Tectonic evolution of the eastern Llano uplift, central Texas: A record of Grenville orogenesis along the southern Laurentian margin

Abstract

Mesoproterozoic metamorphic rocks exposed in the eastern Llano uplift, central Texas, were involved in a ca. 1150-1116 Ma Grenville-age orogenic event along the southern margin of Laurentia. Collision of the exotic Coal Creek arc and a southerly continental block with Laurentia tectonically telescoped and stacked three distinct lithotectonic domains. A major ductile shear zone forms the contact between the Coal Creek ensimatic arc terrane, the southwesternmost domain, and rocks with Laurentian affinities to the north. Directly north of the arc boundary, supracrustal rocks of the Packsaddle domain represent basinal sedimentary and volcanic rocks deposited along the southern Laurentian margin. Farther north, granitic gneisses of the Valley Spring domain, which consist of both plutonic and supracrustal rocks, represent a Laurentian continental-margin arc emplaced beneath the Packsaddle domain along a shear zone. Recent mapping in the northeastern uplift shows that the Valley Spring domain records a polyphase deformational history equivalent to that observed in the Packsaddle domain of the southeastern uplift. Early deformation occurred under uppermost amphibolite-facies metamorphic conditions and was accompanied by partial melting and formation of foliation-parallel leucosomes, consistent with deformation deeper in the orogenic pile. Mylonitic rocks in the shear zone separating these two domains show thrusting to the northeast, similar to that shown by the shear zone separating the Coal Creek arc from the Packsaddle domain. The Valley Spring domain igneous complex adjacent to the shear zone is highly attenuated into thin sheets in this zone. The Grenville event in the eastern uplift is characterized by polyphase ductile deformation synchronous with upper amphibolite-facies dynamothermal metamorphism. Deformation progressed from northeast-directed ductile thrusting and folding 783 *

Tectonic Setting and Evolution of the Grenville Orogen: An Assessment of Progress Over the Last 40 Years

“…These results suggested a high degree of continuity of major tectonic features, such as the ages of the pre-Grenvillian accretion and the timing of the Grenvillian Orogeny, along the 5000 km of exposure in North America (e.g. Mosher et al 2004Mosher et al , 2008Tollo et al 2004;McLelland et al 2010McLelland et al , 2013Rivers et al 2012), and hence that it comprised a single orogen.…”

The Grenville orogenic cycle of southern Laurentia: Unraveling sutures, rifts, and shear zones as potential piercing points for Amazonia

“…Therefore, following the logic of Keppie (1985) regarding shared accretionary history during and/or after amalgamation, most of the southern-central Appalachian massifs (i.e., the Blue Ridge and Sauratown Mtn. terranes), and the terranes of the Llano uplift (e.g., Mosher, 1998;Mosher et al, 2004) formed and/or were accreted to Laurentia during the Shawinigan phase of Grenvillian orogensis. Extensive magmatism, including anorthosite-mangerite-charnockite-granite (AMCG), occurred within the interior magmatic belt along the length of the Canadian Grenville province and Adirondacks (e.g., Rivers, 1997) as well as in the Blue Ridge terrane (e.g., Aleinikoff et al, 1996Aleinikoff et al, , 2000Tollo et al, 2004a,b) Many parts of these Appalachian Grenvillian massifs were subsequently fragmented during the breakup of Rodinia and later reaccreted as micro-continents or parts of larger terranes during the Paleozoic development of the Appalachians.…”

Laurentia-Baltica-Amazonia relations during Rodinia assembly

“…3) is related to localized accretion of outboard cratonic fragments (Boger et al, 2005), although Tohver et al (2004b) proposed this activity reflected initial collision of Amazonia with southern Laurentia prior to sinistral transpressional motion between the two cratons. Mosher et al (2004) ascribed the latter phases of orogenic activity in the Llano uplift, southern Laurentia, to collision of an outboard continent but noted that there was no evidence for transcurrent motion along the margin. In southwest Baltica, the early phases of the Sveconorwegian orogeny, the Arendal phase, also predate the main Grenville event and their restricted regional distribution is consistent with this phase related to localized accretion of the Idefjorden and Telemark terranes.…”

“…The timing of tectonothermal events in the southern Grenville Orogen, exposed within the Llano Uplift of Texas, are earlier than those to the north. Convergent plate interaction in the southern Grenville terminated with deformation and metamorphism of the succession between ~1150 Ma and 1120 Ma, which is related to arc accretion onto the Laurentian margin and collision of an outboard continental block, and was followed by some late tectonic to post tectonic magmatism extending to 1080 Ma (Mosher, 1998;Mosher et al, 2004).…”