DATING THE PALEOMAGNETIC FOLD TEST

The paleomagnetic fold test is a powerful field test to constrain the timing of magnetization. The method relies on the clustering behavior of magnetic directions after (stepwise) restoring tilted beds to their horizontal, depositional orientation. This approach determines timing of (re)magnetization relative to the age of folding, resulting in pre-, syn- or post-folding magnetization behavior. If fold timing is better constrained by absolute dating, then the paleomagnetic fold test would similarly improve. Especially in cases where magnetization coincides with folding, i.e. syn-folding remagnetization, the timing of magnetization can be uniquely determined from absolute fold dating.

We will discuss a method of dating of folds involving Ar-Ar geochronology of clay separates with examples from recent structural and paleomagnetic work in the Mexican fold-thrust belt (FTB). This area preserves evidence for multiple deformation and magnetization events based on fold dating and paleomagnetic fold tests of individual structures. The remagnetizations are chemical in origin and represent discrete fluid pulses during the Cretaceous-Paleogene evolution of the Mexican FTB. Whereas the formation of magnetite in carbonates coincides with neocrystallization of illite in interlayered shale units, regional relationships between deformation pulses and magnetization indicate that a simple mineral reaction from a clay precursor that produces both illite and magnetite is unlikely. Instead, newly formed magnetite and clay minerals are parallel recorders of mineralizing fluid activity. The source of this fluid can be constrained by H stable isotope analysis, which indicates significant influence of surface fluids in this area. Such fluid activity reflects a recharge system that explains regional variability in magnetization and neocrystallization, and tightly dates paleomagnetic syn-folding remagnetization.