Downward host rock transport and the formation of rim monoclines during the emplacement of Cordilleran batholiths

2008 ◽  
Vol 97 (4) ◽  
pp. 397-413 ◽  
Author(s):  
Scott R. Paterson ◽  
David W. Farris
Keyword(s):  
Channel Flow ◽  
Host Rock ◽  
Crustal Thickening ◽  
Magma Ascent ◽  
Field Observations ◽  
Pluton Emplacement ◽  
Common Features ◽  
Roof Strata ◽  
Sufficient Space ◽  
Emplacement Depth

AbstractThe mechanisms by which Cordilleran plutons are emplaced vary widely. However, the present authors have examined a series of plutons ranging from 2-35 km emplacement depth that have many common features, which suggest that downward transport of host rock is the most important mechanism during magma ascent and pluton emplacement. Many of these Cordilleran plutons preserve gently dipping, unfaulted roofs attached to steep walls bordered by narrow ductile aureoles. Flat lying roof strata commonly roll over into steeply dipping rim monoclines and anticlines that young towards and follow the pluton margin. Field observations suggest that such rim monoclines and anticlines formed due to gravitationally driven roof collapse and channel flow along margins. In the examples in this paper, pluton walls are often comprised of narrow steeply dipping ductile aureoles in which the intensity of strain increases downward. Aureole ductile strains are insufficient to account for the volume of magma emplaced, and are typically <40% of pluton volume. However, when aureole strain is combined with minimum estimates of stoping and host rock rotation during rim monoclines formation, sufficient space can be created. The examples suggest that gravitationally driven downward host rock transport by stoping and rigid rotations along roofs and walls and by focused channel flow by ductile strain along walls are common processes during the rise of Cordilleran plutons, and is one process that contributes to crustal thickening and the growth of crustal roots.

Late Caledonian transpression and the structural controls on pluton construction; new insights from the Omey Pluton, western Ireland

2015 ◽  
Vol 106 (1) ◽  
pp. 11-28 ◽  
Author(s):  
William McCarthy ◽  
R. John Reavy ◽  
Carl T. Stevenson ◽  
Michael S. Petronis
Keyword(s):  
Spatial Distribution ◽  
Host Rock ◽  
Shear Zones ◽  
Magma Ascent ◽  
Structural Controls ◽  
Rock Structure ◽  
Granite Complex ◽  
Caledonian Orogeny ◽  
Western Ireland

ABSTRACTThe Galway Granite Complex is unique among the British and Irish Caledonian granitoid terranes, as it records punctuated phases of magmatism from ∼425–380 Ma throughout the latest phase of the Caledonian Orogeny. Remapping of the Omey Pluton, the oldest member of this suite, has constrained the spatial distribution and contact relationships of the pluton's three main facies relative to the nature of the host rock structure. The external contacts of the pluton are mostly concordant to the limbs and hinge of the Connemara Antiform. New AMS data show that a subtle concentric outward dipping foliation is present, and this is interpreted to reflect pluton inflation during continued magma ingress. Combined field, petrographic and AMS data show that two sets of shear zones (NNW–SSE and ENE–WSW) cross-cut the concentric foliation, and that these structures were active during the construction of the pluton. We show that regional sinistral transpression at ∼420 Ma would have caused dilation along the intersection of these two fault sets, and suggest that this facilitated centralised magma ascent. Lateral emplacement was controlled by the symmetry of the Connemara Antiform to ultimately produce a discordant phacolith. We propose that regional sinistral transpression at ∼420 Ma influenced the siting of smaller intrusions over NNW–SSE faults, and that the later onset of regional transtension caused larger volumes of magma to intrude along the E–W Skird Rocks Fault at ∼400 Ma.

Spatial variations of sills and implications for magma dispersal across the Karoo basin

2020 ◽  
Vol 123 (4) ◽  
pp. 511-530
Author(s):  
A. Coetzee ◽  
A.F.M. Kisters
Keyword(s):  
Host Rock ◽  
Sedimentary Facies ◽  
Spreading Rate ◽  
Lateral Spreading ◽  
Large Igneous Province ◽  
Propagation Path ◽  
Magma Ascent ◽  
Subsequent Formation ◽  
Karoo Basin ◽  
Lateral Propagation

