Paul G. Fitzgerald

Uplift history and structure of the Transantarctic Mountains: new evidence from fission track dating of basement apatites in the Dry Valleys area, southern Victoria Land

Fission track analysis of apatites from basement rocks of the Wright Valley in southern Victoria Land provides information about the timing, the amount and hence the rate of uplift of the Transantarctic Mountains in this area. Apatite ages increase systematically with elevation, and a pronounced break in the age versus elevation profile has been recognised at about 800 m on Mt. Doorly near the mouth of Wright Valley. The apatite age of about 50 Ma at this point approximates the time at which uplift of the mountain range began. Samples lying above the break in slope lay within the apatite fission track annealing zone prior to uplift, during a Cretaceous to Early Cenozoic period of relative thermal and tectonic stability. At the lower elevations samples had a zero apatite fission track age before the onset of rapid uplift and have track length distributions indicating rapid cooling. Some 4.8–5.3 km of uplift are estimated to have occurred at an average rate of about 100 ± 5m/Ma since uplift began. From the total stratigraphic thickness known above the uplifted apatite annealing zone it can be estimated that the Late Cretaceous/Early Cenozoic thermal gradient in the area was about 25–30°C/km. The occurrence and pattern of differential uplift across the Transantarctic Mountains can be estimated from the vertical offsets of different apatite fission track age profiles sampled across the range. These show the structure of the mountain range to be that of a large tilt block, dipping gently to the west under the polar ice-cap and bounded by a major fault zone on its eastern side. Offset dolerite sills at Mt. Doorly show the mountain front to be step-faulted by 1000 m or more down to the McMurdo Sound coast from an axis of maximum uplift just inland from Mt. Doorly.

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.

The Transantarctic Mountains of southern Victoria Land: The application of apatite fission track analysis to a rift shoulder uplift

A fission track study of the Transantarctic Mountains (TAM) in the Granite Harbour and Wilson Piedmont Glacier areas of southern Victoria Land reveals information on the timing of uplift, the amount of uplift and erosion, and the structure of the mountains, especially the onshore Transantarctic Mountain Front (TAM Front), which represents the boundary between East and West Antarctica. Apatite ages are < 175 Ma and represent a thermal regime established after heating accompanying Jurassic magmatism. An apatite age profile from Mount England records a break in slope indicating uplift began at ∼55 Ma. Horizontal sampling traverses, plus fieldwork, delineate the structure of the TAM Front as a zone of north-south striking, steeply dipping normal faults, with displacements, dominantly down to the east, of 40–1000 m. The overall structure of the mountains in the area studied can be envisaged as a large tilt block or flexure. Its westerly limb dips gently under the ice cap, compared to its faulted eastern edge, the TAM Front. The bounding structure to the south is the Ferrar fault and to the north is a graben through which the Mackay Glacier drains the polar plateau. The edge of the flexure, or axis of maximum uplift, lies at Mount Termination, ∼30 km west of the McMurdo Sound coast. There has been ∼6 km of uplift since the early Cenozoic and 4.5–5 km of erosion along this axis. The amount of uplift decreases to the west at the same rate as the decrease in dip of the Kukri Peneplain, but the amount of erosion decreases more quickly as indicated by the increasing height of the mountains to the west. The axis of maximum uplift is traced north to Granite Harbour. The axis does not parallel the coast but has a more northerly trend. North-south striking longitudinal faults that delineate the structure of the TAM Front lie at an acute angle to the axis, indicating a dextral component to the dominantly east-west extension in the Ross Embayment. Architecture of the TAM typifies the features of an upper plate passive mountain range, whereas the Ross Embayment has the characteristics of a lower plate. The TAM Front represents an upper plate breakaway zone. Transfer faults may exist up major outlet glaciers that cut the TAM. The inflection point in the coastline at the southern end of McMurdo Sound may be due to the presence of a major transfer fault up or near the Skelton Glacier.

