detachment faulting is best recorded by zircon fission track ages and higher-temperature thermochronometers in the main range and by nearly concordant zircon and apatite fission track ages in the forerange. Slower cooling (3°–7°C Myr−1) of the footwall is recorded by shortened mean confined fission track lengths (<14 μm) and is related to erosional unroofing. Approximately 2 km of late-Tertiary erosion played a significant role in the unroofing of the footwall of the Catalina metamorphic core complex in contrast to metamorphic core complexes in western Arizona, where detachment faulting is the dominant unroofing mechanism.

Timing of events during the late Proterozoic Beardmore Orogeny, Antarctica: Geological evidence from the La Gorce Mountains

The Beardmore Orogeny previously has been designated for deformational and magmatic activity that occurred during the late Proterozoic in the central Transantarctic Mountains. It is recognized in folding of Beardmore Group turbidites, unconformably overlain by Cambrian limestones, and by silicic magmatism dated ∼650 Ma. Until this study, the relationship between the deformation and the magmatism had not been known. In the La Gorce Mountains, La Gorce Formation (Beardmore Group) was tightly folded during a regional metamorphic event producing biotite and limited axial-plane cleavage. The late Precambrian Wyatt Formation is a silicic porphyry with both volcanic and hypabyssal phases, possibly representing the eroded roots of a caldera complex. Wyatt Formation is conformably overlain by Ackerman Formation, a sequence of interbedded volcanics and shallow-marine sedimentary rocks. At a newly discovered locality, Wyatt Formation intrudes folded La Gorce Formation. This relationship demonstrates that the episode of folding and low-grade regional metamorphism was completed prior to the silicic magmatism of the Wyatt and Ackerman Formations. A contact between Ackerman and La Gorce Formations, which previously had been interpreted as conformable, has been shown by this study to be a fault. We advocate that the usage of the term “Beardmore Orogeny” be restricted to the deformational and metamorphic event and that the late Proterozoic magmatism be viewed as the initial stage in an episode of widespread silicic volcanism that continued during the Early and Middle Cambrian in the central Transantarctic Mountains.

Tectonic Model for Development of the Byrd Glacier Discontinuity and Surrounding Regions of the Transantarctic Mountains during the Neoproterozoic — Early Paleozoic

The Byrd Glacier discontinuity is a major tectonic boundary crossing the Ross Orogen, with crystalline rocks to the north and primarily sedimentary rocks to the south. Most models for the tectonic development of the Ross Orogen in the central Transantarctic Mountains consist of two-dimensional transects across the belt, but do not address the major longitudinal contrast at Byrd Glacier. This paper presents a tectonic model centering on the Byrd Glacier discontinuity. Rifting in the Neoproterozoic produced a crustal promontory in the craton margin to the north of Byrd Glacier. Oblique convergence of a terrane (Beardmore microcontinent) during the latest Neoproterozoic and Early Cambrian was accompanied by subduction along the craton margin of East Antarctica. New data presented herein in support of this hypothesis are U-Pb dates of 545.7 ±6.8 Ma and 531.0 ±7.5 Ma on plutonic rocks from the Britannia Range, directly north of Byrd Glacier. After docking of the terrane, subduction stepped out, and Byrd Group was deposited during the Atdabanian-Botomian across the inner margin of the terrane. Beginning in the upper Botomian, reactivation of the sutured boundaries of the terrane resulted in an outpouring of clastic sediment and folding and faulting of the Byrd Group.