Emplacement of multiple magma sheets and wall rock deformation: Trachyte Mesa intrusion, Henry Mountains, Utah
Introduction
In the last ten years evidence has been accumulating that many intrusions were emplaced incrementally from small batches of magma. As a result, the concept of plutons as large crustal magma chambers is being reassessed. For example, using field relationships and AMS data, McNulty et al. (1996) and Mahan et al. (2003) interpreted the emplacement of elongate plutons in the central Sierra Nevada of California as an accumulation of dikes. Elliptical plutons are even interpreted to be a result of several phases of magma inflation (Johnson and Vernon, 2004). Geochronologic data from some classic intrusive suites also support this interpretation. The intrusive suites of the Tuolumne and Mt. Stuart Batholiths have ages that span between ten and five million years (Coleman et al., 2004, Matzel et al., 2006), which is longer than cooling models allow for a large single magma chamber to exist in the middle to upper crust (Glazner et al., 2004). The incremental assembly model of pluton emplacement is also consistent with dike transport of magma, which is now commonly invoked as the dominant mechanism of magma ascent through the middle and upper crust (Clemens and Mawer, 1992, Petford, 1996).
The major difficulty for the incremental assembly model is the lack of evidence for individual pulses. Internal contacts are observed in only a handful of intrusions and mostly aided by differences in composition between pulses (e.g., Wiebe, 2003, Mahan et al., 2003, Harper et al., 2004, Matzel et al., 2006). Work on the Birch Creek pluton, California, has documented the presence of multiple internal contacts of generally similar composition (M. Barton, personal communication). In large, compositionally homogeneous igneous bodies, these types of contact are often not observed. It has been speculated that some internal contacts are destroyed by the emplacement of additional magma pulses or will always remain cryptic as a result of post-emplacement processes (Glazner et al., 2004, Matzel et al., 2006).
In this paper we describe and model the emplacement of a small tabular intrusion that preserves evidence for internal “sheeted” contacts. The Trachyte Mesa intrusion (TMI) is one of many small satellite intrusions in the Henry Mountains of Utah (Fig. 1). Our study focuses on both the fabric within the igneous rock and the deformation of the surrounding wall rocks. Based on contact relationships, we suggest that the TMI is exposed very close to its original emplacement dimensions, providing us with details about its emplacement not ordinarily observed at other intrusions. The unique exposure reveals a series of thin, sub-horizontal shear zones within the intrusion, which we interpret as contacts between magma sheets. Microstructures indicate that the sheets within the TMI were emplaced at very high strain rates. Using the known geometry of the intrusion along with rock magnetic fabrics, we constrain the magma flow patterns during sequential stacked sheet emplacement (e.g. Morgan at al., 2005). This model of stacking magma sheets (Morgan et al., 2005) is in accordance with one of Hunt's original emplacement models (Hunt, 1953) for intrusions in the Henry Mountains. Our data from this small intrusion clearly demonstrate emplacement processes and fabric–wall rock relationships that either do not form or are obscured in larger, more complex intrusions.
Section snippets
Geologic setting and previous work
The Henry Mountains of south-central Utah, USA (Fig. 1), provide a unique setting for the study of igneous emplacement in the shallow crust. The magma bodies intruded the Mesozoic stratigraphy of the Colorado Plateau where high elevations and a lack of vegetation allow the shape of the intrusions and the displacement of the wall rocks to be easily documented (e.g. Gilbert, 1877, Hunt, 1953, Pollard and Johnson, 1973, Jackson and Pollard, 1988, Habert and de Saint Blanquat, 2004, Horsman et al.,
Physiography
The TMI is 1.5 km long and 0.6 km wide (Fig. 2) and varies in thickness between 5 m in the NE to greater than 50 m in the SW. The intrusion defines the top of the mesa and the edge of the intrusion is the edge of the mesa except in the SW where the margin is covered by alluvium. The long axis of the TMI trends NE and is along a line that can be traced 12.6 km SW to the peak of Mount Hillers. The top is generally flat although the NE half can be divided into three geomorphic regions (Fig. 2): (1) a
Description of the WNW Outcrop
At the western end of the northwestern margin of the mesa (Fig. 2), a complete cross section through the deformed sedimentary contact is gradationally exposed along ∼100 m (horizontal distance) (Fig. 4). This is one of two locations where the sedimentary layers at the contact with the lateral margin of the TMI are preserved and not eroded or covered by alluvium. The shear zone at the contact and the outer margin of the intrusion are also well preserved at this location. As we describe in more
Magnetic carriers
Measurement of the anisotropy of magnetic susceptibility (AMS) in igneous rocks provides a proxy for the shape orientation (petrofabric) of the minerals and is commonly used to infer the flow of magma in an intrusion (Bouchez, 1997 and references therein). AMS is approximated by a symmetric 2nd rank tensor, represented by an ellipsoid with three principal axes (K1 ≥ K2 ≥ K3). The long axis of the ellipsoid is generally aligned parallel to the flow direction while the short axis is the normal to the
Sheets
We interpret the thin, planar, sub-horizontal shear zones observed at several locations to represent the boundaries between individual magma sheets. The degree to which the shear zone contacts extend into the interior of the intrusion, and therefore the degree to which individual sheets exist into the interior, is unknown. Field evidence indicates that these contacts exist for at least 30 m into the interior based on the extent of the sheets at the cross-section outcrop.
