Tectonic History of Gros Morne National Park, Newfoundland, Canada

Daniel B. Haug
Spring 2013
Emporia State University
ES 767 Global Tectonics

Introduction
Wilson Cycle
Geologic History of Newfoundland
Humber Zone
Dunnage Zone
Gander Zone
Avalon Zone
Summary
References
Picture References


Introduction

The island of Newfoundland is located at the northeastern extent of the Appalachian Mountains of North America. This unique location is on the southeastern side of the Canadian Shield. The Canadian Shield is an area of Precambrian igneous and metamorphic rocks that forms the geological core of the North American tectonic plate. The Canadian Shield has been subjected to numerous tectonic events that have resulted in accretion of terranes to the North American tectonic plate in the vicinity of Newfoundland. There have been five documented plate collisions of the North American tectonic plate with other continental landmasses in the region occupied by Newfoundland.

The Canadian Gros Morne National Park is one of the unique geologic settings preserved in Newfoundland that provides an unparalleled example of plate tectonics. The Gros Morne National Park achieved recognition as an UNESCO World Heritage site in 1987.

The eastern extent of the Gros Morne National Park contains a portion of the Long Range Mountains, which is a northern extension of the Appalachian Mountains. The Tablelands portion of the park contains the ultramafic rock peridotite. The high concentration of magnesium and heavy metals and the low concentration of calcium in these rocks inhibits the growth of most plant life. (Wikipedia 2013)

The Tablelands seen from Route 431 (Tango 7174, 2009)

The landscape within the park has been heavily glaciated forming steep-sided glacial valleys and fjords. Due to isostatic rebound, fjords such as Western Brook Pond have been isolated from the ocean and are now fresh water bodies.

Western Brook Pond (Tango 7174, 2009)

The national park contains a geologically diverse landscape with Ordovician sedimentary rocks, Precambrian granite and gneiss, Paleozoic serpentinized ultra-basic rocks, gabbros, volcanics, and Lower Paleozoic sedimentary rocks. One of the most accessible exposures of exposed oceanic crust, mantle, and the Mohorovicic discontinuity are showcased within the park. In addition, the park has a nearly complete paleontological record of the Cambrian-Ordovician boundary. This sequence has been proposed as the world stratotype for this geologic period. (Williams 2003)

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The Wilson Cycle

The systematic opening and closing of ocean basins is referred to as the Wilson Cycle (Fichter, 1999). In the opening phase, the stable craton is rifted apart. If the rifting continues, an oceanic basin is formed. The rifted continental crust would then become widely separated with an intervening mid-ocean volcanic spreading center.

Eventually, the process is reversed and oceanic crust is subducted when the ocean basin begins to close. There are only two types of subduction zones and both involve the subduction of oceanic crust. The two types are within oceanic crust - Island Arc type and along a continental margin - Cordilleran type. As the oceanic basin closes, but before the edges of the continental crust begin to collide, the intervening oceanic sediments, volcanics, mafic and ultramafic rocks override the continental crust to form an ophiolites sequence. In the final stages of the collision, the oceanic continental margins will eventually collide to form an overthrusted compressional mountain range. With time these mountains will erode, exposing the underlying rock assemblages, and in older collisions, such as the Grenville Orogeny, the actual suture line and surrounding metamorphosed rocks are exposed.

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Geologic History of Newfoundland

This cyclical process is preserved in the rocks adjacent to and exposed on the island of Newfoundland. Through much of the time period from the Cambrian to the Triassic the North American tectonic plate straddled the northern side of the equator. The first Wilson Cycle occurred with the closing of the Uranus Ocean and the formation of the Grenville Structural Province. These rocks are preserved to the northwest of Newfoundland in southeastern Labrador. The Grenville Orogeny occurred in the late Proterozoic about 1.3 to 0.9 Ga. The Grenville Orogeny extends from the southern edge of the Baltica plate (Scandinavia and other portions of Europe), along the eastern and southern edge of Laurentia (the present North American plate), through the Australia plate, and along the northern extent of the eastern portion of the present Antarctica plate. This assemblage is known as Rodinia. The following drawing shows the relative position of the various tectonic plates along the Grenville Orogeny (Karlstrom K.E., et al, 1999).

Figure 1. Map showing the Grenville Orogeny, Karlstrom, K.E., et al, 1999.

The Taconic Orogeny occurred approximately 450 Ma along the southern and eastern margin of the North American tectonic plate as a result of the closing of the Iapetus Ocean. The Taconic island arc system or microplate originated from rifting along the northwestern portion of South America in the vicinity of Columbia.

The next Wilson Cycle began with the opening of the Iapetus Ocean. At this time the Baltic plate was colliding with the eastern margin of Greenland. This assemblage of plates formed Laurussia. Two distinct orogenies are associated with this cycle of the opening and closure of the Iapetus Ocean.