Abstract Dolerite sill complexes of the Karoo Large Igneous Province (ca. 183 Ma) show systematic variations in emplacement style and size throughout the Karoo basin. These variations are explained in terms of three main, interrelated factors, namely the overburden thickness or emplacement depth, variations in host rock rigidities as a result of sedimentary facies changes in the Karoo basin, and proximity to magma feeders. In the northern parts of the thinner (&lt;500 m) and more coarse-clastic Karoo stratigraphy, sills intrude preferentially below more rigid sandstone horizons that acted as stress barriers causing the arrest of magma ascent and lateral spreading below sandstone beds. The low overburden promotes roof uplift above sills and associated brittle faulting can initiate the formation of inclined sheets that limits the lateral propagation path of inner sills. Roof uplift is further promoted by the proximity to magma feeders in the basement and resulting variations in magma pressure that control the spreading rate and inflation of sills. Localised dyke networks spaced at regular intervals and rooted in underlying sills reflect the stretching of roof rocks above inflating sills. The combination of these effects results in relatively small (&lt;10 km) diameters of sills in the northern parts of the basin. Sills emplaced at intermediate depths (ca. 700 m) in the central Karoo basin are marked by larger diameters (&gt;30 km) and thicknesses of up to 100 m. This reflects the higher overburden pressures and the delay of roof failure and subsequent formation of inclined sheets. Dyke networks in the roof of these sills become more irregular and non-systematic at these greater depths. At even greater depths of up to 2 km in the southern parts of the Karoo basin, mega-sills reach diameters of 50 to 80 km, but thicknesses of only up to 35 m. Thick shale-rich sequences in the southern Karoo basin facilitate sill emplacement through internal host-rock deformation and ductile flow. The thicker overburden and different host rock rigidity delay or suppress roof failure and formation of inclined sheet, thus allowing for the lateral propagation of sills. The deeper-seated sills are typically not associated with local dyke networks.

Structural controls on the emplacement and evacuation of magma from a sub-volcanic laccolith: Reyðarártindur Laccolith, SE Iceland

2021 ◽  
Author(s):  
Vincent Twomey ◽  
William McCarthy ◽  
Craig Magee
Keyword(s):  
Host Rock ◽  
Surface Deformation ◽  
Flow Direction ◽  
Volcanic Eruptions ◽  
Magma Ascent ◽  
Magma Flow ◽  
Existing Structures ◽  
Forced Folding ◽  
Deformation Patterns ◽  
Echelon Fault

&lt;p&gt;Laccoliths play a significant role in the transport and storage of magma in sub-volcanic systems. The construction and geometry of laccoliths can influence host rock and surface deformation patterns that may precede and provide warning of active magmatism and impending eruptions. Yet how laccolith construction and internal magma dynamics controls the location and form of magma ascent conduits (e.g., dykes and inclined sheets), which facilitate magma evacuation and may feed volcanic eruptions, remains poorly documented in natural examples.&lt;/p&gt;&lt;p&gt;The excellently exposed silicic, sub-volcanic Miocene Rey&amp;#240;ar&amp;#225;rtindur Laccolith in SE Iceland offers an opportunity to investigate how magma ascent within inclined sheets, which emanated from the laccolith, related to intrusion construction and deformation in the surrounding host rock. We combine detailed structural mapping with anisotropy of magnetic susceptibility (AMS) analyses, which allow us to map magnetic rock fabrics that reflect magma flow patterns, to show that the laccolith comprises of several distinct magma lobes that intruded laterally towards the south-west. Each lobe intruded, inflated, and coalesced along a NE-SW primary axis facilitated by doming (i.e., forced folding) of the host rock. We also shown that pre-existing NNE-striking, left-stepping, en-echelon fault/fractures, as well as those generated during intrusion-induced host rock uplift, host moderately to steeply inclined rhyolitic/granophyric sheets that emanate from the lateral terminations of some flow lobes.&lt;/p&gt;&lt;p&gt;Based on the observed geometrical relationships between AMS fabrics and the sheet margins where magnetic foliations subparallel sheet contacts, or characterize an imbrication fabric, we suggest that magma evacuated moderately to steeply upward via these fault/fracture-controlled sheets. As these inclined sheets dip towards the laccolith, any eruptions they may have fed would have been laterally offset from the laccolith and any overlying surface deformation driven by forced folding. Our study shows that magma evacuation and ascent from laccoliths can be facilitated by inclined sheets that form at the lateral terminations of magma lobes that are spatially controlled by laccolith construction (e.g., flow direction and doming of the host rock) and the presence of pre-existing structures.&lt;/p&gt;

Incision migration across Eastern Tibet controlled by monsoonal climate, not tectonics?

