Apatite Thermochronology Research Articles

Constraints on exhumation and extensional faulting in southwestern Nevada and eastern California, U.S.A., from zircon and apatite thermochronology

Eastern California and southwestern Nevada represent an area of Tertiary and Quaternary extensional and dextral transtensional deformation. We used zircon and apatite fission-track thermochronology to study the distribution and timing of tectonic exhumation resulting from extensional and transtensional detachment faulting in this area. Sampling efforts were focused on Paleozoic and Precambrian clastic sedimentary and metasedimentary rocks. Sixty-nine new apatite and zircon fission-track cooling ages from 50 samples, analyzed in conjunction with published fission-track data from the region, indicate a distinct population of young (Miocene) fission-track ages and a population of irregularly distributed older (pre-Miocene) fission-track ages. Miocene (young population) fission-track ages become younger toward the west—indicating westward migration of the cooling front, consistent with well-documented Miocene extension of the Basin and Range Province. The younging pattern is also consistent with west-northwest displacement of the hanging wall of a crustal-scale extensional fault system and consequent progressive footwall exhumation. The active trailing edge of the hanging wall of this system generally coincides with Death Valley. Migration rates of the cooling front in the footwall of this system are on the order of 10–11 mm/yr. Based on the distribution of the Miocene fission-track ages, we interpret that the crustal faults that defined the eastern edge of the detachment system originated as separate normal faults that were linked by the formation of a transfer fault. Extrapolation of apatite fission-track closure ages from two transects across the eastern margin of the Death Valley region suggests that exhumation along the eastern margin of the system continues beneath Death Valley today.

Rates and mechanisms of Mesoarchean magmatic arc construction, eastern Kaapvaal craton, Swaziland

The mechanisms and time scales of magmatism and deformation during orogenesis are important for developing models of lithospheric growth. We summarize the first detailed study of a well-preserved Mesoarchean crustal section from the oldest portion of the Kaapvaal craton, southern Africa, which records the complex interactions between deformation and magmatism during craton assembly ca. 3.3–3.2 Ga. We use chemical abrasion–isotope dilution–thermal ionization mass spectrometry (CA-ID-TIMS) U-Pb zircon geochronology and apatite thermochronology in conjunction with geological mapping to show that the tonalitic to granitic Usutu magmatic suite intruded into the ca. 3.66–3.45 Ga Ancient gneiss complex over a period of ∼16 Ma ca. 3236–3220 Ma as discrete pulses of magma with variable intrusive styles. Usutu rocks retain magmatic fabrics that preserve a history of NW-SE regional shortening, consistent with synchronous deformation recorded in the adjacent Barberton greenstone belt. U-Pb zircon dates of ca. 3.28–3.23 Ga and apatite cooling dates from the Nhlangano gneiss SE of the Ancient gneiss complex reveal that they represent an older, largely undocumented period of crustal growth and modification. Synchronicity in magmatism and similarity in kinematics of deformation north and south of a previously suggested continental suture in the Barberton greenstone belt lead us to propose doubly vergent subduction zones that were active from at least ca. 3.28 to 3.22 Ga. Geochemistry of the Usutu suite reveals differences in magma sources from north to south, which, when combined with Nd isotopic signatures, are consistent with their production in a subduction zone. The combination of <1 Ma precision on crystallization dates with field observations and geochemistry allows us to track the evolution of this magmatic system with temporal resolution unprecedented for Archean rocks.

Re-evaluating low temperature apatite thermochronology in slowly cooled terranes

Re-evaluating low temperature apatite thermochronology in slowly cooled terranes

Appraisal of (U-Th)/He apatite thermochronology as a thermal history tool for hydrocarbon exploration: An example from the Taranaki Basin, New Zealand

The Taranaki Basin contains most of New Zealand's commercial hydrocarbon reserves. The southern parts of the Taranaki Basin underwent inversion during the late Miocene, with published AFTA (apatite fission-track analysis) and vitrinite reflectance data suggesting approximately 2.5-3 km of section eroded from the structure on which the Fresne-1 well was drilled, consistent with estimates from reconstructions based on seismic sections. The well-constrained tectonic and thermal history framework for this well section, established by earlier studies, provides an ideal test bed for the thermochronological technique of apatite (U-Th)/He dating. Helium ages determined in apatites from the earlier AFTA study decrease from about 6 Ma at about 400 m depth to about 1 Ma at 2465 m. Although these results are qualitatively consistent with late Miocene cooling, quantitative modeling of (U-Th)/He ages based on best-fit thermal histories derived from new AFTA analyses (using improved methods) suggests that cooling was protracted, involving accelerated cooling during the Pliocene, possibly indicating a discrete Pliocene inversion phase. Integration of AFTA and (U-Th)/He dating can provide improved thermal history constraints in sedimentary basins, particularly on cooling rates and/or resolution of multiple thermal events. By independently defining the timing of peak maturation in relation to formation of potential trapping structures, this information offers the potential for improved assessment of hydrocarbon prospectivity and reduced exploration risk.