Gomaa I. Omar

Fission Track Evidence on the Initial Rifting of the Red Sea: Two Pulses, No Propagation

Fission track analyses indicate that the Red Sea initially opened simultaneously along its entire length. Two distinct pulses of uplift and erosion characterized the early stages of rifting in the Red Sea throughout Egypt and in southwestern Saudi Arabia. The first pulse began at ∼34 million years ago (Ma). The second pulse began in the early Miocene (21 to 25 Ma) and marked the start of the main phase of extension. These data support a rigid plate model for continental extension. These results also indicate that the initiation of rift flank uplift, and therefore rifting, and volcanism occurred nearly simultaneously. This conflicts with classical models of active and passive extension that predict sequential development of these features.

Fission-track analysis of basement apatites at the western margin of the Gulf of Suez rift, Egypt: evidence for synchroneity of uplift and subsidence

Fifty-six apatite fission-track ages and 52 horizontal confined track-length measurements are reported from Precambrian crystalline rocks along the western margin of the Gulf of Suez, Egypt. Ages fall in the range of ca. 11–385 m.y. and older ages often occur within very close geographic proximity to younger ones, indicating non-uniform uplift. The wide range in ages is accompanied by a systematic variation in the distribution of horizontal confined fission track lengths. On the basis of apatite fission track ages and their length distributions, data fall into three distinct groups. Group I: ages ranging from 43 to 385 m.y. Length distributions are all positively skewed and with decreasing age become progressively broader with shorter mean track length. Group II: ages ranging from 23 to 31 m.y. Length distributions are negatively skewed with either a distinct tail or a small peak of short tracks. Group III: ages ranging from 11 to 20.5 m.y. Length distributions are al unimodal, narrow, negatively skewed and have the longest mean lengths among samples studied. Apatite ages from groups I and II are interpreted as “mixed ages” as a result of cooling during uplift from different levels within the apatite partial track annealing zone. Ages from Group III are interpreted as “cooling ages” due to uplift from the apatite total track annealing zone with minor partial annealing. Correcting the ages of the two oldest samples in this group for track-length reduction yields ages of21 ± 2.2and23 ± 1.5m.y. It is proposed that the onset of rift-flank uplift in the Gulf of Suez—northern Red Sea area occurred between 21 and 23 m.y. ago. Fission-track analysis in combination with subsidence data from the Gulf of Suez basin, indicate that commencement of basement uplift postdate the start of rifting and is interpreted as evidence for passive rifting at the Gulf of Suez. Furthermore, this uplift is contemporaneous with, and is directly related to, the process of extension and subsidence at the Gulf of Suez.

An inverse method of modeling thermal histories from apatite fission-track data

Abstract Apatite fission-track (FT) ages and track length distributions are important sources of information about the thermal histories of rocks. Recent advances in the understanding of track annealing in apatite provide a solution to the forward problem of predicting FT ages and track length distributions that result from a given thermal history. In this paper, a method is presented to address the inverse problem—estimation of the thermal history from FT data. The inverse method uses an iterative approach (the downhill simplex) to systematically modify a starting thermal history to achieve satisfactory statistical agreement between the predicted and observed FT age and track length distribution. However, because of analytical uncertainties, a unique thermal history does not exist. Monte Carlo simulations are used to take into account the uncertainties in the data and yield a spectrum of possible thermal histories. The results are based on the isothermal annealing model of Laslett et al. ([1], Chem. Geol. Isot. Geosci. Sect., Vol. 65) but because the method uses forward predictions and not an analytical formula specific to a single annealing model, alternative annealing models could be used within the same framework. This method is an improvement in interpreting fission-track data because the range of thermal histories permitted by the data can be evaluated. To demonstrate the method, it is applied to several sets of test data created by forward modeling of idealized thermal histories. The results show that the age at which the sample cooled through its closure temperature and the most general features of the thermal history are revealed. However, data from partially annealed samples have wide spectra that reflect large uncertainties in the form of the thermal history. For such samples, FT data alone are not likely to provide strong tests of geologic hypotheses. Conversely, geologic information may play a crucial role in constraining the thermal history spectrum. In application to uplift-related cooling, this inverse method can provide much more realistic assessments of the cooling rate and its uncertainty than are otherwise possible.