2020 ◽  
Author(s):  
Katharine Groves ◽  
Mark Allen ◽  
Christopher Saville ◽  
Martin Hurst ◽  
Stuart Jones
Keyword(s):  
Tibetan Plateau ◽  
Transition Zone ◽  
Channel Flow ◽  
East Asian Monsoon ◽  
Crustal Thickening ◽  
Mean Annual Precipitation ◽  
The Tibetan Plateau ◽  
Geomorphic Indices ◽  
Hypsometric Integral ◽  
Early Cenozoic

&lt;p&gt;The formation and uplift history of the Tibetan Plateau, driven by the India-Eurasia collision, is the subject of intense research. Geomorphic indices capture the landscape response to competition between climate and tectonics and reflect the spatial distribution of erosion. We analyse the link between climate and tectonics in the eastern part of the Tibetan Plateau using the mean annual precipitation, digital elevation data, and by calculating the geomorphic indices hypsometric integral (HI), surface roughness (SR) and elevation relief ratio (ZR). This is a region where competing tectonic models suggest either early Cenozoic plateau growth, or a late phase of crustal thickening, surface uplift and plateau growth driven by lower crustal flow (&amp;#8220;channel flow&amp;#8221;).&lt;/p&gt;&lt;p&gt;Swath profiles of rainfall, elevation and the geomorphic indices were constructed, orthogonal to the internal drainage boundary. Each profile was analysed to find the location of maximum change in trend. A broad transition zone is present in the landscape, where changes in landscape and precipitation are grouped and in alignment. The zone cuts across structural boundaries. It represents, from East to West, a sharp decline in precipitation below ~650 mm/yr (interpreted as the western extent of the East Asian monsoon), a change from a high relief landscape to smoother elevations at 4500-5000 m, a transition to low HI (&lt; 0.05), a decrease in SR and an increase in ZR. This zone is not a drainage divide: the main rivers have their headwaters further West, in the interior of the plateau.&lt;/p&gt;&lt;p&gt;We argue that this geomorphic-climatic transition zone represents a change from incised to non-incised landscapes, the location of which is controlled by the western extent of the monsoon. Published low temperature thermochronology data suggest the plateau had reached its modern extent at the Eocene, but has been exhumed since ~15 Ma to the East of the transition zone, at least along major drainage networks. We therefore also suggest that the transition zone is the current position of a long-term wave of incision that has migrated from East to West, driven by late Cenozoic intensification of the monsoon climate. This work supports a model of early Cenozoic growth of the eastern Tibetan Plateau, superimposed by incision driven by climate change; it does not support the channel flow model.&lt;/p&gt;

Emplacement mechanisms of a dyke swarm across the Brittle-Ductile transition

2020 ◽  
Author(s):  
Hans Jørgen Kjøll ◽  
Olivier Galland ◽  
Loic Labrousse ◽  
Torgeir B. Andersen
Keyword(s):  
Host Rock ◽  
Mafic Magma ◽  
Large Igneous Province ◽  
Dyke Swarm ◽  
Field Observations ◽  
Transport Pathways ◽  
Influx Rate ◽  
Brittle Ductile Transition ◽  
Iapetus Ocean ◽  
Rifted Margin

&lt;p&gt;Dykes are the main magma transport pathways through the Earth&amp;#8217;s crust and, in volcanic rifts, they are considered the main mechanism to accommodate tectonic extension. Most models consider dykes as hydro-fractures propagating as brittle tensile, mode I cracks opening perpendicular to the least principal stress. This implies that dykes emplaced in rifts are expected to be sub-vertical and accommodate crustal extension. Here we present detailed field observations of a well-exposed dyke swarm that formed near the brittle-ductile transition at a magma-rich rifted margin during opening of the Iapetus Ocean. It was related to a ca 600 million year-old large igneous province. Our observations show that dykes were not systematically emplaced by purely brittle deformation and that dyke orientation may differ from the typical mode 1 pattern. Distinct dyke morphologies related to different emplacement mechanisms have been recognized including: 1) Brittle dykes that exhibit straight contacts with the host rock, sharp tips, and en-echelon segments with bridges exhibiting angular fragments; 2) Brittle-ductile dykes with undulating contacts, rounded tips, folding of the host rock and contemporaneous brittle and ductile features; 3) Ductile &amp;#8220;dykes&amp;#8221; with rounded shapes and mingling between partially molten host rock and the intruding mafic magma. The brittle dykes exhibit two distinct orientations separated by ~30&amp;#176; that are mutually cross-cutting, demonstrating that the dyke swam did not consist of only vertical sheets oriented perpendicular to regional extension, as expected in rifts. By using the host-rock layers as markers, a kinematic restoration to quantify the average strain accommodating the emplacement of the dyke complex was performed. This strain estimate shows that the dyke swarm accommodated &gt;100% horizontal extension, but also 27% vertical thickening. This suggests that the magma influx rate was higher than the tectonic stretching rate, which imply that magma was emplaced forcefully, as supported by field observations of the host-rock deformation. Finally, observations of typical &amp;#8220;brittle&amp;#8221; dykes that were subsequently deformed by ductile mechanisms as well as dykes that were emplaced by purely ductile mechanisms suggest that the fast emplacement of the dyke swarm triggered a rapid shallowing of the brittle-ductile transition. The abrupt dyke emplacement and associated heating resulted in weakening of the crust that probably facilitated the continental break-up, which culminated with opening of the Iapetus Ocean.&lt;/p&gt;