The Late Ordovician depiction of this orientation of the tectonic plates is shown in a drawing by Scotese, C. R., 1998. Quicktime Computer Animations, PALEOMAP Project.

The Acadian Orogeny occurred approximately 400 Ma.This collision involved an island arc system or micro plate, which collided with the southern and eastern margin of the North American tectonic plate.The Avalon island arc originated form northwestern Africa and / or portions of western Europe.

The Taconic and Avalon island arcs were rifted from the north edge of the Gondwana continental plate and drifted to the north toward the continental margin of Laurussia. The continental margin of Laurussia was partially obducted beneath these two island arc systems. The Iapetus Ocean was closed with these collisions. This sequence of rock is preserved in Gros Morne National Park.

The Early Devonian depiction of Laurussia at the time of the Acadain Orogeny is shown in a drawing by Scotese, C. R., 1998. Quicktime Computer Animations, PALEOMAP Project.

About 356 Ma the Alleghenian Orogeny occurred, which consisted of the collision of Laurussia and Gondwana. Gondwana consisted of the present African, Australian, Indian, Antarctica, South American, New Guinea, and New Zealand plates. This collision closed the Rheic Ocean that had formed between the Taconic and Avalon island arc systems and Gondwana. With the closing of the Iapetus and Rheic Oceans, the supercontinent of Pangaea was formed.

The Late Devonian depiction of Laurussia at the time of the Alleghenian Orogeny is shown in a drawing by Scotese, C. R., 1998. Quicktime Computer Animations, PALEOMAP Project.

The present day Atlantic Ocean began to form with the rifting of the supercontinent Pangaea approximately 200 Ma. The breakup of Pangaea and recent glaciation has resulted in the present configuration of landmasses and topography.

The Late Triassic Map of Pangaea is shown in a drawing by Scotese, C. R., 1998. Quicktime Computer Animations, PALEOMAP Project.

A generalized map of the Newfoundland Appalachians is used as a reference to describe the four tectonic zone present in Newfoundland. The map was compiled by J.P. Hayes in 1987 and Modified by H. Williams in 2004.

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Humber Zone

The western boundary of the Humber Zone is determined by the transition from deformed rocks within the Long Range to the flat-lying interior continental formations to the northwest. The eastern border of the Humber Zone is marked by the transition into the oceanic crust and rocks from the mantle that are present in the Dunnage Zone. This transition is discontinuous and is marked by the Baie Verte Line (Williams 2003). The Humber Zone contains Ophiolite sequences and the Gros Morne National Park.


Dunnage Zone

The Dunnage Zone lies to the east of the Humber Zone and is noted for its exposures of volcanic rocks, oceanic crust and rocks from the mantle. The zone also contains jumbled assemblages of resistance blocks of rock within a shale matrix. Sedimentary rocks within this zone are discontinuous rocks of a deep marine origin. The Dunnage Zone is widest in the north and narrowest in the south. The wider expression of the Dunnage Zone in the north is due to an embayment in the Iapetus margin. This area exhibits two subzones. The western subzone shows a correlation with North American fauna and the eastern subzone shows a correlation with eastern Iapetus fauna. The narrow expression of the Dunnage Zone in the south is due to an Iapetus promontory. The south edge is marked by the occurrence of the Gander River Ultrabasic Belt (Williams 2003).


Gander Zone

The Gander Zone consists of quartz sandstone sequences, siltstones, and shales. These formations grade into deformed and altered rocks. The eastern portion of the Gander Zone contains numerous granitic intrusions and highly deformed and altered rocks. These rocks are interpreted to represent sediment deposited on continental crust from the eastern margin of the Iapetus Ocean (Williams 2003).


Avalon Zone

The Avalon Zone consists of nearly unaltered sedimentary and volcanic rocks, most of which are older than 550 Ma. The fauna contained in these rocks is completely different from the fauna observed in the Humber Zone. The Avalon Zone is visible on the eastern portion of Newfoundland, but also extends nearly 600 km to the east beneath the Atlantic Ocean to the undersea Flemish Cap (Williams 2003).


Summary

The island of Newfoundland is a unique example of plate tectonic theory. The geologic processes preserved in the rocks of Newfoundland are seldom exposed on the surface of the earth in such detail. The complete cycle of the closure of the Iapetus Ocean is not only intact, but accessable on the island. The Gros Morne National Park is a world treasure that has attained the UNESCO World Heritage Site designation. Few geologic sites have been discovered that exhibit such a complete ophiolite sequence, particularly for this time period.

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References

Picture References

Website created by Dan Haug on April 16, 2013

ESU Earth Science Department homepage

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