The Death Valley turtlebacks have been a topic of intense field discussion since the first description. The turtlebacks, given the name by Curry (1938) due to the erosion on the broadly curved shape that has resemblance to the carapace of a turtle, represents the basin and range geology in the area. Curry describes that the turtleback surface is coincident with the anticlinal fold in metasedimentary units that are Precambrian in age and plunges from a height of 865 to 1,350 m northwestward to the playa of Death Valley. Wright, Troxel and Otton describe the turtlebacks as mullions that have developed along planes of weakness in a zone of normal faulting that penetrates the surface. This paper will attempt to describe the major work done on the Death Valley Turtlebacks with emphasis on Mormon Point.
Turtlebacks are features of the Death Valley region. The turtleback feature is characterized by smooth, upward convex topographic surface, a northwest-southwest axis, and the core crystalline basement complex overlain by younger metamorphic lithospheres (Wright and Miller 1990). The first interpretation is by H.D. Curry, who also did some of the earliest work on turtlebacks. Currys work has led him to conclude that the turtlebacks origin is a result of compression folding of the regional thrust fault (1954). Curry argues that the turtlebacks folding represents reverse faulting and he provided clear evidence on this theory. His decision to describe the turtlebacks as the stripped sole of normal thrust faulting is unclear, but may have related the notion to the similar faulting along the Amargosa Fault and also in the Panamint Range. Since normal faulting generally occurs at high angles, Curry describes the turtleback as having low angle faulting that can be observed in reverse faulting. He also argues that there is "chaotic structure, profound and apparent haphazard disorder, and intense crushing of the rocks that lie upon the surface..." which is objected in 1959 by Drewes who argues for orderly crushing in a small area.
Drewes first argued for the normal faulting (1959). His evidence included the presence of normal faults in the area and on the turtlebacks. Drewes also rejects the idea that the turtlebacks are in some way related to the faulting in the Amargosa Fault because of the stratigraphic sequence of rock units of the Turtlebacks is much younger than the units of the Amargosa Fault. Drewes describes the age of the turtleback faults to be Pleistocene in age while the Amargosa fault is to be Tertiary. Drewes pays much attention to the folding and anticlinal structure of the turtlebacks. Drewes describes that the Precambrian rocks underlying the turtlebacks were folded into anticlinal shape with the axis parallel to a nearby syncline. Intruding these folds is the Willow Springs Pluton that is a Precambrian metadiorite that Drewes claims to be undeformed. The folding occurred during the Precambrian time followed by a long period of deposition and erosion until the late Pliocene or early Pleistocene when movement along the Black Mountains fault detached large sedimentary blocks which moved downward along the normal faults (Drewes 1959). The movement and faulting created what can be seen today as the Turtleback surfaces. Drewes did not mention the plate tectonic theory as a cause for the faulting.
The plate tectonic theory is the engine behind the normal faulting. The normal faulting is a result of the strike-slip motion in the Death Valley fault area, which created a pulling apart motion by the two sides of the valley to create the graban. This motion can be seen in figure 2. (Swihart 2006). The normal faults in this area are an expression that motion. This theory is elaborated by Hill and Troxel in a paper "Tectonics of Death Valley Region, California from the Geological Society of America Bulletin where they said the structures of the Death Valley region, including the turtlebacks and Amargosa thrust fault, are manifestations of shortening in a NE-SW direction and relative extension in a NW-SE direction. Hill and Troxel describe the Death Valley area as a product of both compression and extension and an observation of right lateral offset in the Death Valley and Furnace Creek faults and a left lateral motion in the Garlock fault area. This compression force is a result of roughly perpendicular direction of forces supported by a distorted strain ellipse, a single system of deformation that has been active since the Tertiary time that contributed to the turtlebacks. (Hill and Troxel 1966).
Wright et all proposes the theory that the turtleback surfaces are gigantic mullions that developed along planes of weakness in a zone of normal faulting that penetrates the crust. The description presents that a cover of Cenozoic rocks was observed to have been deposited along the fault zone shortly after the faulting began. After the deposited material moved parallel to the axis of the turtleback surface in a downward and towards the northwest to deepen Death Valley. This can be viewed in the above picture.