Pliocene eclogite exhumation at plate tectonic rates in eastern Papua New Guinea

As lithospheric plates are subducted, rocks are metamorphosed under high-pressure and ultrahigh-pressure conditions to produce eclogites and eclogite facies metamorphic rocks. Because chemical equilibrium is rarely fully achieved, eclogites may preserve in their distinctive mineral assemblages and textures a record of the pressures, temperatures and deformation the rock was subjected to during subduction and subsequent exhumation. Radioactive parent–daughter isotopic variations within minerals reveal the timing of these events. Here we present in situ zircon U/Pb ion microprobe data that dates the timing of eclogite facies metamorphism in eastern Papua New Guinea at 4.3 ± 0.4 Myr ago, making this the youngest documented eclogite exposed at the Earths surface. Eclogite exhumation from depths of ∼75 km was extremely rapid and occurred at plate tectonic rates (cm yr-1). The eclogite was exhumed within a portion of the obliquely convergent Australian–Pacific plate boundary zone, in an extending region located west of the Woodlark basin sea floor spreading centre. Such rapid exhumation (> 1 cm yr-1) of high-pressure and, we infer, ultrahigh-pressure rocks is facilitated by extension within transient plate boundary zones associated with rapid oblique plate convergence.

Asymmetric extension associated with uplift and subsidence in the Transantarctic Mountains and Ross Embayment

Abstract Apatite fission track data combined with regional geological observations indicate that the uplift of the Transantarctic Mountains has been coeval with thinning and subsidence of the crust beneath the Ross Embayment. In the Dry Valleys region of south Victoria Land, the mountains have been uplifted about 5 km since the early Cenozoic at an average rate of about 100 m/Ma. During uplift, the crust remained at constant thickness or was slightly thickened by magmatic underplating. In contrast, the crust beneath the Ross Embayment has been extended and consequently thinned beginning in the Late Cretaceous but mainly during Cenozoic times. We suggest here that the uplift of the Transantarctic Mountains and the subsidence of the Ross Embayment are a result of passive rifting governed by a fundamental structural asymmetry defined by a shallow crustal penetrative detachment zone that dips westward beneath the Transantarctic Mountain Front. The localization and asymmetry of this detachment and its unusually deep level expression are attributed to a profound crustal anisotropy inherited from an early Palaeozoic collision along the present site of the mountain range.

ASYMMETRIC EXHUMATION ACROSS THE PYRENEAN OROGEN : IMPLICATIONS FOR THE TECTONIC EVOLUTION OF A COLLISIONAL OROGEN

Abstract The Pyrenees are a collisional mountain belt formed by convergence between the Afro–Iberian and European plates. Apatite fission track thermochronology from three vertical profiles along the ECORS seismic line constrain the exhumation history of the Pyrenean orogen and hence tectonic models for its formation. In the Eocene there is relatively uniform exhumation across the Pyrenees, but significantly more exhumation occurs on the southern flank of the axial zone in the Oligocene. The variation in exhumation patterns is controlled by a change in how convergence is accommodated within the Pyrenean double-wedge. Accommodation of thrusting on relict extensional features that leads to inversion dominated thrust stacking resulted in relatively slow exhumation in the Eocene. However, subsequent crustal wedging and internal deformation in the upper crust under the stacked duplex of antiformal nappes resulted in extremely rapid exhumation on the southern flank in the Oligocene. The Maladeta profile in the southern axial zone records extremely rapid Early Oligocene exhumation followed by dramatic slowing or cessation of exhumation in the middle Oligocene and the formation of an apatite partial annealing zone (PAZ). This PAZ has subsequently been exhumed 2–3 km since the Middle Miocene, supporting the observations of Coney et al. [J. Geol. Soc. London 153 (1996) 9–16] that the southern flank of the range was buried by ≤2–3 km of syntectonic conglomerates in the Oligocene and subsequently re-excavated from Late Miocene to Recent. The present-day topographic form of the Pyrenees is largely a relict of topography that formed in the Eocene and the Oligocene. Comparison with paleoclimatic records indicates that the Eocene–Oligocene exhumation patterns are controlled by tectonic forces rather than resulting from an orographic effect due to uplift of the Pyrenees.

Fission-track geochronology, tectonics and structure of the Transantarctic Mountains in Northern Victoria Land, Antarctica

