Term Paper: Bowers Ridge &amp Shirshov Rise

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Bowers Ridge & Shirshov Rise

Are the Shirshov Rise and Bowers Ridge features old subduction zones or hotspot tracks? That is the question this paper will relate to and attempt to answer based on available research. Not all articles used in this research are necessarily scholarly; but there are two articles from scholarly sources that will be reviewed and critiqued in this paper.

But first, as an introductory background into the geological dynamics of the Shirshov Ridge volcanic belt, the United States Geological Survey explains (http://pubs.usgs.gov) that the ridge consists "chiefly of two units: a relatively older oceanic assemblage of amphibolite, gabbro, diabase, basalt, and chert." The second unit that Shirshov Ridge contains is a "relatively younger volcanic arc assemblage" of the following components: volcaniclastic sedimentary rocks, altered andesite, and shale of Miocene and younger age.

There are a series of normal scarps that face the Komanorsky Basin, and the ridge is segmented into "several north-striking blocks that descent step-by-step toward the basin floor," the USGS reports. The ridge is seen by geologists as one of two things; it is either: "the rifted remnant of an early Tertiary volcanic arc that formed approximately in place"; or a "thrust-thickened stack of oceanic crust that formed in the Neogene"; or a mass of "igneous rock" that makes up an oceanic plateau that was "accreted to the margin of the northeastern Kamchatka Peninsula."

Article: "Shock Dynamics: Alaska" (http://www.newgeology.us/presentation14.html).This publication offers some "far-out" explanations for scientific events and geologic research but it is worth reporting on in the sense of contrasting information with other articles that are available. The writer states that the Shirshov Ridge extends south from Siberia and is 700 km long and as wide as 130 km in some places. The western slope is steep, in some places up to 40 degrees steep but averaging 25 to 30 degrees. How did the Shirshov Ridge get where it is? It is "most probably" a "chaotic piling up of tectonic slabs of the crust of an ancient oceanic basin," the article explains, based on the article "The Bedrock of the Shirshov Ridge" in the journal Oceanology. There was "considerable horizontal compression" associated with the development of Shirshov Rise, the article continues; the ridge was apparently once part of the very edge of the Kamchatka continent and was "torn away" by plate tectonic movement.

But is Bowers Ridge an old subduction zone? Bowers Ridge is considered a volcanic ridge, albeit there is no present evidence of volcanism, there are few earthquakes, and "...no evidence of subduction (remnant slab and trench)," the article asserts, citing Michael Marlow's research with the USGS. As for Shirshov Rise, meanwhile, the research this article embraces seems to indicate that it was labeled a "spreading center, a microcontinent, an uplifted piece of the ocean floor and the remains of an island arc" (Baranov, 1984). The rise broke off Kamchatka after being initially formed through compression over millions of years, but the Bowers Ridge is believed to have been formed about the same time as the Aleutian Ridge (60 million years ago approximately), the article continues.

Did Bowers Ridge form in the Bering Sea, or was it "rafted" into the sea from another oceanic plate? Was it a fragment of an oceanic subduction arc - or, the writer asks, was it formed along a transform fault? There is evidence of "slight compression" in the fairly recent past with regard to both Bowers Ridge and Bowers Swell, but the Bowers Ridge is "...aseismic and lacks evidence of subduction," the article explains.

Article: "Plate-tectonic reconstructions predict part of the Hawaiian hotspot track to be preserved in the Bering Sea" (Geology, 2007). This is an interesting piece that suggests that while Bowers and Shirshov are not now considered hotspots, they may have been formed by "...paleo-Hawaiian hotspot magmatism." The authors (Steinberger, et al.) represent the Center for Geodynamics, Geological Survey of Norway. For the layperson, it seems rather incredible that a plate associated with Hawaii would end up in the Bering Sea, but the authors are not absolutely guaranteeing that this is true. On page 410 of their article they state that the hotspot track crossing the Bering Sea is "a prediction based on current knowledge of plate and hotspot motions." In other words, this is a theory. The relationship between the Hawaiian hotspot and the Shirshov and Bowers Ridges "is plausible although speculative," Steinberger writes. Hopefully, as a result of their research, additional work can be done on this idea "which may corroborate" the proposed relationship.

