Browsing by Subject "Magmatism"

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Mesozoic rifting along the eastern seaboard of North America : insights from the seismic velocity structure of the Newfoundland margin and the northern Gulf of Mexico
(2014-12) Eddy, Drew Richard; Van Avendonk, Harm J. A.
Passive margins along the eastern seaboard of North America formed during early Mesozoic continental rifting and seafloor spreading, tectonic processes that are not fully understood. Seismic refraction and reflection data at the northeastern and north-central Gulf of Mexico and the Grand Banks of Newfoundland, Canada, are used to interpret the deep seismic velocity structure of sediments, crust, and mantle. These interpretations allow for a better understanding of continental rifting, mantle upwelling, magmatism, and seafloor spreading. Magma-poor rifting of the Newfoundland-Iberian margin developed a wide continent-ocean transition zone (COT). I present an analysis of 2-D marine seismic refraction and reflection data from the SCREECH project, including a shear velocity model to constrain the composition of the Newfoundland COT. Comparing SCREECH Line 2 Vp/Vs ratios with depth to potential lithologies supports a COT comprised of hyperextended continental crust and serpentinized mantle. Reconstructions of the opening of the Gulf of Mexico basin are impeded by a lack of seafloor magnetic anomalies and an abundance of sediments that obscure acquisition of seismic refraction datasets. Accordingly, the roles of mantle upwelling, magmatism, and lithospheric extension in this small ocean basin are poorly known. I present new 2-D marine seismic refraction data from the U.S. Gulf of Mexico collected during the 2010 GUMBO project. Rifting in the eastern Gulf of Mexico developed above a zone of anomalously high mantle potential temperatures that led to abundant magmatism. Syn-rift basins in continental crust, high velocity lower crust, a narrow zone of crustal thinning, and seaward-dipping reflectors support this interpretation. Oceanic crust here is thick despite slow seafloor-spreading rates, implying continuation of a thermal anomaly after rifting. In the north-central Gulf of Mexico, transitional crust is consistently thin (~10 km) across a wide zone. Velocity-depth comparisons, asymmetry of the north-central Gulf with the Yucatán margin, and dating of onshore xenoliths support either stretched and magmatically intruded continental crust or a multi-stage episode of seafloor spreading with ridge jumps. I contend that although tectonic inheritance may ultimately influence the location of a passive margin, the rifting process is largely controlled by mantle potential temperature and upwelling rate.
Pliocene-Quaternary deformation and magmatism at the southern margin of the Puna plateau, Argentine Andes
(2005) Baldwin, Austin Kyle; Marrett, Randall
New fault data and mapping of volcanic rocks from northwestern Argentina's Cordillera de San Buenaventura, a Pliocene-Quaternary, E-W trending volcanic range at the southern margin of the Puna plateau adjacent to the modern volcanic arc at 27°S, reveal numerous tectonomagmatic characteristics unique in the region. Faults in the Cordillera de San Buenaventura dominantly strike ENE-WSW, with lengths reaching at least 30-40 km. Fault motion, where measurable, has been normal, commonly with a right-lateral component. Some faults displace alluvium and dated Plio-Quaternary non-explosive lava domes and flows, intermediate in composition, with throws of 1-100 m, whereas other faults do not affect overlying volcanics. Previous studies in the surrounding southern Puna have reported Pliocene-Quaternary NNW-SSE extension along N-S to NNE-SSW striking right-slip faults commonly associated with volumetrically-minor mafic monogenetic cinder cones and flows. Explosive eruptions with intermediate compositions have also characterized the southern Puna during the Pliocene-Quaternary. Volcanics erupted during this time period in the Cordillera de San Buenaventura lack both mafic and explosive tendencies. The new data, combined with existing data, suggest that the Cordillera de San Buenaventura overlies a shallow magma chamber, which thermally weakened the upper crust and promoted the growth of the observed ENE-WSW striking normal and oblique-slip faults during the Pliocene-Quaternary. The new faults created right-stepping extensional transfers by linking preexisting NNE-SSW striking right-slip faults. Decreased horizontal stress at these transfers facilitated magma ascent from the underlying chamber. Periodic breaches of the magma chamber cupola by slip on faults prevented magmatic fluid accumulation and explosive eruptions. Mafic magma ascending beneath the Cordillera de San Buenaventura pooled in the shallow magma chamber, where it assimilated silica-rich crustal material and differentiated, eventually reaching the surface with an intermediate composition. In surrounding areas, where no shallow magma chamber existed, rising magmas reached the surface with mafic compositions. The high topography of the Cordillera de San Buenaventura may be the combined effect of mechanical doming by the shallow magma chamber, thermal uplift, isostatic elevation of an extensional transfer block, and local accumulation of Plio-Quaternary volcanics