A regional fission-track dating study in northern Victoria Land (NVL) provides information on the amount, timing and variable rates of uplift of the Transantarctic Mountains (TAM) at their northernmost extent. Apatite ages increase systematically with elevation and together with confined track length distributions, define a two-stage uplift history, although a variety of thermal histories, resolvable by use of confined track length distributions, exist for different parts of NVL. A pronounced “break in slope” in the apatite age-elevation profile for results from most of NVL occurs at ∼ 50 Ma, approximating the start of uplift of the mountains. This marks the base of an uplifted apatite annealing zone. Prior to uplift, samples above this break lay within the apatite annealing zone whereas those samples below it had an apatite fission-track age of zero. For most of NVL, ∼ 5 km of uplift have been calculated. In the southeastern coastal region, however, uplift of the order of 10 km has been estimated, exposing apatite ages of only 25–35 Ma. Sphene and zircon ages from this area also appear reduced relative to the regional pattern, suggesting that the partial annealing zones for these minerals have been revealed. Confined track length distributions from the lower part of the apatite age profile indicate an initially rapid period of uplift (∼ 200–400 m Ma−1) from ∼ 50 Ma. In the Lichen Hills-Outback Nunataks area, in the west of NVL, apatites have not been completely overprinted by the Jurassic thermal event associated with emplacement of the Ferrar Dolerite, and uplift here is of the order of only 4 km. Block faulting associated with uplift of the TAM is considered to be the same event as Rennick Faulting leading to the formation of the Rennick Graben.

Thermochronologic constraints on post-Paleozoic tectonic evolution of the central Transantarctic Mountains, Antarctica

Built upon the roots of a compressive orogenic belt of late Proterozoic-early Paleozoic age and once adjacent to North America, the present-day Transantarctic Mountains (TAM) represent a rift flank, resulting from episodic uplift in the Cretaceous and Cenozoic. Fault blocks are discernible in present-day topography and subglacial morphology. Fission track results give information on differential block movement (uplift and denudation) and are important in constraining models for the uplift of the range. Apatite fission track thermochronology on samples collected from the central TAM record a complex thermotectonic history for this region over the past 350 m.y. Apatite ages in the Miller Range vary from ∼250 to ∼350 Ma and are from an exhumed apatite partial annealing zone formed following cooling of Cambro-Ordovician granitoids. A period of Cretaceous denudation (≲2 km), beginning at ∼115 Ma, is recorded at Moody Nunatak on the inland side of the TAM. Near the coast, samples along the Beardmore Glacier record rapid cooling indicative of denudation initiated in the early Cenozoic (∼50 Ma). The amount of uplift ∼70 km inland of the coast in the Queen Alexandra Range since the early Cenozoic is ∼7 km, with the likelihood of an additional ∼3 km at the coast. Eastward facing topographic escarpments in the Queen Alexandra Range mark the likely position of steeply dipping normal faults, which offset the apatite ages. Apatite ages on the east side of the Beardmore Glacier mouth are generally younger (average 27 Ma) than on the west side (average 33 Ma), reflecting greater denudation. Assumptions made regarding the use of an assumed paleogeothermal gradient are tested with available geologic evidence. The fission track data neither conflict with nor confirm paleobotanical evidence from the Sirius Group in the central TAM which suggests significant surface uplift (2–3 km) of the TAM since the Pliocene. Results build upon the available fission track database along the TAM and emphasize the subtle variability of uplift along the TAM due to episodic uplift involving differential block movements.

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.

Denudation of metamorphic core complexes and the reconstruction of the transition zone, West central Arizona: Constraints from apatite fission track thermochronology

Apatite fission track thermochronologic results from transects across the Basin and Range and Transition Zone provinces in west central Arizona provide constraints on the denudational history and structural framework of the region. Apatite fission track ages decrease from ∼21 to 14 Ma in the Harcuvar Mountains and from ∼16 to 13 Ma in the Buckskin-Rawhide Mountains in the slip direction (SW - NE) of detachment faults in the lower plates of the metamorphic core complexes. Mean lengths of confined fission tracks from the core complexes are all >14 μm, indicating that the apparent apatite ages record rapid cooling through the apatite partial annealing zone ( 40°C/m.y. for lower plate rocks. Apparent apatite ages in the Transition Zone province generally increase from ∼25 Ma to ∼100 Ma away from the Basin and Range province. This trend of increasing apatite age is disrupted by faulting as many as seven times at the fronts of major mountain ranges and within valleys between the Weaver Mountains and the Colorado Plateau. Gradients of apatite fission track age and confined track length with elevation in the mountain ranges and in the Phillips-Kirkland drill hole reveal parts of denuded Mesozoic and Cenozoic apatite partial annealing zones. These paleopartial annealing zone profiles provide a reference datum for preextension reconstructions of fault blocks. The reconstructions indicate that the major faults in the Transition Zone province have relative displacements of >1 km and that offset on them occurred mostly after 25 Ma. These data also indicate that most of the rocks now exposed in the Transition Zone of west central Arizona were not exposed until Miocene time.