Geomagnetic paleosecular variation for the past 1Ma recorded in lava flows from south Ecuador.

Abstract

High-quality paleosecular variation studies with good geographical and temporal coverage are crucial for understanding the past Earth's magnetic field behavior. In the southern hemisphere, but especially in southern Ecuador, these data are still scarce. Reliable records that can be obtained from volcanic rocks, specifically from lava flows, are required to create more accurate magnetic field models especially for the southern hemisphere. A total of 196 samples from 23 previously dated lava flows from nine different volcanoes were subjected to a detailed rock magnetic and paleodirectional study. Thermomagnetic analysis on the lava flows shows that the main carrier of magnetization is titanomagnetite with variable, but typically low level of Ti substitution. Hysteresis parameters and First Order Reversal Curves (FORCs) indicate that the magnetic grain size varies from SD to PSD and IRM curves reveal the predominance of low-to medium coercivity minerals. Stepwise thermal and alternating field demagnetization were used to obtain 23 site-mean directions. The mean direction from all sites is D = 354.4°, I = -0.7°, and a95 = 10.3°.These paleomagnetic directions are consistent with past works in Ecuador. The mean paleopole direction for all samples is Plat= 85.3° N, and Plong=198.0 E° with an A95=7.8°. Furthermore, two anomalous directions with respect to the GAD (Geocentric Axial Dipole) coincide with two magnetic excursions. Lava flow SE03 (492 ± 9 ka) lies within error as the Orphan Knoll excursion at ~495 ka, previously reported in lavas from Galápagos Island (Ecuador). Furthermore, sample SE21 (30 ±3 ka) falls within the range of the Mono Lake excursion. A comparison between morphotectonic movements from Pallatanga fault and an inconsistent Virtual Geomagnetic Pole (VGP) from Igualatá volcano (SE14: 376 ±10ka) reflects the high tectonic activity at this zone. After applying a Vandamme (1994) cutoff, our results show a dispersion (Sb) of 15.6° (Su = 19.10°; Sl = 11.56°). This new result is similar to the Sb reported previously for the region with a smaller amount of sites and is 1.6° higher than those predicted by G and TK03 geomagnetic field models. Besides the strong tectonic activity, VGP distribution is higher at continental Ecuador due to a strong longitudinal variability associated with important non-dipolar components of the field, which can be associated to the South Atlantic Magnetic Anomaly.