Recharge, decompression, and collapse : dynamics of volcanic processes
(2009-05) Andrews, Benjamin James; Gardner, James Edward, 1963-
Non-linear volcanic and magmatic processes control the occurrence and behavior of volcanic eruptions. Consequently, understanding the responses of volcanic systems to processes of different length scales, timescales, and magnitudes is critical to interpreting ancient deposits, understanding current eruption dynamics, and predicting future activity. Here I present the results of three studies wherein analytical geochemistry, experimental petrology, and turbulent flow analysis describe otherwise obscured volcanic processes. Injections of new magma are common events in magma chambers. Recharging magma can change the chamber composition and temperature and may facilitate assimilation of country rock. Plagioclase phenocrysts provide an opportunity to examine recharge and assimilation processes, because their compositions are sensitive to temperature and their Sr isotopic ratios can record compositional variations in the chamber. Chemical and isotopic microanalyses of crystals from 7 eruptions of El Chichón Volcano, Mexico, reveal that recharge and assimilation events are very common and mixing is efficient, but individual events seldom affect the entire chamber. During every eruption, magma decompresses and ascends through a conduit from a chamber at depth to a vent at the surface. Changes in pumice textures during the 1800 ¹⁴C yr BP eruption of Ksudach Volcano, Kamchatka, suggest that conduit structure changed following caldera collapse. Decompression experiments show that the post-collapse pumice decompressed at ~0.0025 MPa/s, compared to pre-collapse decompression rates of >0.01 MPa/s. By balancing those results with eruptive mass fluxes I quantify the effects of caldera collapse on a conduit, and show that collapse resulted in a conduit with a very broad base and narrow vent. Turbulent air entrainment controls whether an eruption column rises buoyantly or collapses to generate pyroclastic flows. Through extensive re-evaluation of video and photographs of the 18 May 1980 eruption of Mount St. Helens, I report the first measurements of the turbulent velocity field of a volcanic column and show that changes in its turbulence reflect changes in eruption behavior. Those results indicate collapse was caused by a reduction in eddy size and turbulent air entrainment initiated by an increased vent size and the development of a buoyant annulus surrounding a dense, collapsing core.
Tectonothermal effects of mid- and upper-crustal magmatism
(1996) Beam, Eric Christopher, 1966-; Cloos, Mark
This study considers the effects of the addition of heat and fluids to the earth's middle and upper crust by magmatism, as well as methods by which these effects may be distinguished in rock fabrics. A series of axisymmetric finite element transient heat conduction models were constructed to estimate crystallization rates, a proxy for fluid production. Crystallization rate varies from a high of ~80 km³/ky to a low of ~10 km³/ky. The shape of the crystallization rate histories is also highly variable, with thinner batholiths having high, sharp, early peaks. High fluid production rates will favor pooling of magmatic fluids in cupolas at the tops of stocks. In some cases this may trigger explosive eruptions, but in others could lead to economic mineralization. A computer program was written to simulate the formation of inclusion trails in porphyroblasts growing in deforming rocks. This program models synkinematic porphyroblast growth as a series of steps of growth and rotation. Resultant inclusion trails are complex. This complexity is a result of the variable relative rates of rotation of the inclusion and the cleavage. In some cases foliations are generated which could easily be interpreted as an included crenulation cleavage, other cases give an apparent sense of rotation opposite to the actual rotation. Theory describing the rotation of rigid inclusions is applied to biotite, garnet, and amphibole porphyroblasts in order to evaluate the sense of shear, magnitude of strain, and strain path (pure vs. simple shear) in deformed metamorphosed rocks adjacent to a tonalite sill in the Maclaren Glacier Metamorphic Belt (MGMB), south-central Alaska. Mean shear strains determined from biotite populations in thin sections range from γ = 2.4 to 3.3. Coupled temperature-displacement finite element models were constructed to simulate a cooling pluton in a zone of simple shear, using a realistic power law rheology. From these models it is clear that the thermal anomaly associated with a cooling pluton can concentrate deformation not just in the pluton, but into a laterally extensive zone running through the pluton. This may explain the deformation observed in the MGMB.