The Dinkey Creek Pluton is reached by a moderate to strenuous hike into the Dinkey Lakes Wilderness. The area is at an elevation of about 9300 feet. The parking area is accessible by unpaved Forest Service access roads. The roads do get pretty rough, so a high clearance vehicle would be a very good idea. Winter snows will make the EarthCache inaccessible.
The Dinky Creek pluton has been used as an example of a granite pluton that was formed by partial melting of a subducting plate. As the Farallon plate (http://en.wikipedia.org/wiki/Farallon_Plate) subducted (http://www.platetectonics.com/book/page_12.asp) under the North American plate, the subduced oceanic crust began to warm and undergo partial melting (http://www.enotes.com/earth-science/partial-melting).
Partial melting occurs as a result of the different melting points of the various minerals in rocks. As the rock heats up, the minerals that melt at the lowest temperatures begin to melt first forming a magma, while the minerals with high melting points remain solid. The magma is less dense and begins to separate from the solids and moves upward. In this case, the magma that formed from the partial melting of the Farallon Plate was granitic in composition.
An example of partial melting is cheap frozen apple juice concentrate. When opened straight out of the freezer, a portion of material is frozen crystals and some is liquid. The crystals are ice, frozen water. The liquid is a sugar-saturated solution that has a lower melting point than pure water. This example shows how two different materials can exist in two different states (solid and liquid) at the same temperature.
Between 104 and 90 million years ago, this granitic magma migrated up toward the surface along fractures in the overlying rock. Near the surface there was a horizontal plane of weakness between an existing pluton and meta-sedimentary (metamorphic rocks that were sedimentary rocks) that rocks above it. The magma stopped its upward movement and instead began spreading out laterally along this horizontal contact.
Repeated pulses of magma upwelling through the same conduits gradually pushed the previously intruded magma further away from the conduits horizontally. The subsequent pulses also thickened the magma chamber by either uplifting the meta-sedimentary rocks or the dropping the underlying pluton.
Overtime the granite magma cooled forming the granite of the Dinky Creek Pluton. Continued uplift of the Sierra Nevada Mountains increased the erosion on the overlying meta-sedimentary rocks. Erosion continued until all but a few roof pendants (see Dinkey Creek Roof Pendant EarthCache)of the meta-sedimentary rocks remained exposing the Dinky Creek Pluton as we see it today.
One of the lines of evidence for the horizontal movement of the Dinkey Creek magma between the meta-sedimentary rocks and the older pluton comes from the magnetic foliation within the Dinkey Creek Pluton. Magnetic foliation comes from the orientation of small magnetic minerals within the granite. The orientation of these magnetic particles became aligned with the earth’s magnetic field as the magma solidified. The magnetic foliation is measured by advanced instruments that cancel out the relatively powerful magnetic field of the earth so that the weak magnetic field of the minerals can be observed. In magmas, magnetic foliation is used to describe the flow patterns within cooling magmas.
In addition, the age of the granite rocks within the Dinky Creek Pluton supports the formation of the pluton from one or two vertical conduits and spreading out laterally. The youngest granitic rocks within the Dinky Creek Pluton are near the center. Older rocks surround these younger ones.
This process is thought to have taken place over a period of less than 100,000 years instead of the tens of millions of years required by previous theories.
Source
Farallon Plate (http://en.wikipedia.org/wiki/Farallon_Plate
Subduction Zones; Plate Tectonics, 1996 - 2005 platetectonics.com.
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visit link)
Farallon Plate, USGS, (
visit link) Last updated: 05.05.99
The Farallon Plate NASA/Goddard Space Flight Center Scientific Visualization Studio; (
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Martin-Hernandez, F. et al Editors, Magnetic Fabric Methods and Applications, Geological Society, Special Publication 238, , Geological Society of London 2004
Petford, N., Cruden, A., McCaffrey, K and Vigneresse, J-L., Granite magma formation, transport and emplacement in the Earth's crust, Nature, V. 408, p. 669-673, December 2000.