When an ‘inverse’ fabric is not inverse: an integrated AMS‐SPO study in MORB‐like dykes

Hastie, Warwick W.; Aubourg, Charles; Watkeys, M. K.

doi:10.1111/j.1365-3121.2010.00983.x

Cited by 21 publications

(13 citation statements)

References 35 publications

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“…In oblate fabrics, the planar elements of the magnetic fabric dominate and the shape of the susceptibility ellipsoid is flattened in the K 1 – K 2 plane. If the fabric elements at a site are strongly oblate and the 95% confidence ellipses of the K 1 and K 2 axes overlap in the K 1 − K 2 plane, then the K 1 orientation must be treated with caution [ Hastie et al ., ]. Hastie et al .…”

Section: Resultsmentioning

confidence: 99%

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Emplacement of the early Miocene Pinto Peak intrusion, Southwest Utah, USA

Petronis

O’Driscoll

2013

Geochem Geophys Geosyst

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[1] In this contribution, we report rock magnetic, petrographic, and anisotropy of magnetic susceptibility (AMS) data from the Pinto Peak intrusion, all of which bear on volcanic construction. Rock magnetic data indicate that the dominant magnetic mineral phase is low-Ti titanomagnetite of multidomain grain size, the composition of which varies spatially across the intrusion. The intrusion is a porphyritic andesite dominated by Ca-rich plagioclase (>60%) as well as biotite, amphibole, olivine, and opaque minerals. Reflected light petrography reveals mostly euhedral-subhedral (titano)magnetite crystals that often form clustered glomerocrysts and stringers of equant crystals, without exhibiting a consistent mineral alignment fabric. Moderate-to-shallow plunging prolate magnetic susceptibility ellipsoids dominate the northern part of the intrusion while steeply dipping/plunging magnetic susceptibility ellipsoids are generally restricted to the southern part of the intrusion. The vent facies rocks yield moderate-to-steep oblate susceptibility ellipsoids. We interpret the flow pattern in the north to reflect subhorizontal flow of magma, filling a tabular sheet-like body associated with propagation of the intrusion to the north. We argue that the southern part of the intrusion represents the ascent site of the magma rising to shallow crustal levels along a steep feeder system. The oblate magnetic fabrics in the vent area plausibly represent flattening against the conduit walls as evidenced by a weak planar flow foliation observed in the vent conduit rocks. On reaching shallow crustal levels, the magma deformed and uplifted the roof rocks leading to gravitational instability. As the slide mass released from the roof, an explosive eruption ensued resulting in the emplacement of the Rocks of Paradise tuff and associated effusive lava flows. Following eruption, magma pressure decreased and the magma drained northward forming the northern intrusive phase.

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Section: Resultsmentioning

confidence: 99%

“…Hastie et al . [], however, showed that the K 1 lineation may provide information on the flow fabric in planar intrusions even when the magnetic fabrics are oblate and inverse. When the confidence ellipses of K 1 and K 2 axes do not overlap, the K 1 orientation may reflect a flow or stretching direction.…”

Section: Resultsmentioning

confidence: 99%

Emplacement of the early Miocene Pinto Peak intrusion, Southwest Utah, USA

Petronis

O’Driscoll

2013

Geochem Geophys Geosyst

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“…Magnetic fabric may easily reveal mineral distributions that show an imbrication of the magnetic lineation and/or magnetic foliation with respect to the dyke margins, which may be used to infer the sense of flow, as shown in Figure 2. This approach is widely used in the literature to determine magma flow within dykes (Knight & Walker 1988;Varga et al 1998;Aubourg et al 2002;Geoffroy et al 2002;Porreca et al 2006;Soriano et al 2008;Silva et al 2010), even though it is not adopted for all cases (Kissel et al 2010;Hastie et al 2011aHastie et al , b, 2013. This ideal situation may be complicated by the occurrence of …”

