9 research outputs found
Technical Note: Benchmark time-temperature paths provide a shared framework for evaluating and communicating thermochronologic data interpretation
We present a set of six time-temperature (tT) histories, called benchmark paths, that can be used as a shared framework for evaluating the sensitivity of a thermochronologic system to the variables inherent in the interpretation of thermochronologic data (e.g., kinetics models, mineral compositions or geometries, etc.). These benchmark paths span 100 Myr, include monotonic and nonmonotonic histories that represent plausible geologic scenarios, and have a range of cooling rates through different chronometer partial-retention/annealing temperatures. Here, we demonstrate their utility by presenting a method for tuning these paths to 11 different kinetics models for the apatite (U-Th-Sm)/He (n=5), apatite fission-track (n=2), and zircon (U-Th)/He (n=4) systems. These tuned tT paths provide a practical comparison of the kinetics models for each system and the data patterns they predict, thereby offering anyone performing thermal history analysis the ability to consider how their choice of kinetics model may impact their data interpretation. The adoption of benchmark paths for evaluating kinetics models and other variables provides a practical way for the thermochronology community to evaluate and communicate the decision making processes that are inherent in thermochronologic modeling and data interpretation
Thermal history modeling techniques and interpretation strategies: applications using HeFTy
Advances in low-temperature thermochronology, and the wide range of geologic problems that it is used to investigate, have prompted the routine use of thermal history (time-temperature, tT) models to quantitatively explore and evaluate rock cooling ages. As a result, studies that investigate topics ranging from Proterozoic tectonics to Pleistocene erosion now commonly require a substantial numerical modeling effort that combines the empirical understanding of chronometer thermochemical behavior (kinetics) with independent knowledge or hypotheses about a study areaâs geologic history (geologic constraints). Although relatively user-friendly programs, such as HeFTy and QTQt, are available to facilitate thermal history modeling, there is a critical need to provide the geoscience community with more accessible entry points for using these tools. This contribution addresses this need by offering an explicit discussion of modeling strategies in the program HeFTy. Using both synthetic data and real examples, we illustrate the opportunities and limitations of thermal history modeling. We highlight the importance of testing the sensitivity of model results to model design choices and describe a strategy for classifying model results that we call the Path Family Approach. More broadly, we demonstrate how HeFTy can be used to build an intuitive understanding of the thermochronologic data types and model design strategies that are capable of discriminating among geologic hypotheses
Thermal history modeling techniques and interpretation strategies: applications using QTQt
Advances in low-temperature thermochronology have made it applicable to a plethora of geoscience investigations. The development of modeling programs (e.g., QTQt and HeFTy) that extract thermal histories from thermochronologic data has facilitated growth of this field. However, the increasingly wide range of scientists who apply these tools requires an accessible entry point to thermal history modeling and how these models develop our understanding of complex geological processes. This contribution offers a discussion of modeling strategies, using QTQt, including making decisions about model design, data input, kinetic parameters, and other factors that may influence the model output. We present a suite of synthetic data sets derived from known thermal histories with accompanying tutorial exercises in the Supplemental Material. These data sets illustrate the opportunities and limitations of thermal history modeling. Examining these synthetic data helps to develop intuition about which thermochronometric data are most sensitive to different thermal events and to what extent user decisions on data handling and model setup can control the recovery of the true solution. We also use real data to demonstrate the importance of incorporating sensitivity testing into thermal history modeling and suggest several best practices for exploring model sensitivity to factors including, but not limited to, the model design or inversion algorithm, geologic constraints, data trends, the spatial relationship between samples, or the choice of kinetics model. Finally, we provide a detailed and explicit workflow and an applied example for a method of interrogating vague model results or low observation-prediction fits that we call the âPath Structure Approach.â Our explicit examination of thermal history modeling practices is designed to guide modelers to identify the factors controlling model results and demonstrate reproducible approaches for the interpretation of thermal histories
Perspectives on Continental Rifting Processes From Spatiotemporal Patterns of Faulting and Magmatism in the Rio Grande Rift, USA
