Timing, internal flow characteristics, and emplacement mechanisms of the intrusive sheet network on the southern margin of Mount Hillers, Henry Mountains, southern Utah
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The exceptional 3-d exposures of the mid-Tertiary intrusive sheet network on the southern margin of Mount Hillers, Henry Mountains, southern Utah, have undergone no syn- or post-emplacement deformation. The sills and dikes, which formed above the underlying Mount Hillers laccolith, therefore provide an ideal opportunity to study purely magmatic processes in a shallow crustal intrusive sheet network. For this study, field work and laboratory analysis were employed to constrain the timing, emplacement mechanisms, and internal flow characteristics of these sills and dikes. Detailed geologic mapping of cross-cutting relationships, in addition to qualitative textural analysis in the field, indicate that younger, relatively fine-grained dikes cross-cut older, relatively coarse-grained sills. Crystal size distribution, thin section petrography, and major and trace element geochemistry all suggest two distinct batches of magma (one coarse- and one fine-grained) were involved in the construction of the sill/dike complex. Field fabrics and anisotropy of magnetic susceptibility fabrics suggest complex internal flow of the intrusive sheets throughout the growth of the central intrusive igneous body. Field observations indicate that intruding magma exploited radial fractures and bedding planes in the sedimentary host rock. In addition, rigidity contrasts in the host rock were likely an important control on the stratigraphic level of sill emplacement and on intrusive sheet thickness. The proposed construction model for the intrusive sheet network consists of an initial phase of dike-fed sill emplacement in subhorizontal strata. During subsequent growth of the underlying main laccolithic body, which included uplift and rotation of the overburden, continued sill emplacement was followed by radial dike intrusion. This work provides insight into the growth and evolution of shallow crustal magmatic systems, such as those that underlie active volcanoes. ÂKeywords:
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Sheet intrusions represent important magma conduits and reservoirs in subvolcanic systems. Constraining the emplacement mechanisms of such intrusions is crucial to understanding the physiochemical evolution of magma, volcano deformation patterns, and the location of future eruption sites. However, magma plumbing systems of active volcanoes cannot be directly accessed and we therefore rely on the analysis of ancient systems to inform the interpretation of indirect geophysical and geochemical volcano monitoring techniques. Numerous studies have demonstrated that anisotropy of magnetic susceptibility (AMS) is a powerful tool for constraining magma flow patterns within such ancient solidified sheet intrusions. We conducted a high-resolution AMS study of seven inclined sheets exposed along the Ardnamurchan peninsula in northwest Scotland, and examined how magma flow in sheet intrusions may vary along and perpendicular to the magma flow axis. The sheets form part of the Ardnamurchan Central Complex, which represents the deeply eroded roots of an ∼58-m.y.-old volcano. Our results suggest that the inclined sheets were emplaced via either updip magma flow or along-strike lateral magma transport. It is important that observed variations in magnetic fabric orientation, particularly magnetic foliations, within individual intrusions suggest that some sheets were internally compartmentalized, i.e., different along-strike portions of the inclined sheets exhibit subtle differences in their magma flow dynamics. This may have implications for the flow regime and magma mixing within intrusions.
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The Little Minch Sill Complex consists of a series of stacked, multi-leaved Paleocene dolerite sills, which have primarily intruded into Mesozoic sedimentary rocks and Paleocene tuffs/?hyaloclastites within the Sea of Hebrides Basin. The Sea of Hebrides Basin is situated to the west of the Scottish mainland on the NE Atlantic margin. Two previously proposed models for the emplacement of the sill complex have opposing ideas for the location of magma input and the emplacement mechanisms. Both models have been constructed using data primarily from onshore outcrops on the Isle of Skye, Raasay and the Shiant Isles. However, these onshore outcrops only represent a quarter (1040 km 2 ) of the entire extent of the sill complex, which is largely located offshore. To understand how the sill complex as a whole was emplaced within the basin, both onshore and offshore magma transport needs to be considered. Using high-resolution multibeam bathymetry data (up to 2 m resolution) obtained between 2008 and 2011, along with supporting seismic reflection, sparker and pinger data, a new assessment of the offshore extent and character of the sill complex has been constructed. Mapping of the large-scale relationships between intrusions and the host rock, along with morphological features such as magma lobes, magma fingers, transgressive wings, en echelon feeder dykes and the axis of saucer-/half-saucer-shaped intrusions, has indicated the magma flow directions within the intrusive network. Assessing the flow kinematics of the sills has provided insights into magma transport and emplacement processes offshore. Combining data from previously mapped onshore sills with data from our newly constructed model for magma emplacement offshore has allowed us to construct a new model for the emplacement of the Little Minch Sill Complex. This model demonstrates that major basin-bounding faults may have a lesser role in channelling magma through sedimentary basins than previously thought. Applying the knowledge obtained from this study could further progress our understanding of the effect of sill emplacement on fluid flow within volcanic rift basins worldwide, with direct impacts on the exploitation of petroleum and geothermal systems.