The hypothesis by Wright et all is based on many years of observation of the linear features related to the movement along the fault surface of Mormon Point and Copper Canyon turtlebacks. They (turtlebacks) range in scale from minute slickenslides to fault mullions tens or hundreds of meters in amplitude, are all similarly oriented, and tend to lie parallel to the axes of the turtleback surfaces (Wright et al 1974). They also noted that layers of metasedimentary units were on the surface and those layers tend to yield to stress more readily. Wright et al mention that this hypothesis better coincides with the pull-apart hypothesis.
The conceptual view is that Badwater, Mormon Point, and the Copper Canyon turtlebacks share a common early history in a shear zone. Although each turtleback has differences, they all display a doubly plunging antiformal core of metamorphic and igneous rock. The turtlebacks also share the attribute of a brittle fault contact to the northwest that is overlain by Miocene-Pliocene age volcanic and/or sedimentary rock (Miller and Pavlis 2005). Miller and Pavlis also suggest that the turtlebacks are results of several shear faults rather just one.
These entire hypotheses are movements to better understand the fault systems in the Death Valley area. Every hypothesis used the areas tectonic features to describe the creation of the turtlebacks. There is plenty more to be said about this region and the highly debated turtlebacks, I just havent had enough education to know what most of the papers I read were talking about. Since the early 1900s, brilliant dudes have been researching, comparing, and discussing the turtlebacks and their origins, hopefully in the future more information can be learned and researched to produce a more concrete answer on the morphology and origin of the turtlebacks so geologists can gain a better understanding on the Death Valley Fault System.
Cited Resources
Miller, Marli B. and Palvis, Terry J. The Black Mountains Turtlebacks: Rosetta stones of Death Valley tectonics. Earth Science Reviews 73. 115-178p. 2005.
Curry, H.D., "'Turtleback' Fault Surfaces in Death Valley, California." Geological Society of America Bulletin (an Abstract of a presentation), v49, p 1875. 1938.
Hill, Mason L. and Troxel, Bennie W. "Tectonics of Death Valley Region, California." Geological Society of America Bulletin, v77, p435-438. 1966.
Wright, L.A. et al. "Turtleback Surfaces Viewed as Phenomena of Extensional Tectonics." Geology, v.2 p53-54, Geological Society of America, Feb 1974.
Swihart, George. Lecture. Death Valley Field Excursion. 2006.
Turtlebacks are features of the Death Valley region. The turtleback feature is characterized by smooth, upward convex topographic surface, a northwest-southwest axis, and the core crystalline basement complex overlain by younger metamorphic lithospheres (Wright and Miller 1990). The first interpretation is by H.D. Curry, who also did some of the earliest work on turtlebacks. Currys work has led him to conclude that the turtlebacks origin is a result of compression folding of the regional thrust fault (1954). Curry argues that the turtlebacks folding represents reverse faulting and he provided clear evidence on this theory. His decision to describe the turtlebacks as the stripped sole of normal thrust faulting is unclear, but may have related the notion to the similar faulting along the Amargosa Fault and also in the Panamint Range. Since normal faulting generally occurs at high angles, Curry describes the turtleback as having low angle faulting that can be observed in reverse faulting. He also argues that there is "chaotic structure, profound and apparent haphazard disorder, and intense crushing of the rocks that lie upon the surface..." which is objected in 1959 by Drewes who argues for orderly crushing in a small area.
Drewes first argued for the normal faulting (1959). His evidence included the presence of normal faults in the area and on the turtlebacks. Drewes also rejects the idea that the turtlebacks are in some way related to the faulting in the Amargosa Fault because of the stratigraphic sequence of rock units of the Turtlebacks is much younger than the units of the Amargosa Fault. Drewes describes the age of the turtleback faults to be Pleistocene in age while the Amargosa fault is to be Tertiary. Drewes pays much attention to the folding and anticlinal structure of the turtlebacks. Drewes describes that the Precambrian rocks underlying the turtlebacks were folded into anticlinal shape with the axis parallel to a nearby syncline. Intruding these folds is the Willow Springs Pluton that is a Precambrian metadiorite that Drewes claims to be undeformed. The folding occurred during the Precambrian time followed by a long period of deposition and erosion until the late Pliocene or early Pleistocene when movement along the Black Mountains fault detached large sedimentary blocks which moved downward along the normal faults (Drewes 1959). The movement and faulting created what can be seen today as the Turtleback surfaces. Drewes did not mention the plate tectonic theory as a cause for the faulting.
