Figure 1. The western extremity of the Cobequid Chedabucto Fault complex in Nova Scotia. The Cape in the distance is called Cape Chignecto
It is a glorious view from the beach at Advocate Harbour. We look West towards Cape Chignecto. The coastline is straight. Eroding cliffs dip steeply down to the Bay of Fundy. This peninsula, the Cape Chignecto Peninsula, is a Provincial Park and the popular hike around it a tough three day journey. Kayaking around it is possible too even though the tide range averages 15 m here (I’ve done the kayak trip, the hike not yet). We also hope that this stretch of coastline will become a Geopark (I wrote about Geoparks earlier here).
Fig. 2 Google Earth Image showing the location of the photo
Fig. 3 – Relief map of Nova Scotia, Prince Edward Island and part of New Brunswick. The white circle indicates Cape Chignecto.
The relief map above clearly shows several linear relief features extending eastwards from Cape Chignecto. These linear features are a series of faults, collectively known as the Cobequid Chedabucto fault zone, which extends East for more than 300 km from Cape Chignecto. Another name for it is the Glooscap Fault. Glooscap was the Creator God of the Indigenous Mi’kMaq people, the original inhabitants of this part of Canada. The fault is visible in the landscape as a clear, steep scarp.
Fig. 4 – Google Earth Image of the Cobequid-Chedabucto fault (looking East) as expressed in the landscape east of Cape Chignecto. A road runs at the bottom of the scarp that marks the fault.
What happened here?
The fault zone separates the Meguma Terrane, the southern mainland part of Nova Scotia from the Avalon terrane, which stretches – in bits and pieces – to the North. Ok, so – What is a terrane and is that how you spell it?
Fig. 5 – Schematic map showing the southern Nova Scotia Mainland, representing the Meguma Terrane and the Avalon Terrane north of it, separated by the Cobequid-Chedabucto Fault zone.
A terrane (yes, that’s how it’s spelled) is a clearly identifiable fragment of continental crust, usually bounded by faults. So that means that the geologic origin of a terrane is different from that of its surrounding areas. The earth’s crust is formed by continent-size plates (continental and oceanic) that move around throughout geologic time. Moving around also means that plates and plate sections collide and slide along each other, sometimes causing pieces of crust (terranes) to break off from one side of a continental plate and become reattached to another one. Sometimes these terranes move around independently for millions of years before becoming attached again. Dozens of terranes have been recognized and we don’t always know where they came from and how they moved around over time.
Fig. 6 – A very schematic representation of the movement of the continents over the last 225 million years. The Meguma terrane likely broke off from NW Africa (now Mauretania) after Pangea broke up and reattached itself to North America (from Encyclopedia Brittanica).
The Meguma terrane (southern Nova Scotia mainland) was most probably a piece of the margin of the Pangea supercontinent and came from the margin of the African plate, from what we now call Mauretania. When Pangea broke up, this piece of crust broke off and ended up attaching itself to the North American plate. Southern Nova Scotia is the only piece of Meguma terrane anywhere on the planet.
The Avalon terrane is bigger and very fragmented. Pieces of it are recognized from Connecticut all the way to Newfoundland and across the Atlantic Ocean into the UK and as far as Poland (the Atlantic Ocean didn’t exist yet when the Avalon terrane docked on to the continent, so the later opening of the Atlantic Ocean broke it up). It’s not clear where the Avalon terrane came from, but experts do have some evidence that these two terranes traveled together for a while before attaching themselves on the Northeastern margin of the American continent.
The Cobequid Chedabucto Fault zone marks the zone where the Meguma terrane docked and wrenched along the margin of North America after the Avalon terrane had become attached. Such faults are called “strike-slip faults”, a term that indicates that pieces of crust slide along each other (instead of over each other). The fault was probably active for 150 million years, from about 350 to about 200 million years ago. This sliding process wasn’t smooth – it would have been periodic and each movement would have been felt as an earthquake. The friction along these pieces of crust caused heat and fluidization of rocks, injection of hot fluids, and this together led to the concentration of valuable minerals and metals such as Iron, Copper, Cobalt, Barite and Fluorite.
The fault expresses itself in the landscape as a scarp, but when you put your nose to the rocks, the messiness of the grinding, sliding and friction is apparent. Here are a few examples.
Fig. 7 A mixture of granites and dark colored igneous intrusions exposed along the beach close to Cape Chignecto itself. Person for scale
Fig. 8 – Intensely folded and faulted rocks exposed along the Cape Chignecto beach close to Advocate Harbour. Walking pole ca 1 m high
Fig. 9 – A piece that I retrieved from the outcrop of Fig. 8. These are ripples, formed on a beach some 300 million years ago. They became preserved as sedimentary rock and then the fault movement (my finger is on the fault line) displaced the ripple crests!
Is this relevant? You bet. First of all, ancient fault activity like this can cause concentration of valuable minerals and metals. Second, currently active faults like this one are seismic hazard zones. The best known similar strike-slip faults are the San Andreas Fault in California and the Great Anatolia Fault in Turkey. Understanding tectonics like this has lots of practical implications
White, C.E., and Barr, S.M., 2012, Meguma Terrane revisited: stratigraphy, metamorphism, paleontology and provenance. Geoscience Canada, v. 39 no. 1