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Research Article| January 01, 2012 Interconnected sills and inclined sheet intrusions control shallow magma transport in the Ferrar large igneous province, Antarctica James D. Muirhead; James D. Muirhead † 1School of Environment, University of Auckland, Private Bag 92019, Auckland, New Zealand †E-mail: jmuirhead@ymail.com Search for other works by this author on: GSW Google Scholar Giulia Airoldi; Giulia Airoldi 2Geology Department, University of Otago, Leith Street, P.O. Box 56, Dunedin 9054, New Zealand Search for other works by this author on: GSW Google Scholar Julie V. Rowland; Julie V. Rowland 1School of Environment, University of Auckland, Private Bag 92019, Auckland, New Zealand Search for other works by this author on: GSW Google Scholar James D.L. White James D.L. White 2Geology Department, University of Otago, Leith Street, P.O. Box 56, Dunedin 9054, New Zealand Search for other works by this author on: GSW Google Scholar GSA Bulletin (2012) 124 (1-2): 162–180. https://doi.org/10.1130/B30455.1 Article history received: 20 Nov 2010 rev-recd: 25 Mar 2011 accepted: 06 Apr 2011 first online: 08 Mar 2017 Cite View This Citation Add to Citation Manager Share Icon Share Facebook Twitter LinkedIn MailTo Tools Icon Tools Get Permissions Search Site Citation James D. Muirhead, Giulia Airoldi, Julie V. Rowland, James D.L. White; Interconnected sills and inclined sheet intrusions control shallow magma transport in the Ferrar large igneous province, Antarctica. GSA Bulletin 2012;; 124 (1-2): 162–180. doi: https://doi.org/10.1130/B30455.1 Download citation file: Ris (Zotero) Refmanager EasyBib Bookends Mendeley Papers EndNote RefWorks BibTex toolbar search Search Dropdown Menu toolbar search search input Search input auto suggest filter your search All ContentBy SocietyGSA Bulletin Search Advanced Search Abstract Field observations and structural data from intrusive complexes at Allan Hills and Terra Cotta Mountain, South Victoria Land, Antarctica, demonstrate that interconnected sills and inclined sheets transported magma through the shallow subsurface. These sills and sheets represent the upper-crustal (top 4 km) plumbing system of the 183 Ma Ferrar large igneous province. The sheets are short in length (<1500 m), are moderately inclined (47° and 51° means), and show meter-scale variations in attitude; in places, they intruded bedding planes, resulting in stepped sheet-sill geometries. Sheet geometries and their relationship to the surrounding country rock are consistent with peripheral sheet intrusion under local magmatic stresses arising from roof-lift during sill injection. The sheet intrusions thus reflect the intrusive process itself rather than a far-field tectonic stress regime. The sills and sheets, together with local dolerite masses, formed the intrusive network that supplied magma to the Mawson Formation pyroclastic rocks in various parts of South Victoria Land and, by inference, the Kirkpatrick flood basalt lavas. The predominance of inclined sheets rather than steeply dipping dikes indicates a magmatic environment that is unlike the Jurassic rift arm inferred by previous authors. This could be explained using any of the following three scenarios. (1) The axis of the rift, and hence any rift-hosted dikes, lies beyond the current exposure area. (2) The regionally extensive Ferrar sills may have provided rheologically weak horizons that limited mechanical coupling of the basement rocks and overlying Beacon Supergroup, locally detaching the upper 4 km of the crust from possible synmagmatic basement extension below. (3) The Ferrar large igneous province was emplaced in a neutral tectonic setting. In this scenario, broad-scale distribution of magma through the province was controlled by preexisting structure in the basement, and local intrusion geometries reflect the physical interaction of intruding magma with bedding anisotropy of the Beacon Supergroup. You do not have access to this content, please speak to your institutional administrator if you feel you should have access.
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Field and seismic observations show that numerous sills exhibit lobate morphologies. Each lobe corresponds to a distinct igneous segment exhibiting a finger-like shape, the long axis of which is commonly interpreted as a magma-flow indicator. Robust understanding of the emplacement mechanisms of finger-shaped sills, and direct observations supporting finger orientation as magma-flow indicator are lacking. In this paper, we present the results of detailed structural mapping on an exceptional, easily accessible 1-km long outcrop in the Neuquén Basin, Argentina, that exhibits a sill, its contacts and the structures in the finely layered sedimentary host rock. We show that the sill is made of distinct segments that grew, inflated or coalesced. We also demonstrate that the fingers were emplaced according to the viscoelastic fingering or viscous indenter models, with no field evidence of tensile elastic fracture mechanism as commonly assumed in mechanical models of sill emplacement. We identified new structural criteria at the intrusion's contacts for inferring magma flow direction during the magma emplacement. Our small-scale structural observations carried out on a seismic-scale outcrop have the potential to considerably aid the structural interpretation of seismic data imaging igneous sills, i.e. to fill the standard gap between outcrop-scale field observations and seismic-scale geophysical data.
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The structure of upper crustal magma plumbing systems controls the distribution of volcanism and influences tectonic processes. However, delineating the structure and volume of plumbing systems is difficult because (1) active intrusion networks cannot be directly accessed; (2) field outcrops are commonly limited; and (3) geophysical data imaging the subsurface are restricted in areal extent and resolution. This has led to models involving the vertical transfer of magma via dikes, extending from a melt source to overlying reservoirs and eruption sites, being favored in the volcanic literature. However, while there is a wealth of evidence to support the occurrence of dike-dominated systems, we synthesize field- and seismic reflection–based observations and highlight that extensive lateral magma transport (as much as 4100 km) may occur within mafic sill complexes. Most of these mafic sill complexes occur in sedimentary basins (e.g., the Karoo Basin, South Africa), although some intrude crystalline continental crust (e.g., the Yilgarn craton, Australia), and consist of interconnected sills and inclined sheets. Sill complex emplacement is largely controlled by host-rock lithology and structure and the state of stress. We argue that plumbing systems need not be dominated by dikes and that magma can be transported within widespread sill complexes, promoting the development of volcanoes that do not overlie the melt source. However, the extent to which active volcanic systems and rifted margins are underlain by sill complexes remains poorly constrained, despite important implications for elucidating magmatic processes, melt volumes, and melt sources.
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