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The Accretions Of Gander, Nashoba, And Avalon Onto The Laurentian Margin Martha Parsons

2599 words - 10 pages

I. INTRODUCTION

A significant portion of New England was formed as a result of an accretionary orogen. Southeastern New England is marked by a series of terranes that accreted onto the Laurentian supercontinent during the Silurian and Devonian. The Terranes of Gander, Nashoba, Avalon, and Meguma are present from west to east in eastern Massachusetts and all of are Gondwanan provenance. Their modern-day juxtaposition suggests that the marginal Gondwanan micro-continents collided sequentially from west to east, expanding the Laurentian continent with each respective collision. As each subsequent plate collided, an intervening subduction zone died and a new subduction zone was created to the east. The oblique collision of the Avalon Terrane into Laurentia followed the accretions of the Gander and Nashoba Terranes and preceded the accretion of Meguma. The collision was marked by uplift, mylonitic metamorphism, and calc-alkaline Nashoba plutonism as the Iapetus Ocean subducted under the Nashoba and eventually the Avalon collided obliquely into the continental margin.
The area composed of the Gander, Nashoba, Avalon, and Meguma Terranes has been extensively studied for many years. However, it was only recently that the terranes were recognized as distinct geologic entities with unique tectonic histories thus there is still much debate regarding the tectonic model which brought these terranes together (Hon et al., 2007). This paper will address the geology of the peri-Gondwanan terranes and propose a potential tectonic model for the accretional orogenic events. It will also primarily focus on the juxtaposition between the Nashoba and Avalon Terranes.

II. TECTONIC SETTING

Global Tectonics
The most recent complete supercontinent cycle involved the disintegration of Rodinia around 700 million years ago and formation of Pangaea around 300 million years ago (Marshak, 2009). The Proterozoic break-up of Rodinia created two smaller supercontinents, Laurentia to the north and Gondwana to the south, separated by the Iapetus Ocean (Kay, 2012). Subsequently, both continents experienced significant tectonic collision and rifting along their margins. A series of microcontinents, including Gander, Nashoba, Avalon and Meguma, split from Gondwana and accreted onto Laurentia (the ancient equivalent of the North American continent). The accretionary orogeny led to a several periods of significant uplift forming the modern-day northern Appalachians. A significant portion of these collisional belts is known as the Acadian Orogeny. The Acadian Orogeny postdated the Cambrian to Ordovician Taconic orogeny to the south and predated the Alleghenian Orogeny, which brought in the African continent and created the supercontinent Pangaea. Following the formation of Pangaea, the North American and Eurasian plates began to rift apart along the Mid-Atlantic Ridge, creating the Atlantic Ocean.

Regional Tectonics
Western Massachusetts is predominantly composed Laurentian...

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