Apatite fission-track evidence for Laramide and post-Laramide uplift and anomalous thermal regime at the Beartooth overthrust, Montana-Wyoming

The crystalline rocks of the Beartooth overthrust in southwest Montana and northwest Wyoming were uplifted and thrust over the northwest margin of the Bighorn basin during the Laramide orogeny. The timing and geometry of that event is well documented in the synorogenic Paleocene and early Eocene sedimentary rocks that were deposited ahead of the rising Beartooth basement block and partially over-ridden as the block advanced. The absence of post-Laramide sedimentary rocks on the block and in the adjacent Bighorn Basin has, however, precluded the reconstruction of the post-orogenic tectonothermal history of this region. We report here the results of fission-track analysis of apatite from the three components of the Beartooth over-thrust with the aim of reconstructing the Laramide and post-Laramide tectono-thermal history of the southeast margin of the Beartooth Mountains. Apatite fission-track ages and corresponding horizontal confined track length distributions (HCTLDs) from Precambrian basement rocks constituting the upper plate of the Beartooth overthrust indicate that from 7 to 12 km of uplift of the Red Lodge corner of the Beartooth block has occurred since early Paleocene time. This amount of uplift occurred in two stages, with an intervening mid-Tertiary period of tectonic quiescence. The latter was a period of either (1) Oligocene and/or Miocene deposition or (2) tectonic quiescence. Uplift of 4 to 8 km occurred during the first phase of cooling, which lasted from early Paleocene time (∼61 Ma) to early Eocene time (∼52 Ma). During the second phase, which began in late Miocene-early Pliocene time and continues to the present day, about 4 km of uplift occurred. Our fission-track data suggest that the thermal regime in rocks above the Beartooth overthrust was relatively stable during Tertiary time. The maximum geothermal gradient permitted by model thermal histories generated from our observed fission-track data during post-Laramide time is 17 °C/km. This value is the same as that of the present-day geothermal gradient measured in the Amoco Beartooth well and suggests that a low geothermal gradient prevailed throughout Tertiary time. Apatite fission-track ages and HCTLDs from Jurassic and Cretaceous sedimentary rocks beneath the Beartooth overthrust indicate that these rocks have remained 10-20 °C cooler than overlying rocks in the shear zone and the lowermost part of the upper plate since ∼61 Ma, an observation that is not consistent with the stratigraphic position of these rocks. We interpret this temperature to be the result of a persistent thermal regime in which ground water circulating through the sedimentary rocks of the lower plate cooled them and insulated them from conductive heat transfer from hotter overlying rocks; this ground-water circulation may have been responsible for flushing hydrocarbons out of the rock column explored by the Beartooth well. Shear-zone rocks experienced a higher temperature during Cenozoic time than did rocks of the upper and lower plates; this condition was maintained by flow through the shear zone of ground water heated to higher temperatures at deeper levels along the thrust.

Pattern of hydrothermal circulation within the Newark basin from fission-track analysis

Fission-track (FT) data from the Late Triassic-Early Jurassic Newark rift basin indicate long-lived convective downwelling along the border fault. Zircons and apatites from the basin and the surrounding basement yield FT ages averaging ∼180 Ma and ∼140-150 Ma, respectively (i.e., younger than the age of the host rocks). However, along the border fault of the basin, zircon ages exceed 300 Ma and apatite ages are as high as 214 Ma (i.e., older than the depositional ages). The proximity of samples heated to >220 ±40 °C, which reset zircons, and those never heated above 120 ±20 °C across a zone 5-10 km wide indicates an anomalous zone of low temperatures that resulted from the downwelling of water at the border faults. The FT results and other indicators of Hydrothermal flow attest to the circulation of relatively high temperature (100-250 °C) fluids in a pervasive Jurassic hydrothermal convection system. The fluid-convection downwelling along the border fault of the basin may be the consequence of the high topography that bordered the Newark basin in Triassic and Jurassic time. Hydrothermal flow ended by the time coastal-plain sediments covered the basin. Hydrothermal fluids circulating in extensional environments are potentially very important in modifying the temperature structure of the basins and crust.

Post-Alleghanian unroofing history of the Appalachian Basin, Pennsylvania, from apatite fission track analysis and thermal models

