Comprehensive palaeomagnetic study of San Borja and Jaraguay monogenetic volcanic fields, Baja California (28–30°N): considerations on latitudinal corrections

Mapping Intimacies ◽

10.5194/egusphere-egu21-3409 ◽

2021 ◽

Author(s):

Ahmed Nasser Mahgoub ◽

Bernardo Ignacio García-Amador ◽

Luis Manuel Alva-Valdivia

Keyword(s):

Dipole Moment ◽

Late Miocene ◽

Baja California ◽

Temporal Distribution ◽

Magnetic Minerals ◽

Mean Values ◽

Kinematic Evolution ◽

Volcanic Fields ◽

Spatio Temporal ◽

Northwestern Mexico

We report 24 palaeomagnetic directions and 10 high-quality Thellier-derived palaeointensity (PI) values, obtained from 27 sites located in Baja California Peninsula, northwestern Mexico. Sampling was done in four rock units (magnesian andesites, calc-alkaline lavas, ignimbrites, adakites) belonging to San Borja and Jaraguay monogenetic volcanic fields. These units were erupted between ~ 15 and 2.6 Ma (previous K-Ar and 40Ar/39Ar data), hence results are presented in two consecutive periods: middle-late Miocene and Pliocene. Based on previous geological and geophysical records, the kinematic evolution of the region was carefully considered, allowing for the independent restoration of the palaeoposition of each sampled site. The identified main magnetic minerals are titanomagnetite, magnetite, and minor hematite, of variable grain size, present as intergrowths, which reflect varying oxidation/reduction conditions during emplacement of high-temperature magmas. We did not observe a clear relationship between the magnetic properties of the different sites and their success rate for PI experiments. This is with the exception of the FORC analysis which showed a fairly good correlation with PI success. Pliocene (Dec=359.2&#176;; Inc= 47.4&#176;; &#945;95=7.6&#176;; and k= 41.43) and Middle-late Miocene (Dec=353.9&#176;; Inc= 38.5&#176;; &#945;95=9.2&#176;; and k= 28.56) mean directions were calculated from 20 sites (10 sites per period), and PI mean values of 29.2 &#177; 9.1 &#956;T and 23.2 &#177; 6.3 &#956;T were determined for the two periods, respectively. Compiling global filtered PI data, together with our results, indicates that the strength of the geomagnetic field during middle-late Miocene was weak (virtual dipole moment = 5.0&#177;2.2&#215;1022 Am2) compared to Pliocene (6.4&#177;2.8&#215; 1022 Am2), and also relative to the present-day value (7.6 &#215; 1022 Am2). This indicates the global nature of the low dipole moment during the middle-late Miocene, which is consistent with what was previously concluded that from the past 30 Ma to the present time the magnetic field strength has increased. However, issues related to the Spatio-temporal distribution of PI data still present an obstacle to validating these suggestions; therefore, more reliable data are still needed.

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Comprehensive palaeomagnetic study of San Borja and Jaraguay monogenetic volcanic fields, Baja California (28–30° N): considerations on latitudinal corrections

Geophysical Journal International ◽

10.1093/gji/ggab064 ◽

2021 ◽

Author(s):

A N Mahgoub ◽

B I García-Amador ◽

L M Alva-Valdivia

Keyword(s):

Dipole Moment ◽

Late Miocene ◽

Baja California ◽

Temporal Distribution ◽

Magnetic Minerals ◽

Kinematic Evolution ◽

Oxidation Reduction ◽

Volcanic Fields ◽

Spatio Temporal ◽

Northwestern Mexico

Summary We report 24 palaeomagnetic directions and 10 high-quality Thellier-derived palaeointensity (PI) values, obtained from 27 sites located in Baja California Peninsula, northwestern Mexico. Sampling was done in four rock units (magnesian andesites, calc-alkaline lavas, ignimbrites, adakites) belonging to San Borja and Jaraguay monogenetic volcanic fields. These units have erupted between ∼ 15 and 2.6 Ma (previous K-Ar and 40Ar/39Ar data); hence results are presented in two consecutive periods: middle-late Miocene and Pliocene. The identified main magnetic minerals in the sampled sites are titanomagnetite, magnetite, and minor hematite, of variable grain size, present as intergrowths or surrounding grains, which reflect varying oxidation/reduction conditions during emplacement of high-temperature magmas. Based on previous geological and geophysical records, the kinematic evolution was carefully considered in the region, allowing for the independent restoration of the palaeoposition of each sampled site. Previous palaeodirections were also evaluated and corrected for tectonic motion in order to combine them with present data. Accordingly, a number of 15 and 36 directional data are used to calculate palaeopole position for Pliocene and middle-late Miocene periods, respectively, selected from a total of 74 data points. Pliocene (Plat = 87.8°, Plong = 147.5°, K = 41.06, A95 = 6.0°) and middle-late Miocene (Plat = 86.0°, Plong = 172.7°, K = 41.08, A95 = 3.8) palaeopole positions, calculated after tectonic corrections, are not statistically different from expected North American reference pole. Tectonic correction for Middle-late Miocene virtual geomagnetic poles plays an important role in reducing the resultant tilting from 2.7° to -0.8°. PI mean were calculated for Pliocene and middle-late Miocene periods at 29.2 ± 9.1 μT and 23.2 ± 6.3 μT, respectively. Compiling global filtered PI data, together with our results, indicates that the strength of the geomagnetic field during middle-late Miocene was weak (virtual dipole moment = 5.0 ± 2.2 × 1022 Am2) compared to Pliocene (6.4 ± 2.8 × 1022 Am2), and also relative to the present-day value (7.6 × 1022 Am2). This indicates the global nature of the low dipole moment during the middle-late Miocene period. However, issues related to the spatio-temporal distribution of PI data still present an obstacle to validating these suggestions; therefore, more reliable data are still needed.

