Emplacement of multiple magma sheets and wall rock deformation: Trachyte Mesa intrusion, Henry Mountains, Utah

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Abstract

A detailed structural and rock magnetic study of the Trachyte Mesa intrusion and deformed sedimentary wall rocks, Henry Mountains, Utah, indicates that the intrusion grew vertically and horizontally by the accumulation of multiple horizontal magma sheets. 2–3 cm thick shear zones recognized by intensely cataclasized plagioclase phenocrysts define the contact between sheets. Sheets have bulbous and / or steep frontal terminations and are flat on top. The foliation within the interior of the sheets, near the frontal termination, is subvertical. This steep foliation rotates into the subhorizontal shear zones near the top and bottom contacts and provides a magma flow direction indicator. Away from the frontal termination, the interior foliation rotates to become subhorizontal, similar to the fabric in recent analog experimental studies. Sheets are interpreted as being emplaced as plug flows. Both the field fabric and the rock magnetic data collected from 103 locations on the top of the intrusion and from 73 locations along a vertical cross section exposed in a stream gorge support a multi-stage model of intrusion growth. Emplacement begins as narrow magma channels and magma spreads radially outward from the channels to form sheets. Sheets are stacked upon one another and stop at the same lateral termination. The deformation of the sandstones at the margin of the intrusion, which are rotated upward from the margin to become the roof, is partitioned into layer parallel extension, shearing and layer-parallel shortening components. Bulk strain within the thickest sandstone layer indicates ∼20% thinning and microstructures indicate that the thinning was accommodated by grain-scale fracture-induced porosity collapse. Extension occurred as the layer was stretched over the margin of the rising intrusion. Shearing and layer-parallel shortening are a result of coupling with the underlying sheets as they advanced and accommodated through numerous faults parallel to bedding and at low angles to bedding. The deformation of thinner sedimentary layers is consistent with the sedimentary layering immediately in front of an advancing sheet being translated upward and over the top of the sheet as the hinge zone migrates with the front edge of the advancing sheet.

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

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