An integrated palaeomagnetic, palaeointensity and 40Ar/39Ar investigation on a Miocene polarity transition recorded in a lava sequence in la Gomera, Canary Islands

Abstract

A detailed palaeomagnetic, rock-magnetic and palaeointensity study has been carried out on a Miocene volcanic sequence which consists of 39 consecutive lava flows recording a polarity transition in La Gomera (Canary Islands, Spain). In addition, new 40 Ar/ 39 Ar ages were obtained in two flows, yielding 9.63 ± 0.06 Ma in the lower and 9.72 ± 0.08 Ma in the upper part of the sequence. Palaeomagnetic results allowed determining a ChRM direction in all studied lavas: The 25 lowermost flows of the sequence display normal polarity directions and above, a sequence of 14 flows correspond to a transitional geomagnetic regime. If considered together with palaeomagnetic results from a previous study, which were obtained on the flows immediately overlying the upper part of the sequence analysed in this work, these results indicate that the reversal recorded in the Hermigua sequence corresponds to the normal to reverse C4Ar2n to C4Ar3r polarity transition. The lower-lying 25 normal-polarity flows yield a mean direction D = 359.6 • , I = 42.4 • (α 95 = 5.1 • ; k = 33) which agrees well with the expected values. Above, a sequence of 14 flows displays a more irregular directional behaviour, including several transitional directions, suggesting the occurrence of a precursor to the transition. The presence of a virtual geomagnetic pole (VGP) cluster in the western Atlantic Ocean observed in this study coincides with previous records of Miocene transitions. Interestingly, this region corresponds to a near-radial flux centre of the present-day non axial dipole field. Angular dispersion of VGPs calculated for the 25 lowermost normal polarity flows of the sequence shows a lower than expected result. Palaeointensity determinations were carried out using a Thellier type double heating method. 27 of the 48 analysed samples measured yielded successful results. Mean VDMs mean values range from 1.1 ± 0.5 to 8.8 ± 0.9 × 10 22 Am 2 . The intensity values decrease significantly on approaching the directional transitional zone, suggesting an earlier start of the polarity transition in the intensity record, typical of a decreasing axial dipole.

Three‐Dimensional Magnetic Models of La Gomera (Canary Islands): Insights Into the Early Evolution of an Ocean Island Volcano

“…When compared with the direction of the geocentric axial dipole field for this latitude ( I = 47°, D = 0°), which is the average expected direction when the remanence acquisition spans more than 10 5 years, we can note that both directions are close but that they do not exactly coincide. Our results are consistent with the mean paleomagnetic direction obtained by Caccavari et al () in a sequence of 25 normal polarity lava flows belonging to the Old Edifice (see location in Figure (c)), which was defined by an inclination I = 42.4° and a declination D = 359.6° (α 95 = 5.1°). This consistency indicates the reliability of our method for the estimation of the mean magnetization direction in La Gomera.…”

“…After the first modern studies on the geology of La Gomera (Bravo, 1964;Cantagrel et al, 1984;Cendrero, 1970), three reconstructions of the volcanic history of this island are recently proposed (Ancochea et al, 2006;Cueto et al, 2004;Paris et al, 2005). In the last 15 years, work has focused on the study of abundant dike swarms (Ancochea et al, 2003;Ancochea et al, 2008;Márquez et al, 2018;Rodríguez-Losada & Martínez-Frías, 2004), the study of marine geomorphological features (Llanes et al, 2009), the characterization of gravitational landslides by structural analysis (Casillas et al, 2010;Fernández et al, 2015), and paleomagnetic data for studying a Miocene geomagnetic polarity transition (Caccavari et al, 2015). This wealth of knowledge has provided us with a rather good picture of La Gomera subaerial volcanic history, although little is known about the construction of the submarine edifice.…”

“…The red stars mark the positions of the four volcanic centers (from C1 to C4, in order of decreasing age) identified by Ancochea et al () through the geometric analysis of different dike swarms. The dark blue cross shows the location of the Hermigua lava sequence studied by Caccavari et al () for paleomagnetic purposes. See the text for details.…”

Evidence of Unusual Geomagnetic Regimes Recorded in Plio‐Pleistocene Volcanic Sequences from the Lesser Caucasus (Southern Georgia)

“…Specifically, lava flow sequences can be particularly helpful in revealing the behavior of the EMF during an extended time. This type of records can be found in sequences characterized by sequential image lava flow eruptions (e.g., Caccavari et al, ; Camps et al, ; Chauvin et al, ; Herrero‐Bervera & Valet, ; Jarboe et al, ; Kissel et al, ; Leonhardt et al, ; Mankinen et al, ; Moulin et al, ; Prévot et al, ).…”

A paleointensity test of the geocentric axial dipole (GAD) hypothesis

“…Because the Precambrian part of PINT was recently updated for the analysis by Biggin et al (2015), most of our additions are Phanerozoic. They include data from various locations, such as the Pacific Ocean (Yamazaki and Yamamoto, 2014), South Korea (Chang et al 2013),Namibia(Sprain et al 2016), Iceland(Cromwell et al 2015;Tanaka and Yamamoto, 2016), the Canary Islands(Caccavari et al 2015) and Ethiopia(Ahn et al 2016).…”