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2018 Publication Archive

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Broken Sheets—On the Numbers and Areas of Tectonic Plates - McElroy

The sizes and numbers of tectonic plates are thought to record the importance of plate division, amalgamation, and destruction at divergent and convergent margins. Changes in slope apparent on log area versus log frequency plots have been interpreted as evidence for discrete populations of plate sizes, but the sizes of lithospheric plates are also closely approximated by a continuous density function in which diameters of individual plates are exponentially distributed; such size frequencies are dependent only on the total area and number of designated elements.

Cover of Geosphere Volume 14, Issue 3

2.7 Ga high-pressure granulites of the Teton Range: record of Neoarchean continent collision and exhumation -Swapp, Frost, C., & Frost B.

Continent-continent collisional orogens are the hallmark of modern plate tectonics. The scarcity of well-preserved high-pressure granulite facies terranes minimally obscured by later tectonic events has limited our ability to understand how closely Archean tectonic processes may have resembled better-understood modern processes. Here we describe Neoarchean gneisses in the Teton Range of Wyoming, USA, that record 2.70 Ga high-pressure granulite facies metamorphism, followed by juxtaposition of gneisses with different protoliths, and then by intrusion of leucogranites generated through decompression melting in response to post-collisional uplift. This evidence is best explained as the result of a 2.70–2.68 Ga Himalayan-style orogeny, and suggests that, although subduction may have been occurring earlier in the Archean, doubling of continental thickness by continent-continent collisions may date back to at least 2.7 Ga.

Cover of Geosphere Magazine

Synextensional dike emplacement across the footwall of a continental core complex, Chemehuevi Mountains, southeastern California

We characterize the composition, timing, geometry, and deformation style of the syntectonic Miocene Chemehuevi dike swarm exposed in the footwall of the regionally developed low-angle Chemehuevi detachment fault system (southeastern California, USA). Our data support mafic to felsic dike emplacement from ∼1.5 ± 1 to 3.8 ± 1 m.y. after initiation of regional extension (ca. 23 Ma), followed by rapid slip and denudation with minor magmatism. Pb/U zircon ages indicate intermediate to felsic dike emplacement adjacent to the Mohave Wash fault, part of the regional fault system, as it was active across the upper limit of the brittle-plastic transition, from 21.45 ± 0.19 to 19.21 ± 0.15 Ma. Intermediate to felsic dikes are undeformed at structurally shallow levels (<9 km minimum paleodepth), but are rotated and locally folded, and host a well-developed mylonitic foliation and lineation at deeper structural levels (≥9 km paleodepth), even where the country rock is nonmylonitic.

Cover of Geosphere magazine

Neoarchean tectonic history of the Teton Range: Record of accretion against the present-day western margin of the Wyoming Province

Although Archean gneisses of the Teton Range crop out over an area of only 50 × 15 km, they provide an important record of the Archean history of the Wyoming Province. The northern and southern parts of the Teton Range record different Archean histories. The northern Teton Range preserves evidence of 2.69–2.68 Ga high-pressure granulite metamorphism (&gt;12 kbar, ∼900 °C) followed by tectonic assembly with isotopically juvenile quartzofeldspathic metasedimentary rocks under high-pressure amphibolite-facies conditions (∼7 kbar, 675 °C) and intrusion of extensive leucogranites. Together, these events record one of the oldest continent-continent collisional orogenies on Earth. Geochemical, thermobarometric, and geochronological data from the gneisses of the southern Teton Range show that this part of the uplift records a geologic history that is distinct from the northern part.

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