Paleocurrent direction measurements in a volcanic setting by means of anisotropy of magnetic susceptibility: A case study from the Lower Miocene Tepoztlán Formation (Transmexican Volcanic Belt, Central Mexico)

Abstract

Sources of ancient volcanic rocks are often unknown if they are either eroded and/or covered by younger deposits. This problem, as well as the provenance of reworked volcaniclastic, fluvial and mass-flow deposits, can be partially solved by the application of anisotropy of the magnetic susceptibility (AMS). For massive and poorly sorted volcaniclastic rocks in particular this may be the only way of finding reliable transport directions and therefore allowing for paleogeographic reconstructions. Here, we present a data set of 428 AMS measurements and 249 measurements of sedimentary paleocurrent indicators from the Miocene Tepoztlán Formation at the southern edge of the Transmexican Volcanic Belt (Central Mexico). The highest degree of reliability of AMS measurements is gained for data from lava samples and the lowest from mass flows. Sedimentary structures in sandstones and conglomerates such as trough cross-stratification, asymmetric ripple marks, and the shape of scours and channels could be used to calibrate the results from AMS data and to prove their reliability. AMS data on fluvial deposits point to a drainage system with a W-E flow direction, indicating an outflow of the river system into the ancient Gulf of Mexico.Keywords: Anisotropy of magnetic susceptibility; paleocurrent direction; volcaniclastic; Transmexican Volcanic Belt; Miocene; Mexico 3 In ancient volcanic settings, one challenge in the investigation of volcanic and volcaniclastic deposits is the determination of source vent locations, since former volcanic centers may be either eroded and/or covered by younger deposits. Furthermore, the determination of the provenance of fluvial and mass-flow deposits, arising from the volcanic ring plain itself (Manville et al., 2009, and references therein) or rivers in distal reaches, can also be problematic. In lava flows, both vesicle and crystal preferred orientations have been used extensively for this purpose (e.g., Waters, 1960;Smith and Rhodes, 1972;Walker, 1989;Cashman and Kauahikaua, 1997;Manga, 1998;Iezzi and Ventura, 2002). However, petrological structures such as foliation and lineation due to the movement of a lava flow are often difficult to observe and measure (Bascou et al., 2005). Massive pyroclastic rocks such as ignimbrites and block-and-ash flow deposits rarely show any measurable alignment or structure that can help determine flow direction. However, the magnetic fabric can be used to determine their paleocurrent direction, and the speed, precision, cheapness and range of applicability make it unique in this regard (Tarling and Hrouda, 1993).All materials, even if they do not acquire remanent magnetization, have a magnetic susceptibility (Liu et al., 2001), which is not always isotropic in the rock (Ising, 1942). This spatial susceptibility variation is defined as the anisotropy of magnetic susceptibility (AMS) and reflects the preferred orientation of magnetic minerals in the rock or sediments, i.e. its magnetic fabric (Hrouda, 1982;Tarling and Hrouda, 1993), which yields thr...

Paleoflow directions of a subaqueous lahar deposit around the Miocene Keserűs Hill lava dome complex (North Hungary) as constrained by photo-statistics and anisotropy of magnetic susceptibility (AMS)

“…Samples with extreme Pj values are characterized by anomalously high magnetic susceptibility (Km), which indicates the contribution of strongly magnetic lithic fragments in such samples. The role of abundant lithoclasts in weakening the magnetic fabric in volcanogenic mass-flow deposits is supported by the results of Lenhardt et al (2013) who showed the weakening of magnetic fabric in volcanogenic sedimentary sequences (conglomerates) with increasing grain size. Conglomerates show weaker fabrics than those found in PDC deposits with fine-grained matrix and in volcanogenic sandstones.…”

“…Recent studies (Capaccioni and Sarocchi 1996, Capaccioni et al, 1997, Valentini et al, 2008, Biró 2013) applied computer-assisted image analysis to determine directional fabric. Another widely used approach is the application of anisotropy of magnetic susceptibility (AMS), which gives information on the directional fabric in an indirect way by measuring the alignment of magnetic and paramagnetic minerals (e. g. Ellwood, 1982;MacDonald and Palmer, 1990;Lamarche and Frogatt, 1993;Fisher et al, 1993;Cagnoli and Tarling, 1997;Le Pennec et al, 1998;Ort et al, 2003;Porreca et al, 2003;Caballero-Miranda et al, 2009;LaBerge et al, 2009;Dedzo et al, 2011;Lenhardt et al, 2013). Studying the directional fabric in such deposits is advantageous for two reasons.…”

Determining flow patterns and emplacement dynamics from tsunami deposits with no visible sedimentary structure

“…Anisotropy of magnetic susceptibility is a geophysical technique that has been widely used to infer depositional patterns and tectonic evolution in hard rocks (Hamilton and Rees, 1970;Rathore, 1979;Borradaile and Henry, 1997;Dragoni et al, 1997;Lüneburg et al, 1999;Li et al, 2014). This technique has also been applied to unconsolidated sediments to deduce flow direction during deposition (Rees and Woodall, 1975;Ellwood, 1980;Hassold et al, 2009;Lenhardt et al, 2013;Park et al, 2013;Wassmer et al, 2015). Here we use the abbreviation MF (Magnetic Fabric) to refer to Anisotropy of Magnetic Susceptibility, to avoid confusion with the AMS acronym applied to radiocarbon dating.…”

Palynomorph Preservation in Volcaniclastic Rocks of the Miocene Tepoztlan Formation (Central Mexico) and Implications for Paleoenvironmental Reconstruction

“…In an attempt to cover the entire sedimentary succession of the Tepoztlán Formation, The sediments within the two stratigraphic sections accumulated in proximal to median environments (not more than 5 -10 km from the source area), in flank and apron settings of a volcanic ring plain (shedding its debris from north to south), interfingering with an axial W-E trending braided river system (Lenhardt et al, 2010;2011;2013). While the lower part of the San Andrés section is still predominantly characterized by sandstones and conglomerates of the axial braided river system, the influence of the volcanic ring plain increases up-section and completely dominates the Tepozteco section (ca.…”

“…The initial volcanic activity of the early TMVB is dated to ca. 22 Ma and is documented by the Tepoztlán Formation (Lenhardt et al, 2010(Lenhardt et al, , 2013Lenhardt and Götz, 2011), cropping out in Malinalco (Mexico State) and in Tepoztlán and Tlayacapan (Morelos). The geology and geography of Morelos in Central Mexico controlled the evolution of the local flora and fauna (c.f., Lenhardt et al, 2006;Graham, 2010Graham, , 2011.…”