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Ancient magnetization(s), often recorded by hematite (Fe2O3), provide key paleomagnetic constraints on plate interactions through time. Primary remanent magnetizations may be modified or overprinted by secondary processes that complicate interpretations of paleomagnetic data. Hematite (U‐Th)/He (hematite He) dating has the potential to resolve when secondary magnetizations were acquired. Here, we compare hematite He data and paleomagnetic results in Paleoproterozoic crystalline rocks, meters below a major nonconformity in the Colorado Front Range, USA. Prior work and new rock magnetic data indicate that pervasive hematite alteration records a secondary chemical remanent magnetization (CRM) during the Permo‐Carboniferous Reverse Superchron, coincident with the Ancestral Rocky Mountain orogeny. We target minor hematite‐coated faults cutting basement for (U‐Th)/He analyses because they are of sufficient hematite purity to yield geologically meaningful dates. Two samples yield overlapping and scattered individual hematite He dates ranging from ∼138 to 27 Ma (n = 33), significantly younger than the age of the late Paleozoic CRM. Scanning electron microscopy, electron probe microanalysis, and Raman spectroscopy indicate that aliquots have variable grain size distributions and fluorocarbonate impurities. Thermal history models support hematite on fault surfaces mineralized coeval with CRM acquisition during the late Paleozoic, and hematite He data scatter reflects variable He loss during Mesozoic burial owing to differences in grain size distribution from fault slip comminution and in chemistry among aliquots. Our results underscore the differences in temperature sensitivity and sampling requirements between paleomagnetic and hematite He investigations and illustrate that hematite He dates will usually be younger than preserved remanent magnetizations.
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This content will become publicly available on March 1, 2026
Magnetic Mineralogy and Paleomagnetic Record of the Nama Group, Namibia: Implications for the Large‐Scale Remagnetization of West Gondwanaland and Its Tectonic Evolution
Abstract The late Ediacaran to early Cambrian witnessed significant Earth system changes, including animal life diversification and an enigmatic paleomagnetic record. This study focuses on the Nama Group, a key geological unit for understanding the Ediacaran‐Cambrian transition. Previous paleomagnetic studies in the Nama Group identified complex remagnetization patterns but lacked a detailed examination of remanence carriers. To address this, we conducted a series of rock magnetic experiments on unweathered borehole core samples to better constrain the remagnetization mechanisms. Thermal demagnetization identified two magnetic components.C1, a recent viscous remanent magnetization, used for borehole core orientation, andC2, a stable remagnetization component carried by single‐domain (SD) pyrrhotite and magnetite. Magnetic mineralogy and paleomagnetic data suggest that the remanence acquisition mechanism ofC2is best explained by thermoviscous remanent magnetization (TVRM) and thermal remanent magnetization (TRM), rather than chemical remanent magnetization (CRM). The presence of low unblocking temperatures, coupled with thermochronological evidence of prolonged heating during tectonic collisions and subsequent cooling, supports this interpretation. The remagnetization event is linked to the final consolidation of West Gondwanaland during the late stages of megacontinent assembly (∼490–480 Ma), coinciding with regional uplift and a stable geomagnetic field during the Moyero reverse superchron. These findings challenge the CRM hypothesis, as the quasi‐synchronous remagnetization across cratonic blocks and the predominance of single reverse polarity are better explained by thermal processes. This study highlights the critical role of thermoviscous relaxation in large‐scale remagnetization and provides new insights into the tectonic evolution of West Gondwanaland.
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- Award ID(s):
- 2153786
- PAR ID:
- 10655576
- Publisher / Repository:
- American Geophysical Union
- Date Published:
- Journal Name:
- Journal of Geophysical Research: Solid Earth
- Volume:
- 130
- Issue:
- 3
- ISSN:
- 2169-9313
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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