Results of apatite fission track analyses on 29 Ordovician through Permian sandstones from the Appalachian Basin in Pennsylvania are presented. Ages range from 111±17 to 184±10 Ma. Mean track lengths of 10.71±0.29 to 13.10±0.17 μm with unimodal, negatively skewed length distributions are indicative of slow cooling. The data separate into two groups on an age versus mean length plot. The younger group (111–144 Ma) is found in the structural depressions of the Anthracite Basin and the Broad Top Basin and adjacent Appalachian Plateau. The older group (144–184 Ma) is found in the structurally higher southwestern Appalachian Plateau and Juniata Culmination and adjacent central plateau. Fission track data suggest that the basin cooled slowly after the Alleghanian Orogeny, with culminations cooling earlier than depressions. Cooling histories modeled from apatite fission track data, with maximum temperatures constrained by vitrinite reflectance, indicate cooling beginning soon after the Alleghanian Orogeny except in the Juniata Culmination, which apparently experienced synorogenic cooling and unroofing during formation of the underlying duplex. Model cooling histories and available geologic information indicate that the foreland basin did not experience Mesozoic reheating. Unroofing histories were modeled from fission track cooling histories using heat flow estimates and burial depths derived from vitrinite reflectance profiles. The models suggest that the unroofing history of the Appalachian Basin in Pennsylvania can be divided into three episodes. An initial episode of relatively rapid cooling and unroofing (Late Permian-Early Jurassic) is attributed to flexural rebound of the foreland in response to erosional removal of Alleghanian topographic load. Initial unroofing rates are higher in eastern Pennsylvania than in the west, consistent with a flexural model. An episode of little to no unroofing (Middle Jurassic-late Oligocene) began contemporaneously with the inception of drift at the Atlantic continental margin. At this time, unloading of the orogen was replaced by subsidence and sedimentation on the new margin. Without flexural rebound the driving force for unroofing of the basin was removed and unroofing slowed greatly. An episode of rapid unroofing over the full width of the basin occurred from the Miocene to the present. Although the driving mechanism for unroofing at this time has not been identified, it is consistent with increased sedimentation rates in the middle Atlantic offshore basins for the same period.

Fission-Track Dating of Haughton Astrobleme and Included Biota, Devon Island, Canada

Haughton Astrobleme is a major extraterrestrial impact structure located on Devon Island in the Canadian Arctic Archipelago, Northwest Territories. Apatite grains separated from shocked Precambrian gneiss contained in a polymict breccia from the center of the astrobleme yielded a fission-track date of 22.4 million � 1.4 million years before the present or early Miocene (Aquitanian). This provides a date for the impact event and an upper limit on the age of crater-filling lake sediments and a flora and vertebrate fauna occurring in them. A geologically precise date for these fossils provides an important biostratigraphic reference point for interpreting the biotic evolution of the Arctic.

Deformational history of the central Brooks Range, Alaska: Results from fission‐track and 40Ar/39Ar analyses

The Brooks Range is the northernmost orogenic belt in Alaska. From south to north it consists of a thin belt of oceanic basalt and chert, followed by two belts of high-pressure/low-temperature metamorphic rocks (the schist and central belts), a basement-cored anticlinorium (the Doonerak Window), a fold and thrust belt, and a foreland basin (the Colville Basin). We have used apatite and zircon fission-track (FT) and 40Ar/39Ar white mica analyses of a N-S transect of the central Brooks Range to study the cooling history. These data are used with a balanced cross section to constrain the timing of deformational events in northern Alaska. The oldest cooling ages from this study were clusters of ∼185 Ma and ∼135 Ma zircon FT ages from the fold and thrust belt. Based on the similarity of the 185 Ma ages with the age of crystallization of the western Brooks Range ophiolites [Wirth et al., 1993], we suggest that the Brooks Range orogeny in northern Alaska had begun prior to this time. Cooling rates in the fold and thrust belt were very slow from 185 to 135 Ma, suggesting little or no deformation was occurring. Apatite and zircon FT analyses from the fold and thrust belt indicate that a substantial amount of cooling (caused by uplift and erosion) occurred from 135 to 95 Ma. We suggest that an increase in the rate of cooling at ∼135 Ma was caused by contractional deformation associated with renewed subduction with the opening of the Canada Basin. The 40Ar/39Ar white mica cooling ages of 130 to 120 Ma from the schist belt rocks of the southern Brooks Range indicate that peak metamorphism must have occurred prior to 130 Ma. An 40Ar/39Ar muscovite age of 113 Ma from the southernmost Brooks Range is interpreted to be the age of movement on normal faults juxtaposing lower-grade phyllite belt rocks against schist belt rocks. At the same time, rapid deposition of Brooks Range-derived sedimentary sequences began to the north, in the Colville foreland basin, and to the south, in the Yukon-Koyukuk basin. Unstrained high-level granitic plutons, which intruded the Yukon-Koyukuk basin and Ruby terrane from 107 to 95 Ma, indicate the end of collision with the Yukon-Koyukuk arc. By 95 Ma, cooling rates in the Brooks Range had slowed and little or no deformation was occurring. At 60 Ma, a major episode of rapid cooling occurred throughout northern Alaska, when the active plate margin in Alaska was more than 500 km to the south. In the Colville Basin, cooling was caused by thrust faulting and folding, and hence erosion, within the Albian-age and younger sedimentary rocks. Within the interior of the Brooks Range, a large-scale basement-involved anticlinorium (the Doonerak Window) became active at this time. At 25 Ma, another episode of rapid cooling occurred within the Doonerak Window region. Both of these events may be related to shallow subduction of the Kula plate from the southern Alaskan plate margin.

