The Liquiñe-Ofqui fault zone (LOFZ) is a major~1000 km long dextral shear zone of southern Chile, likely related to strain partitioning of Nazca Plate oblique convergence with South America. To understand block rotation pattern along the LOFZ, we paleomagnetically sampled 55 sites (553 samples) between 38°S and 41°S. We gathered Oligocene to Pleistocene volcanics and Miocene granites at a maximum distance of 20 km from the LOFZ, and at both sides of it. Rotations with respect to South America, evaluated for 36 successful sites, show that crust around the LOFZ is fragmented in small blocks,~1 to 10 km in size. While some blocks (at both fault edges) undergo very large 150°-170°rotations, others do not rotate, even adjacent to fault walls. We infer that rotations affected equidimensional blocks, while elongated crust slivers were translated subparallel to the LOFZ, without rotating. Rotation pattern across the LOFZ is markedly asymmetric. East of the fault and adjacent to it, rotations are up to 150°-170°clockwise, and fade out~10 km east of fault. These data support a quasi-continuous crust kinematics, characterized by small rigid blocks drag by the underlying ductile crust flow, and imply 120 km of total fault offset. Conversely, crust west of the LOFZ is cut by seismically active NW-SE sinistral antithetic faults, and yields counterclockwise rotations up to 170°at 8-10 km from LOFZ, besides the unrotated blocks. Further data from the Chile fore arc are needed to understand block rotation kinematics and plate dynamics west of the LOFZ.

Although the oldest volcanic rocks exposed at\ud Pantelleria (Strait of Sicily) are older than 300 ka, most of\ud the island is covered by the 45–50 ka Green Tuff\ud ignimbrite, thought to be related to the Cinque Denti\ud caldera, and younger lavas and scoria cones. Pre-50 ka\ud rocks (predominantly rheomorphic ignimbrites) are exposed\ud at isolated sea cliffs, and their stratigraphy and chronology\ud are not completely resolved. Based on volcanic stratigraphy\ud and K/Ar dating, it has been proposed that the older La\ud Vecchia caldera is related to ignimbrite Q (114 ka), and that\ud ignimbrites F, D, and Z (106, 94, and 79 ka, respectively)\ud were erupted after caldera formation. We report here the\ud paleomagnetic directions obtained from 23 sites in ignimbrite\ud P (133 ka) and four younger ignimbrites, and from an\ud uncorrelated (and loosely dated) welded lithic breccia thought\ud to record a caldera-forming eruption. The paleosecular\ud variation of the geomagnetic field recorded by ignimbrites is\ud used as correlative tool, with an estimated time resolution in\ud the order of 100 years. We find that ignimbrites D and Z\ud correspond, in good agreement with recent Ar/Ar ages\ud constraining the D/Z eruption to 87 ka. The welded lithic\ud breccia correlates with a thinner breccia lying just below\ud ignimbrite P at another locality, implying that collapse of the\ud La Vecchia caldera took place at ~130–160 ka. This caldera\ud was subsequently buried by ignimbrites P, Q, F, and D/Z.\ud Paleomagnetic data also show that the northern caldera\ud margin underwent a ~10° west–northwest (outwards) tilting\ud after emplacement of ignimbrite P, possibly recording magma\ud resurgence in the crust

We report on 33 new paleomagnetic directions obtained from 16 lava flows emplaced in the last 3 ka on São Miguel, the largest island of the Azores. The data provide 27 well‐dated directions from historical or 14C dated flows which, together with 6 directions previously gathered from the same flows by Johnson et al. (1998), yield the first paleomagnetic directional record of the last 3 ka from the Atlantic Ocean. Within‐flow directions are consistent, suggesting that inclination swings from 60° to 25° and declination changes between −10° to 20° reflect variations in the geomagnetic field over the last 3 ka. To a first approximation, the declination record is consistent with predictions from CALS3k.4 and gufm1 global field models. Conversely, inclination values are lower than model predictions at two different ages: 1) four sites from the 1652 AD flow yield I = 48° instead of I = 63° predicted by gufm1; 2) data from several flows nicely mimic the inclination minimum of 800–1400 AD, but inclination values are lower by ∼10° than CALS3k.4 model predictions. By interpolating a cubic spline fit on declination / inclination versus age data, we tentatively infer the directional evolution of the geomagnetic field at the Azores from 1000 BC to 1600 AD. The obtained curve shows three tracks in virtual overlap during the 1000–800 BC, 800–500 BC, and 400–700 AD time spans.

