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On my second visit to Lake Cowal back in early December, I was told about a place that was of great interest to geologists visiting the nearby gold mine. This trip I headed out to have a look.
Following is the information I was given to explain what I was going to see.
"The outcrop is part of the Booberoi Fault and features sheared Late Silurian-Early Devonian Edols Conglomerate (Sherwin, 1996) showing stretched quartzite and vein-quartz pebbles in a quartz white mica-chlorite matrix, which was probably a muddy sandstone. Mesoscopic kinematic indicators show at least two movement episodes: a horizontal sinstral movement and a near vertical reverse (west side up) movement. The movement direction can be discerned from the asymmetry of the matrix surrounding the pebbles and the mineral elongation within the matrix."
Video - youtu.be/gFIb1JXFTb8
Hollister, California
In the San Francisco Bay area there are three major faults, from west to east the San Andreas, the Hayward, and the Calaveras; all are part of the San Andreas fault system. All of these are "right-lateral strike-slip faults," which means that the motion is predominantly horizontal, with the land on the west side of the fault moving north.
South of the Bay Area the Hayward and Calaveras merge into the San Andreas. Hollister is located just north of where this happens, right on top of the southern end of the Calaveras fault.
What makes Hollister particularly interesting is that from San Juan Bautista to just north of Parkfield the faults in the San Andreas system are not "stuck": instead of moving only during major earthquakes, they continuously "creep." As a result of this creep, Hollister is being slowly ripped in two, for the most part along a remarkably narrow zone running right through the middle of town.
The rate of creep is inconsistent, and has been measured at between 6 and 15mm per year at various times throughout the 20th century. Underground pipes, road paving, curbing, and foundations all show signs of being gradually shifted apart. Notice how the distortion always bends structures to the right- no matter from which side of the fault the photo was taken. Also notice that the motion is horizontal: the ground is remaining level as it moves. Together, these two observations define right-lateral strike-slip motion.
On my second visit to Lake Cowal back in early December, I was told about a place that was of great interest to geologists visiting the nearby gold mine. This trip I headed out to have a look.
Following is the information I was given to explain what I was going to see.
"The outcrop is part of the Booberoi Fault and features sheared Late Silurian-Early Devonian Edols Conglomerate (Sherwin, 1996) showing stretched quartzite and vein-quartz pebbles in a quartz white mica-chlorite matrix, which was probably a muddy sandstone. Mesoscopic kinematic indicators show at least two movement episodes: a horizontal sinstral movement and a near vertical reverse (west side up) movement. The movement direction can be discerned from the asymmetry of the matrix surrounding the pebbles and the mineral elongation within the matrix."
Video - youtu.be/gFIb1JXFTb8
On my second visit to Lake Cowal back in early December, I was told about a place that was of great interest to geologists visiting the nearby gold mine. This trip I headed out to have a look.
Following is the information I was given to explain what I was going to see.
"The outcrop is part of the Booberoi Fault and features sheared Late Silurian-Early Devonian Edols Conglomerate (Sherwin, 1996) showing stretched quartzite and vein-quartz pebbles in a quartz white mica-chlorite matrix, which was probably a muddy sandstone. Mesoscopic kinematic indicators show at least two movement episodes: a horizontal sinstral movement and a near vertical reverse (west side up) movement. The movement direction can be discerned from the asymmetry of the matrix surrounding the pebbles and the mineral elongation within the matrix."
Video - youtu.be/gFIb1JXFTb8
Hollister, California
In the San Francisco Bay area there are three major faults, from west to east the San Andreas, the Hayward, and the Calaveras; all are part of the San Andreas fault system. All of these are "right-lateral strike-slip faults," which means that the motion is predominantly horizontal, with the land on the west side of the fault moving north.
South of the Bay Area the Hayward and Calaveras merge into the San Andreas. Hollister is located just north of where this happens, right on top of the southern end of the Calaveras fault.