The cooling history of Silurian to Cretaceous alkaline ring complexes, south Eastern Desert, Egypt, as revealed by fission-track analysis

Abstract Forty fission-track ages of apatite, zircon and sphene, and nine horizontal “confined” track-length distribution patterns in apatite have been used to establish the cooling history of nine Silurian to Late Cretaceous alkaline ring complexes which intrude Precambrian basement in the southern Eastern Desert of Egypt. Zircon or sphene fission-track ages were determined from three complexes for whichK/Ar andRb/Sr ages on the same samples were also available, these ages are concordant and are interpreted as emplacement ages resulting from rapid cooling following high level crustal intrusion into relatively thick volcanic piles. Average apatite ages for each of the eight ring complexes range from 33 to 167 m.y. Track-length distribution patterns for apatites taken together with their ages invite subdivision into two groupings. Those complexes yielding Early Oligocene apparent apatite ages suggest cooling from the total track annealing zone followed by a relatively lengthy residence near the base of the partial annealing zone whereas those with Late Cretaceous ages indicate cooling from a shallower level in the partial annealing zone. Variations in cooling history resulted from differential uplift between fault-bound blocks. One block, that containing the Late Cretaceous Abu Khruq complex, was relatively stable and the different degrees of partial resetting recorded in apatites of this complex are attributed to the thermal effect of localised Tertiary dyke intrusion. Fission-track analysis in combination with geologic data indicates that in the south Eastern Desert of Egypt a phase of uplift commenced in Late Oligocene time and was accompanied by paleogeothermal gradients of ca. 40–50°C/km. Uplift was more pronounced (at least 2–2.5 km) in areas within about 100 km from the present Red Sea coast. This uplift, which is viewed as part of a broader regional tectonism related to the opening of the Red Sea, occurred along a northwest fracture pattern and was controlled by pre-existing lines of weakness in the basement complex.

Magmatism and deformation, southern Revillagigedo Island, southeastern Alaska

The mid-Cretaceous, cale-alkaline, LREE-enriched plutons on southern Revillagigedo Island, southeastern Alaska, form two distinct groups: (1) biotite-epidote-hornblende tonalite of the Moth Bay pluton and (2) biotite-bearing, hornblende-absent leucotonalite. Higher total REE and lower Na 2 O, Al 2 O 3 , and Sr distinguish the Moth Bay tonalite from the leucotonalite. The plutons are surrounded by narrow contact metamorphic aureoles containing kyanite and staurolite. Pressure estimates for emplacement based on metamorphic conditions of contact rocks and from igneous amphibole compositions are ∼8-9 kbar. Detailed analyses of pluton/host rock relationships indicate that pluton emplacement was synchronous with orogen deformation. Pre-emplacement thrusting placed the metavolcanic and metasedimentary rocks of the Taku terrane structurally above the younger metasedimentary rocks of the Gravina belt. The contact between these units was subsequently cut by the Moth Bay pluton. The southern margin of the Moth Bay pluton has been transformed to blastomylonite. Contact metamorphic textures and undeformed dikes which cut the blastomylonite indicate that shearing did not outlast crystallization of the pluton. A still later phase of deformation transposed the pluton-related structures and generated a northeast-dipping, ductile thrust fault (Southern Revillagigedo shear zone) along the southern shore of Revillagigedo Island. The shear zone and earlier structures all trend approximately northwest. Fission-track age determinations and track-length measurements in apatite were used to determine the cooling histories of the plutonic units. Fission track ages from the western part of the study area are 51.3 ± 4.0 Ma (apatite), 57.8 ± 4.4 Ma (zircon), and 69.8 ± 4.2 Ma (sphene); eastward, toward the Coast batholith, fission track ages are ∼10 m.y. younger. These younger ages may result from a tectonic/thermal event associated with batholith emplacement. This same event may have generated north-trending structures that overprint older northwest-trending structures on the east side of the study area.