Vertical AMS variation within basalt flow profiles from the Xitle volcano (Mexico) as indicator of heterogeneous strain in lava flows

• Published in: Journal of volcanology and geothermal research Vol. 311; pp. 9 - 28
• Main Authors: Caballero-Miranda, C.I., Alva-Valdivia, L.M., González-Rangel, J.A., Gogitchaishvili, A., Urrutia-Fucugauchi, J., Kontny, A.
• Format: Journal Article
• Language: English
• Published: Elsevier B.V 01-02-2016
• Subjects: AMS lava flow zones; Anisotropy; Anisotropy of magnetic susceptibility (AMS); Dipping; Grains; Heterogeneous strain models; Lava; Magnetic fabric; Magnetic properties; Rock; Strain; Trans-Mexican volcanic belt; Volcanoes; Xitle lava flows; Mexico; Anisotropy of magnetic susceptibility (AMS); Xitle lava flows; Trans-Mexican volcanic belt; AMS lava flow zones; Magnetic fabric; Heterogeneous strain models
• ISSN: 0377-0273; 1872-6097
• DOI: 10.1016/j.jvolgeores.2016.01.003

The within-flow vertical variation of anisotropy of the magnetic susceptibility (AMS) of three basaltic flow profiles from the Xitle volcano were investigated in relation to the lava flow-induced shear strain. Rock magnetic properties and opaque microscopy studies have shown that the magnetic mineralogy is dominated by Ti-poor magnetite with subtle vertical variations in grain size distribution: PSD grains dominate in a thin bottommost zone, and from base to top from PSD-MD to PSD-SD grains are found. The vertical variation of AMS principal direction patterns permitted identification of two to three main lava zones, some subdivided into subzones. The lower zone is very similar in all profiles with the magnetic foliation dipping toward the flow source, whereas the upper zone has magnetic foliation dipping toward the flow direction or alternates between dipping against and toward the flow direction. The K1 (maximum AMS axis) directions tend to be mostly parallel to the flow direction in both zones. The middle zone shows AMS axes diverging among profiles. We present heterogeneous strain ellipse distribution models for different flow velocities assuming similar viscosity to explain the AMS directions and related parameters of each zone. Irregular vertical foliations and transverse to flow lineation of a few samples at the bottommost and topmost part of profiles suggest SD inverse fabric, levels of intense friction, or degassing effects in AMS orientations.
•Detailed vertical within-flow variation of AMS in basaltic lava flow profiles.•Identification of 2 to 3 AMS lava zones within each lava flow profile.•Correspondence of AMS lava zones to flow-induced shear strain in each profile.•Vertical flow velocity variation in strain ellipse models explains AMS lava zones.•Irregular AMS fabric as a degassing process or dragging flow mechanism result.