Thursday, July 26, 2007
3D Map, oriented to N, shows N-S lineation (Inlet)
This Spring occurs on a Peninsula, E of a N-S Inlet
Maquinna Hot Spring, Vancouver. B.C.
(North of Tofino, Pacific coast)
The SW coast of Vancouver Island, B.C. lies in a terrane (a body of geologic rock foreign to its neighbors- and therefore moved from its source), which was evidently driven into the main island in Tertiary time. The similarity to the Eocene beds which comprise the Olympic Mountains is a factor for understanding the terrane, but the main comparison is made by extrapolation of the fault trends from across the Strait of Juan de Fuca and in the American islands in Washington State.
The complications to these generalizations are due to the circumstance of three different movements of the earth’s Crust in this general area where Canada bounds the United States, in the Strait of JdF area. The continental (USA)plate is driving westward, as noticed in the E-W fault boundary in the Deception Pass, Whidbey Island metasediments; next, the Juan de Fuca plate is diving eastward under the continent, as evidenced by the line of N-S volcanoes (the Cascades- vulcanism occurs perpendicular to the subduction direction); and finally, the Pacific plate is driving north towards Alaska, influencing the boundary region as far inland as Whidbey island (from indications of N-S shear in the cliffs there, even though this plate shear should mainly be noticed near the trough west of Vancouver Island).
Placing all this in a frame of reference, where the layman can understand it without getting bogged down in the usual Geologic terminology requires visualization of the movement in three dimensions, but I hope to furnish maps and photos which will help (see the first one in the photo above).
I took a trip to Tofino, B.C. to determine whether a simplistic view of the earth’ movements could easily be attained. My first priority was to determine whether the dominant fractures and their orientations could be used to simplify this view, and to get away from the inordinate classification- which is the usual memorization necessary to determine the meaning of the “hand waving” used by Geologists. Understanding is usually frustrated with the system of words (rather than mathematical or physical descriptions) which are presented by the standard geologic paper or presentation. Maps or cross-sections are more helpful, since these deals with facts (really nomenclature- which can’t be argued, since how can one be averse to terminology or classification?).
The personal reason for using this technique is to determine whether a person with a small amount of Earth Science training can go into the field and gain understanding of the earth movements which shape our world. I believe that these movements are responsible for the location of mineral deposits and travel of fluids vertically in the Crust. Consequently, one should not have to wade through a mass of terminology to get to the important assessments necessary to determine what lies below one’s feet. One should be able to cope with the world by appraising the subtle and sometimes obvious clues that surround us.
The following includes the notes taken from a five day trip to the West Coast of Vancouver Island, where metasediments dominate the scene, and where one can easily measure fracture orientation and the freshness or earth activity as noted by erosion and appearance of the Rocks. Try your own analysis of the Earth and its Crust by noting the fresh rocks and their orientations in road cuts and stream valleys as you travel about the beautiful West.
After reading all of my comments above, I realized that I was guilty of writing similarly to that I accused the geologists of exhibiting. So I will just write my conclusions outright:
1.The Vancouver Island is oriented NW-SE, which I suspected was due to the stress field exhibited previous to times more than two million years ago. This I had found out in my fieldwork in AZ and UT, and I noted from orientations of rivers and lineations that the fractures had re-oriented to that of N-S sometime after that 2 mybp (million years before present) time. Looking at the gradually changing orientation of the Gulf and San Juan Islands on a geographic map, in the sound to the east, one can see the interaction of the westward push of the continental movement causing the shift of the island trend to the north as it encountered the push of the JdF block moving eastward. This creates an area of uncertainty, where the transition zone is being dominated by the SdF subduction overcoming the westward continental movement. Look at the geographical map shown below, in the earlier Blog, to see these trends. Since Vancouver Island has rocks dating all the way back to the Devonian, and the three or more terranes all tend to align NW-SE, this indicates that whatever created the orientation of the island has done it to all of them (as with movement of masses of the crust- as opposed to stresses operating on one of them at a time). The conclusion here is that Vancouver Island has taken its orientation throughout time (starting at least in the Devonian), as the result of earth movements down to the present time.
2. The Maquinna fault and hot spring do not orient similarly to the island proper, rather they orient N-S, even though they occur on the western-most coast of the island (next to the Pacific). Since the spring is still hot, it must have an opening to the deep heat source, and it must represent the most recent fracturing and faulting pattern. The N-S fracture system must be the younger, and this is similar to that found at the Hurricane fault, Hawaiian Island re-orientation since Maui formation, and the youngest rivers of the west (Colorado, Rio Grande, and locally in Utah- the Coal Pits Creek.
Orthogonal Fractures, E-W, allow exit of Hot Water
3. The Vancouver SW coast is well east of the JdF and Explorer ridges (occurring at depth in the Pacific Ocean), which would be expected to dominate the fracturing and faulting near them. This indicates that this fracture system in effect now is that of the ridges well offshore, and might even have some influence on that at Whidbey Island (in occasional surges of movement).
4. One can evaluate present movements of the Crust without having to resort to naming the rocks (or even determining which rocks are present- since the fractures and faulting are independent of the rock type present). Knowledge of the existing and fossil stress states in advance of terminology determinations may help to find which type of rock to expect and simplify the naming process- particularly with metamorphic rocks.
Try your hand at understanding the movements of the Earth’s Crust as you drive through the newly-created road cuts and along recently eroded stream valleys. This exercise will help to pass the time (when are we going to get there?) and gradually allow you to formulate a method of assessing the changes which have occurred along your path.