Additionally, individual
Conclusions
The Trachyte Mesa intrusion was emplaced into the upper crust as a series of sub-horizontally stacked magma sheets. Contacts between sheets are observed at the intrusion margins and are defined by thin shear zones. Shear zones are defined by plagioclase phenocrysts that have undergone severe cataclasis. Sheets vary in their shape but most are meters thick and have steep frontal terminations. Geomorphic and structural data also support the presence of sheets across the top of the TMI.
AMS fabric
Acknowledgements
We thank Nicholas Koepke, Lisa Bishop, Andrew Nugent and Brandi Boyd for field assistance. Dave Dilloway and the Hanksville B.L.M. office provided essential logistical assistance. Charlie Onasch is thanked for all his assistance with the strain analyses. Mike Jackson is thanked for assisting magnetic analyses at the Institute for Rock Magnetism at the University of Minnesota. Daming Wang and Josep Pares assisted with magnetic analyses at the University of Michigan. Calvin Miller and Daniel Holm
References (49)
- et al.
Intra-crater activity, AA-block lava, viscosity and flow dynamics: Arenal volcano, Costa Rica
Journal of Volcanology and Geothermal Research
(1984) - et al.
Granitic magma transport by fracture propagation
Tectonophysics
(1992) - et al.
Mechanisms and duration of non-tectonically assisted magma emplacement in the upper crust: the Black Mesa pluton, Henry Mountains, Utah
Tectonophysics
(2006) Normalized center-to-center strain analysis of packed aggregates
Journal of structural Geology
(1988)Random point distributions and strain measurements in rocks
Tectonophysics
(1979)- et al.
Effects of magnetic interactions in anisotropy of magnetic susceptibility: discussion on models and experiments—implications for the quantification of rock fabrics
Tectonophysics
(2006) - et al.
Emplacement-related fabric in a sill and multiple sheets in the Maiden Creek sill, Henry Mountains, Utah
Journal of Structural Geology
(2005) - et al.
Foliation development and progressive strain-rate partitioning in the crystallizing carapace of a tonalite pluton; microstructural evidence and numerical modeling
Geological Society of America Bulletin
(2004) - et al.
Toward more realistic formulations for the analysis of laccoliths
Journal of Structural Geology
(1998) - et al.
Monoclinal bending of strata over laccolithic intrusions
Tectonophysics
(1981)
Non-scaled analogue modelling of AMS development during viscous flow: A simulation on diapir-like structures
Tectonophysics
Mechanics of growth of some laccolith intrusions in the Henry mountains, Utah, II; bending and failure of overburden layers and sill formation
Tectonophysics
Damage zone and slip-surface evolution over μm to km scales in high-porosity Navajo sandstone, Utah
Journal of Structural Geology
Pressure-induced microckracking and grain crushing in Berea and Baise sandstones: acoustic emission and quantitative microscopy measurements
Mechanics of Materials
Analogue 3D simple shear experiments of magmatic biotite subfabrics
Granite is never isotropic: an introduction to AMS studies of granitic rocks
Rethinking the emplacement and evolution of zoned plutons; geochronologic evidence for incremental assembly of the Tuolumne Intrusive Suite, California
Geology
Igneous rocks and constituent hornblendes of the Henry Mountains, Utah
Geological Society of America Bulletin
Are plutons assembled over millions of years by amalgamation from small magma chambers?
GSA Today
Explosive volcanism may not be an inevitable consequence of magma fragmentation
Nature
Shape anisotropy versus magnetic interactions of magnetite grains; experiments and application to AMS in granitic rocks
Geophysical Research Letters
Cited by (87)
Magnetic fabrics reveal three-dimensional flow processes within elongate magma fingers at the margin of the Shonkin Sag laccolith (MT, USA)
2023, Journal of Structural GeologyRe-evaluation of the role of volatiles in the rupture of magma chambers and the triggering of crystal-rich eruptions
2023, Journal of Volcanology and Geothermal ResearchSediment deformation triggered by underlying magma intrusion
2022, Journal of Asian Earth Sciences