Our very own graben

Fig. 7:This diagram shows the periods in deep time

Image from: http://geoscape.nrcan.gc.ca/ottawa/time_e.php

Located in Ontario, in the Ottawa Valley, The Ottawa-Bonnechere graben "was formed hundreds of millions of years ago [approximately 175 million years ago during the Mesozoic period] and is associated with the breakup of an ancient continent." (Eyles, 2002). It is known as a 'failed rift'. A failed rift is a graben that no longer widens and grows larger over time. A rift which fails to grow any further will eventually fill with either water or some type of sediment.

Fig.7:Map of Ontario shows location of the graben

Image from: geology.com/canada/ontario-map.gif (modifications made by author)

Fig. 8: The shape and location of the graben

Image from: http://geoscape.nrcan.gc.ca/ottawa/time_e.php

It is located along the Gatineau Hills, running Northward from Ottawa and Westward from Montreal. The graben runs approximately 35km wide and reaches depths of 75 feet. Like similar grabens around the globe (the East African Rift Valleys for example), Alkaline Igneous rocks can be found at the Ottawa-Bonnechere Graben. Sedimentary rocks from the Paleozoic era (from 544 to 250 million years ago) are also found there as well as in the surrounding St. Lawrence Valley (Rimando and Benn, 2005).

Seismicity in the Ottawa-Bonnechere Graben and Surrounding Area: This table shows us that these plates are continuously moving and are still active. We see that not all earthquakes have been damaging or even threatening, though we do know they are there!

Fig. 9: Table of seismicity surrounding the graben.

Location

Magnitude

Date

Montreal

5.8

September 16, 1732

Ottawa

--

July 12, 1861

Timiskaming

6.2

November 1, 1935

Cornwall-Massena

5.6

September 5, 1944

Cornwall

3.3

July 4, 1981

Timiskaming

4.3

August 13, 1982

North Gower (near Ottawa)

4.1

October 11, 1983

Timiskaming

5.2

January 1, 2000

Table information from: Rimando, Rolly E, and Keth Benn, 2005, http://www.sciencedirect.com/science_ob=ArticleURL&_udi=B6V9X4FNDS431&_user=10&_rdoc=1&_fmt=&_orig=search&_sort=d&view=c&_acct=C000050221&_version=1&_urlVersion=0&_userid=10&md5=1ab79f1c6fe07817f3435dd8fc617a78

Fig.10:The Barron Canyon fault crossing the graben

Image from: Bob & Diana McElroy. www.mcelroy.ca/bushlog/images/10c-3153.jpg

There are many fault lines running perpendicular to and across the graben as well. The Barron Canyon is an examples of one of those faults.