What is the Hawaiian-Emperor Chain? it's basically a portion of the Pacific plate, and the authors of this article suggest that the ocean basin in the Bering Sea just north of the Aleutian Trench is understood to have been formed - a "captured remnant" - by the Kula plate's movement. The Kula plate has been subducted (subduction is when an oceanic plate moves under a continental plate), the authors say, and in the process of subduction the Kula plate (which subducted under Alaska) "...may have preserved older parts of the Hawaiian hotspot track."

So are the Bowers and Shirshov rises / ridges old subduction zones? For starters, the age of the Shirshov and Bowers Ridges are "unknown," Steinberger writes. But age notwithstanding, Bowers is a "volcanic arc at a fossil subduction zone" - and Steinberger justifies that assertion based on seismic, magnetic, and gravity support research (data). He speculates, "A hotspot track localized the later Bowers and Shirshov Ridges" based on "marine magnetic anomalies." In fact, there are indications that the "Hawaiian hotspot" has moved and was at one time "farther north in the geologic past."

At one time in geologic history, the Hawaiian hotspot was located beneath the Izangi and Kula plates, Steinberger continues on pages 408-409. The movement of the Hawaiian hotspot, indeed the trajectory of the Hawaiian hotspot could be located near Bowers and Shirshov ridges if, Steinberger continues, the "ocean crust was part of the Kula plate and became attached to the North American plate." If that portion of the Kula plate could be studied, it might by up to 90 million years old. What geologic evidence there is reflecting that Bowers Ridge was at one time a volcanic arc in the Tertiary "...could mean that the proximity of predicted track and observed ridges is pure coincidence," writes Steinberger on page 409.

According to Rabinowitz (1974), the Shirshov and Bowers Ridges "may be structurally unrelated," Steinberger goes on. And moreover, a strike-slip zone roughly north-south in direction "...may have formed at the location of a pre-existing oceanic plateau after subduction was initiated in the eastern part of the Aleutian Arc," he posits. Following that formation of a strike-slip zone, the separate Shirshov and Bowers Ridge "developed from the originally continuous and straight strike-slip zone." And that "supposed" oceanic plateau was originally part of the Hawaiian hotspot, Steinberger believes. The reconstruction of ancient geologic events, as Steinberger and his colleague Carmen Giana have suggested, including plate motion in the Bering Sea, means that Alaska may have "accommodated" the westward movement of the Bering Sea in relation to the North American plate. If this theory could be proved, it would show that the Hawaiian hotspot indeed moved toward Shirshov and Bowers Ridges - and is now buried by subduction of the two plates.

And so, this article offers a long answer to the question of whether or not the Shirshov and Bowers Ridges are subduction-related or hotspots: yes they were at one time hotspots but now are subduction zones.

Article: "Viewing the Tectonic Evolution of the Kamchatka-Aleutian (KAT) Connection with an Alaska Crustal Extrusion Perspective" (Scholl, 2007). The thrust of this article is that the Shirshov and Bowers Ridges are found within subduction zones. Scholl writes that the Pacific Rim tectonic dynamics (crashing together) created (in the Aleutian-Bering Sea region) the "geometry of the offshore subduction zones of the Aleutian-Shirshov-Bowers system..."

Moreover, on pages 15-16, Scholl writes that based on dredge samplings and geophysical data (some coming from advanced GPS technologies), the Shirshov and Bowers Ridges are seen as "the massifs of volcanic arcs." The two ridges were "constructed by arc volcanism" but for "unknown reasons" (Scholl, 25) - in the early Miocene period - the volcanic period ended at the Shirshov and Bowers subduction zones and the ridges "subsided below wave-base erosion" down about 2,000 meters into the ocean. Scholl says nothing about the possibility of the Hawaiian hotspot being buried by later geologic action of the ridges. But Scholl does say in his Concluding Statement that there is unfortunately an "absence of constraining age and paleolatitude information" for Shirshov and Bowers Ridges (among other uncertainties) combine to "vex assessing the soundness of the Pacific Rim, extrusion part of the coupled model." In other words, like other scientists, Scholl insists that more research is needed to fully uncover the mysteries related to the Shirshov and Bowers Ridges.

Article: "The Disappearing Island" (Hubbard, 1932). There is no mystery, however, as to the fact that the… [END OF PREVIEW]

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Bowers Ridge &amp Shirshov Rise.  (2008, June 4).  Retrieved May 27, 2019, from https://www.essaytown.com/subjects/paper/bowers-ridge-shirshov-rise/3543512

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"Bowers Ridge &amp Shirshov Rise."  Essaytown.com.  June 4, 2008.  Accessed May 27, 2019.
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