Analysis of spatiotemporal patterns of faulting and magmatism in the Rio Grande rift (RGR) in New Mexico and Colorado, USA, yields insights into continental rift processes, extension accommodation mechanisms, and rift evolution models. We combine new apatite (UâThâSm)/He and zircon (UâTh)/He thermochronometric data with previously published thermochronometric data to assess the timing of fault initiation, magnitudes of fault exhumation, and growth and linkage patterns of rift faults. Thermal history modeling of these data reveals contemporaneous rift initiation at ca. 25 Ma in both the northern and southern RGR with continued fault initiation, growth, and linkage progressing from ca. 25 to ca. 15 Ma. The central RGR, however, shows no evidence of Cenozoic faultârelated exhumation as observed with thermochronometry and instead reveals extension accommodated through Late Cenozoic magmatic injection. Furthermore, faulting in the northern and southern RGR occurs along an approximately northâsouth strike, whereas magmatism in the central RGR occurs along the northeast to southwest trending Jemez lineament. Differences in deformation orientation and rift accommodation along strike appear to be related to crustal and lithospheric properties, suggesting that rift structure and geometry are at least partly controlled by inherited lithosphericâscale architecture. We propose an evolutionary model for the RGR that involves initiation of faultâaccommodated extension by oblique strain followed by block rotation of the Colorado Plateau, where extension in the RGR is accommodated by faulting (southern and northern RGR) and magmatism (central RGR). This study highlights different processes related to initiation, geometry, extension accommodation, and overall development of continental rifts.Plain Language SummaryWe identify patterns of faulting and volcanism in the Rio Grande rift (RGR) in the western United States to better understand how continental rifts evolve. Using methods for documenting rock cooling ages (thermochronology), we determined that rifting began around 25 million years ago (Ma) in both the northern and southern RGR. Rift faults continued to develop and grow for another 10 to 15 million years. The central RGR, however, shows that rift extension occurred through volcanic activity both as eruptions at the surface and as magma injection below the surface since ~15 Ma. Interestingly, RGR faulting in the north and south parts of the rift occurs on a northâsouth line, while volcanism in the central RGR is along a northeast to southwest line. The differences in the location and orientation of faulting and volcanic activity may be related to the thickness of the lithosphere beneath different parts of the rift. Using these patterns of faulting and magmatism, we propose the RGR evolved through a combination of (1) oblique strainâextension diagonal to the rift and (2) block rotationâwhere the Colorado Plateau is the rotating block. This detailed study highlights different processes related to the accommodation of extension and the overall development of continental rifts.Key PointsInitiation of the Rio Grande rift appears to be synchronous ~25 Ma and does not support a northward propagation modelExtension is accommodated by faulting in the northern and southern Rio Grande rift and by magmatic injection in the central Rio Grande riftDifferent rift accommodation mechanisms may be controlled by preexisting weaknesses and lithospheric properties (i.e., thickness)Peer Reviewedhttps://deepblue.lib.umich.edu/bitstream/2027.42/152704/1/tect21226.pdfhttps://deepblue.lib.umich.edu/bitstream/2027.42/152704/2/wrcr21226-sup-00001-2019TC005635-SI.pdfhttps://deepblue.lib.umich.edu/bitstream/2027.42/152704/3/tect21226_am.pd
Aqueous alteration processes in Jezero crater, Marsâimplications for organic geochemistry
The Perseverance rover landed in Jezero crater, Mars, in February 2021. We used the Scanning Habitable Environments with Raman and Luminescence for Organics and Chemicals (SHERLOC) instrument to perform deep-ultraviolet Raman and fluorescence spectroscopy of three rocks within the crater. We identify evidence for two distinct ancient aqueous environments at different times. Reactions with liquid water formed carbonates in an olivine-rich igneous rock. A sulfate-perchlorate mixture is present in the rocks, which probably formed by later modifications of the rocks by brine. Fluorescence signatures consistent with aromatic organic compounds occur throughout these rocks and are preserved in minerals related to both aqueous environments
Coronal Heating as Determined by the Solar Flare Frequency Distribution Obtained by Aggregating Case Studies
Flare frequency distributions represent a key approach to addressing one of
the largest problems in solar and stellar physics: determining the mechanism
that counter-intuitively heats coronae to temperatures that are orders of
magnitude hotter than the corresponding photospheres. It is widely accepted
that the magnetic field is responsible for the heating, but there are two
competing mechanisms that could explain it: nanoflares or Alfv\'en waves. To
date, neither can be directly observed. Nanoflares are, by definition,
extremely small, but their aggregate energy release could represent a
substantial heating mechanism, presuming they are sufficiently abundant. One
way to test this presumption is via the flare frequency distribution, which
describes how often flares of various energies occur. If the slope of the power
law fitting the flare frequency distribution is above a critical threshold,
as established in prior literature, then there should be a
sufficient abundance of nanoflares to explain coronal heating. We performed
600 case studies of solar flares, made possible by an unprecedented number
of data analysts via three semesters of an undergraduate physics laboratory
course. This allowed us to include two crucial, but nontrivial, analysis
methods: pre-flare baseline subtraction and computation of the flare energy,
which requires determining flare start and stop times. We aggregated the
results of these analyses into a statistical study to determine that . This is below the critical threshold, suggesting that Alfv\'en
waves are an important driver of coronal heating.Comment: 1,002 authors, 14 pages, 4 figures, 3 tables, published by The
Astrophysical Journal on 2023-05-09, volume 948, page 7