Introduction to New England Geology

New England Tectonics and Mountain Building

This blog will take you through a reconstruction of New England Tectonic history by looking at a number of field trip stops near Williamstown, Massachusetts. On each page, you will find field notes, an interpreted summary of each stop, and a brief summary of the geologic significance of each week's theme. 

By the Dates:

Mesoproterozoic:

1.1 Ga, Grenville Orogeny (Basement rocks)

- See field trips from weeks 1 and 2

Neoproterozoic to Early Cambrian:

 560 Ma: Post-Grenville rifting

Continental Rift to Drift Transition

Breakup of Rodinia

-See field trips from weeks 1, 2, and 3

Cambrian-Ordovician:

Opening of Proto-Atlantic (Iapetus Ocean)

475 Ma: Formation of the Shelburne Falls Arc

Rifting of Gondwana

-Weeks 1, 2, 3, and 4

Ordovician:

460 Ma: Taconic Orogeny

440 Ma: Formation of the Bronson Hill Volcanic Arc

Convergence of Avalonia

Devonian:

400-375 Ma: Acadian Orogeny

Avalonia and Laurentia Collide

Permian:

300-250 Ma: Alleghanian Orogeny

Laurentia and Gondwana collide to form supercontinent Pangea

Triassic-Jurassic:

200-180 Ma: Mesozoic rifting

Breakup of Pangea

Present day:

Passive margin of opening Atlantic

What's an "orogeny?"

An orogeny is a mountain-building event. They occur when the lithosphere, or earth's crust, is deformed by tectonic activity. They generally produce "orogenic belts" that have various components. 

So What's the Story?

The Grenville Orogeny created the crystalline basement rocks that underlie much of New England. After the Grenville Orogeny, Post-Grenville rifting occurred.

Wait, tell me more about rifting!
Rifting is an extensional tectonic event where the lithosphere is pulled apart. Mid-ocean ridges are a common location for rifting because new crust forms as the old crusts moves outward at this divergent boundary.

i.1: This model, taken from a paper by Lister et al., shows 3 different models for rifting, also known as "continental extension". Note the lack of parallel extension in the second and third model compared to the first "pure-shear model."

Since the orogenies that built New England result in passive margins and other features such as uplift, the claims made by Lister et al. that a symmetrical model (first image in i.1) is not applicable to many geomorphic landscapes applies to New England. Rather, their descriptions of a detachment model of extension (the Wernicke and delamination models shown above) are more relevant when thinking about New England Geology. Detachment models are more likely to create passive margins, rift valleys, half-grabens, and other geomorphic structures that are common in our area.

Back to the story...

Post-Grenville rifting resulted in the breakup of the supercontinent Rodinia, and it allowed for the opening of the Proto-Atlantic, also known as the Iapetus Ocean. The rift phase transitioned to the drift phase, and the edge of the continent became the Paleozoic passive margin. During 480-475 Ma, crust began to be subducted as an east-dipping subducting plate, closing the Proto-Atlantic and forming the Shelburne Falls Arc. During the Taconic Orogeny, the Shelburne Falls Arc collided with the Laurentian margin. Previously, it was thought that the magmatic arc that underwent the collision was the Bronson Hill Arc, but Karabinos and others corrected this model using dated zircons from the area to conclude that it was the Shelburne Falls arc that collided with Laurentia. The Bronson Hill Arc shows no evidence of having undergone Taconic deformation. In addition, the cooling ages of 40Ar/39Ar rocks from the Laurentian margin range in age from 470 to 460 Ma, which is 15 to 30 million years older than dated rocks related to the Bronson Hill Arc (Karabinos 1998). The methods used by Karabinos et al. will be discussed in more detail in the blog post for week 4.

i.2: Tectonic model of Massachusetts during the Taconic Orogeny, Karabinos et al. (1998). This model shows a reversal in subduction polarity between the Early Ordivician (East-dippng) and the Late Ordovician (West-dipping). This represents the change that occurred after the Shelburne Falls Arc (Figure A) collided with Laurentia and created an active margin that formed the Bronson Hill Arc (Figure B).

After the collision of the Shelburne Falls Arc, the Bronson Hill Arc became active on the margin of Laurentia and subduction resumed, west-dipping. Marine sediments from Avalonia collided with Laurentia during the Acadian orogeny. During the Alleghenian orogeny at the end of the Paleozoic, Gondwana (Africa) collided with Laurentia, forming the supercontinet Pangea. During the Mesozoic (Triassic-Jurrasic), rifthing began again, causing Pangea to breakup. A passive margin formed as the Atlantic opened. This passive margin and the rifting of the Atlantic continues today.

i.3: Lithotectonic Map showing the geology of the region discussed in this blog, formed by the tectonic activity described above. This diagram will be used throughout the blog to show where each field trip and the associated formations relate to the tectonic history as a whole.

Sources: Reference material provided by Paul Karabinos and Bud Wobus
"Taconian Orogeny in the New England Appalachians: Collision Between Laurentia and the Shelburne Falls Arc," Karabinos et al., 1998
"Diachronous Rifting, Drifting, and Inversion on the Passive Margin of Central Eastern North America: An Analog for Other Passive Margins," Withjack et al, 1998