These two projects focus on the origin and dynamics of the arcuate bends of the Himalayan range located in northern Pakistan and southeastern Tibet.
The shape of mountains can focus precipitation and cause focused erosion. This in turn causes rapid ascent of hot crustal material, weakening the crust and stabilizing rapid uplift. This tectonic aneurysm is characterized by high relief with rapid exhumation. Our group has modeled the interplay of erosion and tectonics at Nanga Parbat in northwest Pakistan, Namche Barwa in southeast Tibet and St Elias in southeast Alaska.
Microstructures in Deformed Rocks
Rocks are made of grains of minerals and their size, shape, orientation and geometrical arrangement relative to one another constitutes the rock’s microstructure. From microstructure, we can infer processes of metamorphism and deformation and a rock’s bulk response to strain. One of the methods we use in microstructural studies is modeling. Here are a few results of past projects using various modeling software.
Coupling of Deformation and Metamorphism, ELLE/basil
This movie, which runs to a shear strain of approximately 2.0, shows the effect of
prograde metamorphism and porphyroblast growth on integrated strength. The
model mineralogy is initially “quartz” and relatively weak
“mica” in the central part of the model (bright colors).
Porphyroblasts of “garnet” grow in this central, mica-bearing layer.
As the movie starts, notice that the mica-rich layer is relatively weak. As the
porphyroblasts begin to grow, notice how strain rate decreases in the mica-rich
layer, and increases in the quartz layers above and below. Notice also that the
vorticity in the mica-rich layer changes from being higher than in the
surrounding layers, to being lower once the porphyroblasts start to grow. This
strength evolution as a function of metamorphism was the central focus of Wes
Groome’s PhD work – see the published work (Groome et al., 2006).
Initiation of Ductile Shear Zones, FLAC3D
This movie shows the initiation of ductile shear zones in a two-phase material (“mica” and “plagioclase”). The movie has some explanatory frames, but the idea is that gradients in differential stress and strain rate across the margins of weak and strong minerals cause the strong minerals to fracture, leading to connected zones of plastic flow and the establishment of stable zones of weakness. General non-coaxial flow with total shear strain approximately 0.03. This work was published the Journal of Structural Geology (Johnson et al., 2004).
Reconstruction of “Millipede Microstructure,” Mathematica
This movie shows a reconstruction of the 5 foliation surfaces that pass through a plagioclase porphyroblast showing “millipede microstructure.” Serial sections were taken through the rock at around 1 mm spacing, and the program Mathematica was used to reconstruct the geometry. This reconstruction clearly illustrates the heterogeneous stretching that occurs around porphyroblasts during crenulation cleavage development. See Johnson and Moore (1996) for full story.
Metamorphism is an intrinsic part of orogenesis, a major process affecting Earth’s crust. Hence, understanding the various aspects of the metamorphism in any orogen is critical for understanding the evolution of mountain belts. Extensive metamorphism has affected the southern half of Maine, primarily of the type commonly referred to as Low-P/High-T metamorphism. Of particular importance is the fact that, the mineral assemblages have closely approached chemical equilibrium with respect to the last thermal event to which they were subjected, enabling accurate mapping of isograds and defining of mineral reactions. Not only has this enabled study of the manner in which the minerals adjust crystallochemically in response to their chemical variation, but it has also enabled application of various cation exchange geothermobarometry which can then be compared with the P-T estimates obtained via the petrogenetic grid approach.
The light elements lithium, beryllium, boron and fluorine are important constituents of several major rock-forming minerals, and are trace constituents in virtually all rock-forming minerals. All four elements can provide information on fluid activity and shed light on the origin of wide range of metamorphic rocks. Zoned tourmaline from a contact aureole around a pegmatite at Mount Mica has rims enriched in lithium due to its metasomatic introduction from the pegmatite into the aureole.
Another study concerns beryllium-bearing pegmatites from the ultrahigh-temperature granulite-facies Napier Complex of Enderby Land, East Antarctica. Beryllium-bearing sapphirine in the pegmatites is inferred to have crystallized at 1000-1100 ºC from anatectic melt, and to have been subsequently metamorphosed at 800-900 ºC, when the sapphirine broke down to garnet, sillimanite and the beryllosilicate surinamite, occasionally leaving a distinctive atoll texture. Study of the beryllium-bearing pegmatites is expected to lead to a better understanding of the role of light elements in anatectic processes in the Earth’s lower crust.
Igneous intrusions commonly make up 25-75% of exposed rocks in eroded arcs, yet there is no consensus regarding how large volumes of granitoid magma ascend through the crust and become emplaced as middle- to upper-crustal plutons. There are two possible mechanisms that have been proposed: ascent as whole large bodies of magma through diapirism or growth in situ by expansion from the blunted tips of one or more dikes. Ascent by diking and diapirism have very different implications for the physical and chemical evolution of Earth’s crust. For example, the different emplacement mechanisms would strongly influence the strain rates experienced by the host rocks surrounding the plutons. Using field data from plutons across western North America, we have determined strain rates surrounding these plutons with numerical modeling studies. Scott Johnson has been involved in pluton-related research in Baja California with colleagues from the USA, Australia and México. Although we are not certain about whether the magma reached the emplacement level in dikes or multiple diapirs, the structural data clearly indicates that the body expanded in-situ to the north during emplacement.
Magma Chamber Processes
The Coastal Maine Magmatic Province (CMMP) provides a spectacular natural laboratory to understand magma chamber processes through the study of plutons. The well-exposed plutons display dramatic field relations that allow study of the episodic influx of mantle-derived magma into felsic chambers and the dynamics of crystallization within the felsic magma chambers themselves. NSF sponsored research focuses on the Vinalhaven, Deer Isle and Mt Waldo plutons. Our studies combine textural, compositional and isotopic studies of zoned feldspars and accessory minerals (using detailed petrography, electron and ion-microprobe techniques) and whole-rock geochemical and isotopic data for the granites, enclaves and schlieren layers. By assessing the nature and extent of the mantle component in these plutons, we can enhance our understanding of magma chamber dynamics and growth, enable recognition of the contributions of mantle and crustal components in granite petrogenesis and evaluate models for the growth and evolution of the continental lithosphere.
STEEP is a collaborative project aimed at investigating coupling between tectonics and surface processes in an extremely active region.
The University of Maine ECM is a teaching and research tool for conducting equilibrium climate change experiments at 1 km resolution for anywhere in the world.
IDEAS: Inquiry-based Dynamic Earth Applications of Supercomputing, is an ITEST program that is focused on integrating computational modeling with the existing science curriculum at the middle school level.