scholarly journals Formation and transfer of stoped blocks into magma chambers: The high-temperature interplay between focused porous flow, cracking, channel flow, host-rock anisotropy, and regional deformation

Geosphere ◽  
2012 ◽  
Vol 8 (2) ◽  
pp. 443-469 ◽  
Author(s):  
Scott R. Paterson ◽  
Valbone Memeti ◽  
Geoffrey Pignotta ◽  
Saskia Erdmann ◽  
Jiří Žák ◽  
...  
Keyword(s):  
High Temperature ◽  
Channel Flow ◽  
Host Rock ◽  
Magma Chambers ◽  
Porous Flow ◽  
Regional Deformation ◽  
Rock Anisotropy

Monsoon-driven incision and exhumation of the Eastern Tibetan Plateau

2021 ◽  
Author(s):  
Katharine Groves ◽  
Mark Allen ◽  
Christopher Saville ◽  
Martin Hurst ◽  
Stuart Jones
Keyword(s):  
Tibetan Plateau ◽  
Channel Flow ◽  
Crustal Thickening ◽  
Mean Annual Precipitation ◽  
The Tibetan Plateau ◽  
Eastern Tibetan Plateau ◽  
Geomorphic Indices ◽  
Surface Uplift ◽  
Hypsometric Integral ◽  
Early Cenozoic

&lt;p&gt;The formation and uplift history of the Tibetan Plateau, driven by the India-Eurasia collision, is the subject of intense research. We analyse the link between climate and tectonics in the central and eastern Tibetan Plateau using geomorphic indices of surface roughness (SR) hypsometric integral (HI) and elevation-relief ratio (ZR) and mean annual precipitation, thermochronology and erosion rate data. Geomorphic indices capture the landscape response to competition between climate and tectonics and reflect the spatial distribution of erosion. This is a region where competing tectonic models suggest either early Cenozoic plateau growth, or a late phase of crustal thickening, surface uplift and plateau growth driven by lower crustal flow (&amp;#8220;channel flow&amp;#8221;). Swath profiles of rainfall, elevation and the geomorphic indices were constructed, orthogonal to the internal drainage boundary. Each profile was analysed to find the location of maximum change in trend. We identify a broad &amp;#732;WSW-ENE trending transition in the landscape where changes in landscape and precipitation are grouped and in alignment. It represents, from east to west, a sharp decline in precipitation (interpreted as the western extent of the East Asian monsoon), a change to a low relief landscape at 4500-5000 m elevation, an increase in ZR and a transition to low HI and SR. This zone cuts across structural boundaries and is not a drainage divide: the main rivers have their headwaters further West, in the interior of the plateau. We argue that this geomorphic-climatic transition zone represents a change from incised to non-incised landscapes, the location of which is controlled by the western extent of the monsoon. Modern erosion rates are lower in the non-incised region, west of the monsoon extent (mean 0.02 mm/yr), than the incised region (mean 0.26 mm/yr). Compiled thermochronology data shows an increase in exhumation from &amp;#732;25 Ma in the incised area but no evidence of this increased exhumation in the non-incised area. This pattern supports a model of early Cenozoic growth of the eastern Tibetan Plateau, superimposed by incision driven by Miocene monsoon intensification. Our results do not support the channel flow model, which would predict an eastwards wave of surface uplift and therefore erosion and exhumation during the Miocene, which are not present in the data.&lt;/p&gt;

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