The plate tectonic theory is the engine behind the normal faulting. The normal faulting is a result of the strike-slip motion in the Death Valley fault area, which created a pulling apart motion by the two sides of the valley to create the graban. This motion can be seen in figure 2. (Swihart 2006). The normal faults in this area are an expression that motion. This theory is elaborated by Hill and Troxel in a paper "Tectonics of Death Valley Region, California from the Geological Society of America Bulletin where they said the structures of the Death Valley region, including the turtlebacks and Amargosa thrust fault, are manifestations of shortening in a NE-SW direction and relative extension in a NW-SE direction. Hill and Troxel describe the Death Valley area as a product of both compression and extension and an observation of right lateral offset in the Death Valley and Furnace Creek faults and a left lateral motion in the Garlock fault area. This compression force is a result of roughly perpendicular direction of forces supported by a distorted strain ellipse, a single system of deformation that has been active since the Tertiary time that contributed to the turtlebacks. (Hill and Troxel 1966).
Wright et all proposes the theory that the turtleback surfaces are gigantic mullions that developed along planes of weakness in a zone of normal faulting that penetrates the crust. The description presents that a cover of Cenozoic rocks was observed to have been deposited along the fault zone shortly after the faulting began. After the deposited material moved parallel to the axis of the turtleback surface in a downward and towards the northwest to deepen Death Valley. This can be viewed in the above picture.
The hypothesis by Wright et all is based on many years of observation of the linear features related to the movement along the fault surface of Mormon Point and Copper Canyon turtlebacks. They (turtlebacks) range in scale from minute slickenslides to fault mullions tens or hundreds of meters in amplitude, are all similarly oriented, and tend to lie parallel to the axes of the turtleback surfaces (Wright et al 1974). They also noted that layers of metasedimentary units were on the surface and those layers tend to yield to stress more readily. Wright et al mention that this hypothesis better coincides with the pull-apart hypothesis.
The conceptual view is that Badwater, Mormon Point, and the Copper Canyon turtlebacks share a common early history in a shear zone. Although each turtleback has differences, they all display a doubly plunging antiformal core of metamorphic and igneous rock. The turtlebacks also share the attribute of a brittle fault contact to the northwest that is overlain by Miocene-Pliocene age volcanic and/or sedimentary rock (Miller and Pavlis 2005). Miller and Pavlis also suggest that the turtlebacks are results of several shear faults rather just one.
These entire hypotheses are movements to better understand the fault systems in the Death Valley area. Every hypothesis used the areas tectonic features to describe the creation of the turtlebacks. There is plenty more to be said about this region and the highly debated turtlebacks, I just havent had enough education to know what most of the papers I read were talking about. Since the early 1900s, brilliant dudes have been researching, comparing, and discussing the turtlebacks and their origins, hopefully in the future more information can be learned and researched to produce a more concrete answer on the morphology and origin of the turtlebacks so geologists can gain a better understanding on the Death Valley Fault System.
Cited Resources
Miller, Marli B. and Palvis, Terry J. The Black Mountains Turtlebacks: Rosetta stones of Death Valley tectonics. Earth Science Reviews 73. 115-178p. 2005.
Curry, H.D., "'Turtleback' Fault Surfaces in Death Valley, California." Geological Society of America Bulletin (an Abstract of a presentation), v49, p 1875. 1938.
Hill, Mason L. and Troxel, Bennie W. "Tectonics of Death Valley Region, California." Geological Society of America Bulletin, v77, p435-438. 1966.
Wright, L.A. et al. "Turtleback Surfaces Viewed as Phenomena of Extensional Tectonics." Geology, v.2 p53-54, Geological Society of America, Feb 1974.
Swihart, George. Lecture. Death Valley Field Excursion. 2006.
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