Wednesday, June 20, 2007

Active Tectonics at Whidbey Island




Thrust Faulting on Whidbey Island

There are thrust faults on the island- those that are well-known include the NW-SE trending left-lateral faults. But, first for memory:

TERMS: a fault is a slice into the earth, with obvious movement (displacement) either along the slice, up or down the slice or a combination of these two possibilities. Normally the slice creates a straight line along the surface of the Earth, but over a significant distance the slice may be curved, jagged, or irregular:
a. A normal fault is the most likely occurrence, and it is characterized by movement down into the earth (relative to the upthrown block). It is a result of a stretching of the earth’s crust, so that gravity causes the block to fall into the void created by the tension. The angle of dip of the fault plane may be 45 degrees to the earth’s surface up to almost vertical. This type of fault will drag along the opposing wall or plane, sometimes creating slickensides.
b. A reverse or thrust fault is less likely, being a result of compression of the earth’s crust- which occurs with great tectonic events, such as mountain building, salt domes, vulcanism, or subduction. However, I have seen small thrusts occurring over sinkholes, caused by prying of the roof as it is dropped into the earth.
The photos above show thrusting on a small scale, due to part of the crust being dragged horizontally. The fault plane angle may be almost horizontal, up to vertical.
c. A lateral fault is one which slices along the earth’s crust, due to the underlying sediments being moved by two separate forces (or directions); This is the type common to regions where large sections of the earth’s crust are moving at angles to each other, as with the Continental block moving west on Whidbey Island while the southern part of the island is moving northward (being in a separate Pacific block). Standing on either side of the fault line, it is left-lateral if the block across from the observer moves left.
d. There are other categories of faults, but it is best not to over-classify this slicing due to earth movements. These three types listed fit in with a physical categorization of mechanics, where there occurs Tension, Compression, or Shear (or combinations of these strains). Rock samples may be investigated with testing machines, by compressing, stretching or torsion, to yield numerical measurements such as moduli or strain/stress under Compression, Tension, or Shear, and failure end points for crushing, stretching, or shear. Classification does not increase understanding, and may cause confusion when it becomes voluminous- which is the usual case with geological categorization (which could be accused of creating terminology to suppress layman’s interference).
Large-scale movements on Whidbey Island The extreme northern end of W.I. has an obvious junction of the westward moving Continental plate (at Deception Pass), against the younger movement of the rest of the Island. This is noted, by merely looking at the hard green rock at the pass, compared to the young till and other glacial deposits dumped to the south by the glaciers of the last million years. When you drive over the bridge, notice the hard rock of Mesozoic age (greater than 65 m.y.). You cannot carve these metasediments with a knife, while anything south of these outcrops may be scratched with the fingernail.
All of this is known and well-documented, but what is not known is the influence of the Pacific plate as it interferes with Whidbey Island- which is well within the continental deposits. Although the Pacific plate occurs west of Vancouver Island (evidenced by soundings offshore), there is some evidence that it lurches occasionally into the border zone, which is Cascadia. This would be northward movement towards Alaska, as opposed to the general trend of most evidence as NW-SE (as is the trend of the shape of Vancouver Island, the California coast, and many large-scale geological features in the western USA.
So what is the evidence for north-south thrusting, on Whidbey Island? The first photo above shows large displacement (in a west facing cliff of 100 meters displacement or so) along a plane- almost horizontal- south of Hancock Lake. When one looks at the evidence, it is not clear whether the overlying sediments moved southward over the deeper, or whether the underlying moved northward under the shallower. This could be sorted out by using the following rules:
1. When the thrust dies out with depth, it can be assumed to have originated from the top- this would be the case for a sheet of ice from the young Glaciation sliding south (which actually happened- from independent studies). The thick layer of ice, being some thousands of feet thick, would drag the immediately underlying layers (being soft and lubricant-like) along with it- causing the simplest type of thrust.
2. When the thrust dies out toward the surface of the ground, it likely originated at depth. This would be the case of a plate of rock being moved at great depth, dying out as it reached soft sediments which do not show the sliding planes.
A large vertical section of cliff is necessary, for the above rules to indicate the correct analysis. This rarely occurs in nature, since one only finds an expression of thrusting in canyons, road cuts, or in vertical drill holes (with pictures for viewing). The vertical wall necessary usually slopes away from the feature of interest, also. I tend to think that the Hancock thrusting occurred mostly from glaciation, but it may have masked a simultaneous North-to-Alaska movement of Pacific crust.
Small Thrusting at Ebby’s Landing cliffs
The photos below the Hancock thrust are not as graphic, but they show small thrusts parallel to each other, with a seep occurring below them. There is also an obvious buckle, where the sediments rode over their deeper location. All three of these features together strongly indicate a thrust fault. Again, does the layer move from below toward the north, or is it the reverse (upper toward the south)? The first possibility would indicate a non-glacier action. Some subtle facts help the analysis;
The small seep indicates that a fluid path is open, and has not been sealed since glaciation time. This hints that stress operates now, not just when the glaciers were sliding.
Secondly, there are orthogonal fractures in the cliff, and this occurs with a large-scale tectonic action (such as is noted all over the west USA). This feature is not definitive, however.
Thirdly, there is an inflection in the bedding above the seep, and this is indicative of thrusting overall. This feature is at the bottom of the fault lanes, hinting that it is of non-glacial origin.
Overall, there is some evidence that Whidbey Island has some shear occurring from south to north, which would not be of glacier origin, but likely from a lurching of the northward Pacific plate influencing the island far to the east of the plate proper. But this is overwhelmed by evidence that the main faults occur due to interference of the westward-moving North-American plate and the transition zone to the west, short of the Pacific plate.
Make your own analysis from the photos, and then hike south from Lagoon Point County Park about a mile, to see the cliffs just short of the Hancock Lake.