Description: The volcanic origin of the Samoan archipelago can be explained by one of three models, specifically, by a hot spot forming over a mantle plume, by lithospheric extension resulting from complex subduction tectonics in the region, or by a combination of these two processes, either acting sequentially or synchronously. In this paper, we present results of 36 high-resolution 40Ar/39Ar incremental heating age analyses for the initial (submarine) phase of Samoan volcanoes, ranging from 13.2 Ma for the westernmost Samoan seamounts to 0.27 Ma in the eastern Samoan volcanic province. Taken as a whole, our new age data point to a hot spot origin for the shield-building volcanism in the Samoan lineament, whereby seamounts younger than 5 Ma are consistent with a model of constant 7.1 cm/yr plate motion, analogous to GPS measurements for the Pacific Plate in this region. This makes our new 40Ar/39Ar ages of the submarine basalts all older compared to recent absolute plate motion (APM) models by Wessel et al. (2008), which are based on the inversion of twelve independent seamount trails in the Pacific relative to a fixed reference frame of hot spots and which predict faster plate motions of around 9.3 cm/yr in the vicinity of Samoa. The Samoan ages are also older than APM models by Steinberger et al. (2004) taking into account the motion of hot spots in the Pacific alone or globally. The age systematics become more complicated toward the younger end of the Samoan seamount trail, where its morphology bifurcates into two en echelon subtracks, termed the VAI and MALU trends, as they emanate from two eruptive centers at Vailulu'u and Malumalu seamount, respectively. Spaced ∼50 km apart, the VAI and MALU trends have distinct geochemical characters and independent but overlapping linear 40Ar/39Ar age progressions since 1.5 Ma. These phenomena are not unique to Samoa, as they have been observed at the Hawaiian hot spot, and can be attributed to a geochemical zoning in its underlying mantle source or plume. Moreover, the processes allowing for the emergence of two distinct eruptive centers in the Samoan archipelago, the stepped offset of these subtracks, and their slight obliqueness with respect to the overall seamount trail orientation may very well be controlled by local tectonics, stresses, and extension, also causing the rejuvenated volcanism on the main islands of Savai'i, Upolu, and Tutuila since 0.4 Ma.

Description: Constraining the long-term variability and average of the Earth's magnetic fi eld strength is fundamental to understanding the characteristics and behavior of the geomagnetic field. Questions remain about the strength of the average field, and the relationship between strength and reversal frequency. The dispersion of data from key time intervals reflects the complexity in obtaining absolute paleointensity values. Here, we focus on the Cretaceous Normal Superchron (CNS; 121-84 Ma), during which there were no reversals. We present new results from 42 submarine basaltic glass (SBG) sites collected on the Nicoya Peninsula and Murci elago Islands, Costa Rica and new and revised 40Ar/39Ar ages along with biostratigraphic age constraints from previous studies that indicate ages ranging from 141 to 112 Ma. One site with a 40Ar/39Ar age of 135 +/-1.5 Ma (2 ) gave a reliable intensity result of 34 +/- 8 uT (equivalent to a paleomagnetic dipole moment, PDM, value of 88 +/- 20 ZAm2), while three sites between 121 and 112 vary from 21 +/- 1 to 34 +/- 4 uT (53 +/- 3 to 87 +/- 10 ZAm2) spanning the onset of the CNS. These results from the CNS are all higher than the long-term average of 42 ZAm2 and similar to data from Suhongtu (46-53 ZAm2) and the Troodos Ophiolite (81 ZAm2, reinterpreted using the same criteria of this study). Together with the reinterpreted data, the new Costa Rica results suggest that the strength of the geomagnetic eld was about the same before and after the onset of the CNS. Therefore, the data do not support a strict correlation between polarity interval length and the strength of the magnetic fi eld.

Description: Published

Description: e2020GC009605

Description: 1A. Geomagnetismo e Paleomagnetismo

Description: JCR Journal