How Determine the Overall Uplift Rate of the Alps
Tuesday, 10. November 2009, 21:38:30
A couple of days ago I posted about the uplifting of the alps. A paper in the newest issue (Volume 21, Number 6, December 2009) of Basin Research describes how you can get a reasonable estimate of the uplift rate - or in other words determine the average Alpine exhumation rate. Exhumation is a term for the process that returns deeply buried rocks to the surface.
In the study fission-track thermocronology was used to reconstruct the long-term exhumation history. I described how fission-track dating works in a post back in March this year (2009), and invite you to read that post, if you have problems with the following discussion of the so-called lag time concept.
The following figure modified after Bernet (2009) hopefully illustrates the basic idea.
In uranium bearing minerals (e.g. zircon) spontaneous fission of uranium-238 releases energy, with the two fission products speeding away in opposite directions and electrons fleeing away from their path and leaving a single trail - a fission-track - in the crystal. The older the crystal, the more tracks. Fission-tracks are however sensitive to heat. If the temperature is sufficiently high for a sufficiently long time, any existing fission track will disappear. This “closure temperature” lies around 240°C for zircon. At temperatures lower than this the fission-track clock starts running. So on its way upwards when a rock with zircons cools below the closure temperature of the zircon fission-tracks system the clock is started and continues to tick to the surface. At the surface the rock is weathered and eroded, and the resulting grains are transported by glaciers and rivers down to their final deposition as sediments. The “lag time” is the time between the time of closure and the time of deposition and mainly represents the time needed to exhume the rock to the surface.
The depositional age is derived from the sediment in which the zircons are sampled, and in this case based on biostratigraphy (biozones) - with an estimated error margin of one million years.
So time of closure (“cooling age”) minus time of deposition (“depositional age”) = time of exhumation (the time of erosion and transport is regarded as geologically instantaneous).
This concept was described in details by Garver et al. (1999). There are 3 generl requirements for the use of this method.
1) No subsequent heating (higher than the closure temperature) that could have reset the “clock”
2) The detrital grains can be dated with sufficient precision
3) No active volcanism during the time of deposition (with possible input of much younger volcanic zircons contaminating the sediment samples).
In the study in question 24 medium to coarse-grained sandstone samples were collected from different stratigraphic horizons from the latest 36 million years. Obviously the result is an average, and maybe I should mention that different parts of the Alps have been uplifted at different rates (at different times). The overall average exhumation rate was calculated to be between 200 and 300 meters per million year on a regional scale.
References:
Bernet et al.
Exhuming the Alps through time: clues from detrital zircon fission-track thermochronology
Basin Research (2009) 21, 781–798
doi: 10.1111/j.1365-2117.2009.00400.x
Garver et al.
Exhumation history of orogenic highlands determined by detrital fission track thermocronology.
In: Exhumation Processes: Normal Faulting, Ductile Flow, and Erosion. Special Publication from the Geological Society of London 1999
In the study fission-track thermocronology was used to reconstruct the long-term exhumation history. I described how fission-track dating works in a post back in March this year (2009), and invite you to read that post, if you have problems with the following discussion of the so-called lag time concept.
The following figure modified after Bernet (2009) hopefully illustrates the basic idea.
In uranium bearing minerals (e.g. zircon) spontaneous fission of uranium-238 releases energy, with the two fission products speeding away in opposite directions and electrons fleeing away from their path and leaving a single trail - a fission-track - in the crystal. The older the crystal, the more tracks. Fission-tracks are however sensitive to heat. If the temperature is sufficiently high for a sufficiently long time, any existing fission track will disappear. This “closure temperature” lies around 240°C for zircon. At temperatures lower than this the fission-track clock starts running. So on its way upwards when a rock with zircons cools below the closure temperature of the zircon fission-tracks system the clock is started and continues to tick to the surface. At the surface the rock is weathered and eroded, and the resulting grains are transported by glaciers and rivers down to their final deposition as sediments. The “lag time” is the time between the time of closure and the time of deposition and mainly represents the time needed to exhume the rock to the surface.
The depositional age is derived from the sediment in which the zircons are sampled, and in this case based on biostratigraphy (biozones) - with an estimated error margin of one million years.
So time of closure (“cooling age”) minus time of deposition (“depositional age”) = time of exhumation (the time of erosion and transport is regarded as geologically instantaneous).
This concept was described in details by Garver et al. (1999). There are 3 generl requirements for the use of this method.
1) No subsequent heating (higher than the closure temperature) that could have reset the “clock”
2) The detrital grains can be dated with sufficient precision
3) No active volcanism during the time of deposition (with possible input of much younger volcanic zircons contaminating the sediment samples).
In the study in question 24 medium to coarse-grained sandstone samples were collected from different stratigraphic horizons from the latest 36 million years. Obviously the result is an average, and maybe I should mention that different parts of the Alps have been uplifted at different rates (at different times). The overall average exhumation rate was calculated to be between 200 and 300 meters per million year on a regional scale.
References:
Bernet et al.
Exhuming the Alps through time: clues from detrital zircon fission-track thermochronology
Basin Research (2009) 21, 781–798
doi: 10.1111/j.1365-2117.2009.00400.x
Garver et al.
Exhumation history of orogenic highlands determined by detrital fission track thermocronology.
In: Exhumation Processes: Normal Faulting, Ductile Flow, and Erosion. Special Publication from the Geological Society of London 1999
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http://www3.interscience.wiley.com/journal/122221627/abstract?CRETRY=1&SRETRY=0
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http://my.opera.com/nielsol/blog/2009/03/04/fission-track-dating
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http://my.opera.com/nielsol/blog/2009/11/06/alps-growing-or-shrinking
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