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Satellite-Derived Spatio-Temporal Distribution and Parameters of North Atlantic Polar Lows for 2015–2017

Atmosphere ◽

10.3390/atmos12020224 ◽

2021 ◽

Vol 12 (2) ◽

pp. 224

Author(s):

Pavel Golubkin ◽

Julia Smirnova ◽

Leonid Bobylev

Keyword(s):

North Atlantic ◽

Barents Sea ◽

Auxiliary Information ◽

Low Frequency ◽

Temporal Distribution ◽

Study Region ◽

Polar Lows ◽

Mean Values ◽

Polar Low ◽

Spatio Temporal

A list of North Atlantic polar lows was compiled for 2015–2017. A total of 131 polar lows were found by analyzing the Moderate Resolution Imaging Spectroradiometer (MODIS) infrared imagery and auxiliary information. The study region was additionally divided by the 20° W meridian to assess possible differences in the polar lows occurring in the western and eastern parts of the region. The highest polar low activity was found over the Barents Sea and the northern Norwegian Sea. A large number of polar lows over this region were dual or multiple. When considering such systems as a single event, more polar lows were found in 2015 over the Labrador Sea and southern Davis Strait, which is the region with the second highest number of polar lows. High interannual variability of polar low frequency was noted, which was more pronounced in the western part of the region. During the analyzed period, the largest number of polar lows occurred in January for the western part of the region and in February for the eastern part. The main polar low parameters were similar within the region, with the mean values slightly higher in the western part of the region, but all extreme high values were observed in the eastern part.

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Effects of environmental factors on the spatio-temporal distribution of striped marlin catch rates off Cabo San Lucas, Baja California Sur, Mexico

Fisheries Research ◽

10.1016/j.fishres.2014.09.007 ◽

2015 ◽

Vol 166 ◽

pp. 47-58 ◽

Cited By ~ 9

Author(s):

Sofía Ortega-García ◽

Edgardo Camacho-Bareño ◽

Raúl O. Martínez-Rincón

Keyword(s):

Environmental Factors ◽

Baja California ◽

Temporal Distribution ◽

Baja California Sur ◽

Spatio Temporal ◽

Catch Rates

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Spatio-temporal distribution and abundance patterns of red crab Pleuroncodes planipes related to ocean temperature from the Pacific coast of the Baja California Peninsula

Fisheries Science ◽

10.1007/s12562-015-0938-8 ◽

2015 ◽

Vol 82 (1) ◽

pp. 1-15 ◽

Cited By ~ 4

Author(s):

Juan Antonio De Anda-Montañez ◽

Susana Martínez-Aguilar ◽

Eduardo F. Balart ◽

Tania Zenteno-Savín ◽

Lía Méndez-Rodríguez ◽

...

Keyword(s):

Baja California ◽

Pacific Coast ◽

Temporal Distribution ◽

Baja California Peninsula ◽

Ocean Temperature ◽

Abundance Patterns ◽

The Pacific ◽

Spatio Temporal

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Morphological and genetic variation of black-tailed jackrabbit (Lepus californicus) populations separated by rivers

Therya ◽

10.12933/therya-21-1104 ◽

2021 ◽

Vol 12 (2) ◽

pp. 257-274

Author(s):

Consuelo Lorenzo ◽

Maricela García-Bautista ◽

Coral Rosas-Ronzón

Keyword(s):

Genetic Variation ◽

Baja California ◽

Phylogenetic Reconstruction ◽

Habitat Characteristics ◽

Baja California Peninsula ◽

Geometric Morphometric ◽

Mean Values ◽

Rio Conchos ◽

Lepus Californicus ◽

Northwestern Mexico

Two rivers in the hot desert of northwestern México have been considered as filter barriers in the distribution of mammals: Río Conchos in Chihuahua and Río Nazas in Durango. Between both rivers, the black-tailed jackrabbit, Lepus californicus, shows significant differences in external morphological traits. We investigated if these differences are supported by phylogenetical signals and compared them with populations living at similar latitudes in the Baja California Peninsula to determine the importance of the genetic variation caused by the rivers. An external mophology, and a cranial geometric morphometric analysis were performed using the dorsal, ventral, and lateral views of the skull; and a genetic analysis of cytochrome b gene. Measurements and fur color patterns of specimens from two continental groups, north of Río Conchos (NRC) and south of Río Nazas (SRN), were compared to four groups (A-D) inhabiting different latitudes of the Baja California Peninsula (BCP). The parietal region, zygomatic arch, and auditory bullae were identified as the main cranial structures related to skull shape; however, no differences were observed in size and shape between groups. The phylogenetic reconstruction of L. californicus showed that it is a monophyletic species, with high branch support values (100). It is represented by two polyphyletic subclades, one with haplotypes of the SRN and NRC populations and the other with haplotypes of the BCP populations. The average genetic distance (p-distance) and genetic differentiation (FST) between SRN and NRC were low (0.8 % and 0.09, respectively), with higher mean values between the BCP groups (1.23 % and 0.30, respectively). The statistical parsimony network of Cyt b did not identify a clear geographic genetic structure between haplotypes of SRN and NRC and they did not share haplotypes with the BCP populations. There are neither cranial geometric morphometric nor genetic differences between L. californicus populations related to either the rios Conchos or Nazas; thus, these rivers cannot be considered geographic barriers. However, there are morphological differences between the populations in Chihuahua and Durango and the populations inhabiting Baja California Peninsula, which may be associated with evolutionary distance and local habitat characteristics.

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