The Chile fore arc at 37°S–47°S represents the coseismic deformation zone of the 1960 Mw 9.5 Valdivia earthquake. Here we report on the paleomagnetism of 43 Oligocene‐Pleistocene volcanic sites from the fore‐arc sliver between 38°S and 42°S. Sites were gathered west of the 1000 km long Liquiñe‐Ofqui dextral fault zone (LOFZ) that represents the eastern fore‐arc sliver boundary. Nineteen reliable sites reveal that the fore arc is characterized by counterclockwise (CCW) rotations of variable magnitude, except at 40°S–41°S, where ultrafast (>50°/Myr) clockwise (CW) rotations occur within a 30 km wide zone adjacent to the LOFZ. CCW rotation variability (even at close sites) and rapidity (>10°/Myr) suggest that the observed block rotation pattern is related to NW‐SE seismically active sinistral faults crosscutting the whole fore arc. According to previously published data, CW rotations up to 170° also occur east of the LOFZ and have been related to ongoing LOFZ shear. We suggest that the occurrence and width of the eastern fore‐arc sliver undergoing CW rotations is a function of plate coupling along the subduction zone interface. Zones of high coupling enhance stress normal to the LOFZ, induce high LOFZ strength, and yield a wide deformation zone characterized by CW rotations. Conversely, low coupling imply a weak LOFZ, a lack of CW rotations, and a fore arc entirely dominated by CCW rotations related to sinistral fault kinematics. Our locking inferences are in good agreement with those recently derived by GPS analysis and indicate that seismotectonic segment coupling has remained virtually unchanged during the last 5 Ma.

The AD 1761 eruption on Terceira was the only historical subaerial event on the island and one of the last recorded in the Azores. The eruption occurred along the fissure zone that crosses the island and produced a trachybasalt lava flow and scoria cones. Small comenditic trachyte lava domes (known as Mistérios Negros) were also thought by some to have formed simultaneously on the eastern flank of Santa Bárbara Volcano. Following a multidisciplinary approach, we combined geological mapping, paleomagnetic, petrographic, mineral and whole-rock geochemical and structural analyses to study this eruption. The paleomagnetic dating method compared geomagnetic vectors (directions and intensities) recorded by both the AD 1761 lava flow and Mistérios Negros domes and revealed that the two events were indeed coeval. Based on new data and interpretation of historical records, we have accordingly reconstructed the AD 1761 eruptive dynamics and distinguished three phases: (1) a precursory phase characterized by decreased degassing in the fumarolic field of Pico Alto Volcano and a gradual increase of seismic activity, which marked the intrusion of trachybasalt magma; (2) a first eruptive phase that started with phreatic explosions on the eastern flank of Santa Bárbara Volcano, followed by the inconspicuous effusion of comenditic trachyte (66 wt% SiO 2 ), forming a WNW-ESE-oriented chain of lava domes; and (3) a second eruptive phase on the central part of the fissure zone, where a Hawaiian to Strombolian-style eruption formed small scoria cones (E-W to ENE-WSW-oriented) and a trachybasalt lava flow (50 wt% SiO 2 ) which buried 27 houses in Biscoitos village. Petrological analyses show that the two batches of magma were emitted independently without evidence of interaction. We envisage that the domeforming event was triggered by local stress changes induced by intrusion of the trachybasalt dyke along the fissure zone, which created tensile stress conditions that promoted ascent of comenditic trachyte magma stored beneath Santa Bárbara Volcano.

Paleointensity data from the Atlantic Ocean are rare. We present new paleointensity data from São Miguel (Azores Islands, Portugal) based on 20 paleomagnetic sites from 13 lava flows emplaced over the last 3000 years. Ten lava flows are radiocarbon dated, whereas three flows were paleomagnetically dated and one site was dated using stratigraphic relations. All the samples, previously investigated to recover paleodirections, were subjected to IZZI experiments. Importantly, the new data are internally consistent, agree with Moroccan and European datasets, and offer new constraints for global geomagnetic field models. Some of the ages of the paleomagnetically dated lava flows have been revised based on the intensity data presented here. The inferred Virtual Axial Dipole Moments (VADMs) range from 68.2 to 163.5 ZAm 2. A peak in field strength with an estimated age of around 600 BC is well supported by two sites from the same flow (Furna), and is comparable to the high intensity values found in Portugal for the same age and the earlier field peak at about 1000 BC in the Levant. A gradient in VADM values with latitude from northwestern Africa and across Europe between 100 and 1000 AD is confirmed as well as its absence from between 0 to 100 AD.

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Constraining the long‐term variability and average of the Earth's magnetic field 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, due to the dispersion of data from key time intervals. Here, we focus on the Cretaceous Normal Superchron (CNS; 121‐84 Ma), during which there were no reversals. We present new intensity results from 41 submarine basaltic glass (SBG) sites collected on the Nicoya Peninsula and Murcièlago Islands, Costa Rica. New and revised 40Ar/39Ar and biostratigraphic age constraints from previous studies indicate ages from 141 to 65 Ma. One site with an age of 135.1 ± 1.5 Ma (2σ) gave a reliable intensity result of 34 ± 8 µT (equivalent to a virtual axial dipole moment, VADM, value of 88 ± 20 ZAm2), three sites from 121 to 112 Ma, spanning the onset of the CNS, vary from 21 ± 1 to 34 ± 4 µT (53 ± 3 to 87 ± 10 ZAm2). These results from the CNS are all higher than the long‐term average of ∼42 ZAm2 and data from Suhongtu, Mongolia (46–53 ZAm2) and are similar to the Troodos Ophiolite, Cyprus (81 ZAm2, reinterpreted in this study). Together with the reinterpreted data, the new Costa Rica results suggest that the strength of the geomagnetic field was approximately the same both 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 field.