Section: Methods and Laboratory Analysesmentioning

confidence: 99%

“…In particular, amphibole phenocrysts and vesicles orientations were statistically treated by means of the image analysis of photographs taken in these natural sections in different orientations, either parallel or orthogonal to the dyke plan (Launeau et al 2010). The SPO technique is widely used for studying rock fabric and is generally applied to microscopic images (thin section and SEM images) when several cases of 'inverse' magnetic fabric are recognized in the samples (Archanjo et al 2002;Launeau et al 2010;Hastie et al 2011a, b and references therein). In our case, except for a very few specimens, most of the AMS data show a 'normal' fabric: that is, with magnetic foliation (K max 2 K int ) parallel to the dyke plane.…”

Section: Methods and Laboratory Analysesmentioning

confidence: 99%

Magma flow within dykes in submarine hyaloclastite environments: an AMS study of the Miocene Cabo de Gata volcanic units

Porreca

Cifelli

Soriano

et al. 2014

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The Miocene Cabo de Gata volcanic arc in SE Spain comprises a wide variety of volcanic\ud facies and eruptive styles in subaqueous to subaerial environments. In the SW sector of the\ud area, 5–100 m-thick, NNW–SSE-orientated dykes feed and intrude submarine hyaloclastite\ud deposits. We analysed the anisotropy of magnetic susceptibility (AMS) of six dykes and five hyaloclastite\ud sites from three volcanic units: the Cerro Can˜adillas, Los Frailes, and El Barronal formations.\ud The main magnetic minerals are primary low-Ti titanomagnetite and magnetite. The\ud AMS ellipsoids in the dykes are generally oblate-triaxial in shape, with magnetic foliations subparallel\ud to the dyke walls. Kinematic field evidence supports the inferred flow directions deduced from\ud magnetic lineation and imbrication of magnetic foliation. The geometric relationships between\ud dyke margins and AMS axes indicate that dykes at El Barronal were emplaced via prevalent subvertical\ud upward magma flow. The inferred flow directions are reproduced well by analogue models\ud of AMS simulating magma migration in dykes with a diapiric geometry. The other dykes were\ud emplaced by lateral magma propagation. Conversely, hyaloclastite shows a large scatter of the\ud AMS axes reflecting different degrees of fragmentation. We observe a gradual increase in\ud scatter in the AMS from confined dykes to fragmented hyaloclastite

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“…In this case, additional investigations of the magnetic mineralogy and other independent observations are necessary to understand its origin (e.g. Archanjo et al 2002;Chadima et al 2009;Kissel et al 2010;Hastie et al 2011).…”

Section: Anisotropy Of Magnetic Susceptibility (Ams) As Magmatic Flowmentioning

confidence: 99%

The use of palaeomagnetism and rock magnetism to understand volcanic processes: introduction

Ort

Porreca

Geissman

2015

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This Special Publication provides a snapshot of our understanding of volcanic processes through the use of palaeomagnetic and rock magnetic techniques. Here, we provide a context for the book, placing individual chapters within the milieu of previous work, including some magnetic techniques that were not used in the particular studies described herein. Thermoremanent magnetization is a powerful tool to understand processes related to heating and cooling of rocks, including estimating the temperature of emplacement of pyroclastic deposits, which may allow us to better understand the rates of cooling during eruption and transport. Anisotropy of magnetic susceptibility and anisotropy of remanence are used primarily to investigate rock fabrics, and allow the interpretation of flow dynamics in dykes, lava flows and pyroclastic deposits, as well as the location of the eruptive vents. Rock magnetic characteristics can help in the correlation of volcanic deposits but also provide means to date volcanic deposits and to better understand the processes of cooling of the deposits, as the magnetic minerals can change with temperature. In addition, volcanic rocks may be key recorders of past magnetic fields, allowing a better understanding of changes in field intensity and, perhaps, providing clues of how the magnetic field is formed.

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When an ‘inverse’ fabric is not inverse: an integrated AMS‐SPO study in MORB‐like dykes

Cited by 21 publications

References 35 publications

Emplacement of the early Miocene Pinto Peak intrusion, Southwest Utah, USA

Emplacement of the early Miocene Pinto Peak intrusion, Southwest Utah, USA

Magma flow within dykes in submarine hyaloclastite environments: an AMS study of the Miocene Cabo de Gata volcanic units

The use of palaeomagnetism and rock magnetism to understand volcanic processes: introduction

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