What makes Hollister particularly interesting is that from San Juan Bautista to just north of Parkfield the faults in the San Andreas system are not "stuck": instead of moving only during major earthquakes, they continuously "creep." As a result of this creep, Hollister is being slowly ripped in two, for the most part along a remarkably narrow zone running right through the middle of town.
The rate of creep is inconsistent, and has been measured at between 6 and 15mm per year at various times throughout the 20th century. Underground pipes, road paving, curbing, and foundations all show signs of being gradually shifted apart. Notice how the distortion always bends structures to the right- no matter from which side of the fault the photo was taken. Also notice that the motion is horizontal: the ground is remaining level as it moves. Together, these two observations define right-lateral strike-slip motion.
Hollister, California
In the San Francisco Bay area there are three major faults, from west to east the San Andreas, the Hayward, and the Calaveras; all are part of the San Andreas fault system. All of these are "right-lateral strike-slip faults," which means that the motion is predominantly horizontal, with the land on the west side of the fault moving north.
South of the Bay Area the Hayward and Calaveras merge into the San Andreas. Hollister is located just north of where this happens, right on top of the southern end of the Calaveras fault.
What makes Hollister particularly interesting is that from San Juan Bautista to just north of Parkfield the faults in the San Andreas system are not "stuck": instead of moving only during major earthquakes, they continuously "creep." As a result of this creep, Hollister is being slowly ripped in two, for the most part along a remarkably narrow zone running right through the middle of town.
The rate of creep is inconsistent, and has been measured at between 6 and 15mm per year at various times throughout the 20th century. Underground pipes, road paving, curbing, and foundations all show signs of being gradually shifted apart. Notice how the distortion always bends structures to the right- no matter from which side of the fault the photo was taken. Also notice that the motion is horizontal: the ground is remaining level as it moves. Together, these two observations define right-lateral strike-slip motion.
On my second visit to Lake Cowal back in early December, I was told about a place that was of great interest to geologists visiting the nearby gold mine. This trip I headed out to have a look.
Following is the information I was given to explain what I was going to see.
"The outcrop is part of the Booberoi Fault and features sheared Late Silurian-Early Devonian Edols Conglomerate (Sherwin, 1996) showing stretched quartzite and vein-quartz pebbles in a quartz white mica-chlorite matrix, which was probably a muddy sandstone. Mesoscopic kinematic indicators show at least two movement episodes: a horizontal sinstral movement and a near vertical reverse (west side up) movement. The movement direction can be discerned from the asymmetry of the matrix surrounding the pebbles and the mineral elongation within the matrix."
Video - youtu.be/gFIb1JXFTb8
On my second visit to Lake Cowal back in early December, I was told about a place that was of great interest to geologists visiting the nearby gold mine. This trip I headed out to have a look.
Following is the information I was given to explain what I was going to see.
"The outcrop is part of the Booberoi Fault and features sheared Late Silurian-Early Devonian Edols Conglomerate (Sherwin, 1996) showing stretched quartzite and vein-quartz pebbles in a quartz white mica-chlorite matrix, which was probably a muddy sandstone. Mesoscopic kinematic indicators show at least two movement episodes: a horizontal sinstral movement and a near vertical reverse (west side up) movement. The movement direction can be discerned from the asymmetry of the matrix surrounding the pebbles and the mineral elongation within the matrix."
Video - youtu.be/gFIb1JXFTb8
On my second visit to Lake Cowal back in early December, I was told about a place that was of great interest to geologists visiting the nearby gold mine. This trip I headed out to have a look.
Following is the information I was given to explain what I was going to see.
"The outcrop is part of the Booberoi Fault and features sheared Late Silurian-Early Devonian Edols Conglomerate (Sherwin, 1996) showing stretched quartzite and vein-quartz pebbles in a quartz white mica-chlorite matrix, which was probably a muddy sandstone. Mesoscopic kinematic indicators show at least two movement episodes: a horizontal sinstral movement and a near vertical reverse (west side up) movement. The movement direction can be discerned from the asymmetry of the matrix surrounding the pebbles and the mineral elongation within the matrix."
Video - youtu.be/gFIb1JXFTb8