GP31B-01
Thermally Enhanced Magnetic Fabrics of Basaltic Dikes from Kapaa Quarry, Koolau Volcano, Oahu, Hawaii, USA.
Progressive thermal treatment has been used to investigate the anisotropy of magnetic susceptibility (AMS) of a wide range of lithologies. Initial results on e.g., red sandstones, glacial tillites, granites and gneisses showed that laboratory stepwise heating resulted in thermal enhancement of AMS, showing the potential of thermal treatment in studying weak AMS and masked or cryptic fabrics. Studies have however shown that heating induced changes in AMS may be more complex that simple enhancement of the magnetic fabric In general, thermal induced magneto-mineralogical alterations are complex and not well understood, and further investigation of heating induced effects in mineralogy, grain size and texture systematically investigated for different lithologies is needed. For our experiment we have used a suite of samples from eight basaltic dikes from the Kappa Quarry, Koolau volcanic range in Oahu, Hawaii. The AMS fabric was determined as part of a study to investigate the influence of hydrothermal alteration by Krasa and Herrero-Bervera (2005). They found that hydrothermal alteration changes the bulk susceptibility and anisotropy degree, but AMS ellipsoid principal axes are not affected. Since hydrothermal alteration transforms the primary Ti-poor titanomagnetites into granular intergrowths of titanomagnetites, titanomaghemite and hematite, and that samples show varying degrees of alteration, the samples react differently to laboratory stepwise heating permitting study of thermal effects on the magnetic mineralogy, and AMS parameters and principal susceptibility axes. Further, thermal treatment results in fabric enhancement with reduced axial scatter associated with weak bulk susceptibilities and anisotropy degrees in the dikes. For the AMS experiment samples were heated progressively to temperatures up to 400° C or 560° C and the AMS measured after each step. AMS parameters and bulk susceptibility show changes with increasing temperature while the AMS ellipsoidal axes remain stable or with small angular changes. The dike fabrics are defined by maximum k1 axes within the dike planes and minimum k3 axes lying oblique to dike planes. Results from the thermal treatment experiment permit improved definition of directional AMS patterns, with in some cases less scatter in the principal axes.
GP31B-02
Analysis of the Magnetic Susceptibility Anisotropy of the Cantera Ignimbrite, San Luis Potosé Volcanic field, Mexico
Anisotropy of Magnetic Susceptibility (AMS) results from a group of 17 - 18 sites (286 - 312 specimens) from the Cantera Ignimbrite - of Oligocene age and part of the San Luis Potosí Volcanic Filed (SLPVF), México - are presented and analysed in order to help to determine the source and flow directions. In each site a flow direction is inferred based on AMS results. As the Cantera Ignimbrite is generally dipping, AMS was structural corrected. So two sets of geographical and paleo-geographical (structural corrected) inferred-flow directions were obtained. Both sets are analysed trying to define if the source of the ignimbrite is related to a calderic (concentric structure) or to the NW-SE faulting and jointing. Geographical AMS results mostly give SW flow directions, the southernmost sites give to SSE. Meanwhile structural corrected results give a wider range of flow directions, a group of them to NW and another northerly group mostly to NE. AMS was measured in a KLY2 appliance, Jelinek and other statistics and density distributions were performed, giving all very similar results in each site. Mean susceptibilities range from 147 to 27200 x10-6 SI (average = 5713 x10-6 SI). Anisotropy degree (Pj) range from 1.011 to 1.055 with two sites of 1.134-1.254 (Pj average = 1.046). Shape is mostly oblate ranging the T-factor from 0.843 to 0.409 and only one site mainly prolate: T of -0.277 (T average = 0.550).
GP31B-03
Magnetic Fabrics in Chicxulub Crater Impact Breccias: Results From UNAM-5 Breccia Sequence
Chicxulub crater is our best example of a large complex impact structure that preserve the ejecta deposits. Their study is important in understanding cratering, fragmentation and collapse mechanisms, firewall deposits, post- cratering events, etc. Our objective is to investigate fabric type and characteristics fabric in the impact breccias by means of anisotropy of magnetic susceptibility (AMS). This study is made in core samples from the UNAM-5 drill hole located 110 km from the structure centre. In UNAM-5, impact breccias have 172m thickness of polymictic ‘suevitic' breccias with clay matrix rich in carbonate, impact glass, melt, basament fragments and melt-matrix breccias. From macroscopic and textural analyses of fragment size, color, matrix we defined four breccia subunits: subunit-1 (332-347m), subunit-2 (347-412m), subunit-3 (412-464m), subunit-4 (464-503.9m). AMS study gives two fabric types characterized by: (1) Prolate fabrics (332.05-450.7m), 55 percent lineation, and variable low-field susceptibility (250-1000x10-6 SI). (2) Oblate fabrics (450.72-503.9m), and high low-field susceptibilities (~700-1500x10-6 SI). In general the anisotropy degree is less than 1.1, in the range for sedimentary and volcanic rocks. Principal susceptibility axial distributions show relatively complex patterns, which can be linked to the textural characteristics of breccias and relative amounts of melt, basement clasts and matrix type. We observe that in the first 118m amount of melt is small, and for the next 53m amount of melt and fragments increases. Secondary alteration processes affecting the fabric in each subunit, including hydrothermal processes and weathering may have been important and their effects on the magnetic fabric are examined.