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Magmatic fluxes 508_2k13_lec17
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Magmatic fluxesMagmatic fluxes
508_2k13_lec17508_2k13_lec17
FluxesFluxes
Rates of magmatism (vol/time or similar) to be compared to rates of other tectonics processes;
Can be applied to rates of everything geologic.
Rates of magmatism (vol/time or similar) to be compared to rates of other tectonics processes;
Can be applied to rates of everything geologic.
10
8
6
4
2
-2
0
-4
-6
-8
40 80 120 160
Age (Ma)εNd
Isotope -time relationships, but how relevant are the samples outlined in red?
TechniquesTechniques
1. Magmatic thickening rate (MTR)= Thickness/ time (km/My) - magmatic addition in a vertical view;
2. Apparent igneous flux (AIF) - km2/time, where the area is in plan view;
Magmatic addition rate (km3/kmMy) (MAR), volume of intrusive material per time per km leght of feature (ridge, arc, etc).
1. Magmatic thickening rate (MTR)= Thickness/ time (km/My) - magmatic addition in a vertical view;
2. Apparent igneous flux (AIF) - km2/time, where the area is in plan view;
Magmatic addition rate (km3/kmMy) (MAR), volume of intrusive material per time per km leght of feature (ridge, arc, etc).
MTRMTR
Applies to areas that contain volcanics or sill-like geometries;
Can be “translated” into the other flux parameters by assuming a certain width of the arc;
Applies to areas that contain volcanics or sill-like geometries;
Can be “translated” into the other flux parameters by assuming a certain width of the arc;
Example-ZagrosExample-Zagros
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0.3 km/My
From MTR to MARFrom MTR to MAR
Assume an arc width , e.g. 100 km 0.3 x 1 km x 100 km = 30 km3/km My = 1
AU
Assume an arc width , e.g. 100 km 0.3 x 1 km x 100 km = 30 km3/km My = 1
AU
AIFsAIFs
Conservative calculations for areas in which depth info is limited;
Rely on surface areas of plutonic/volcanic rocks and their ages;
AIR and MAR are not the same even though they both have the same units, area per time.
Conservative calculations for areas in which depth info is limited;
Rely on surface areas of plutonic/volcanic rocks and their ages;
AIR and MAR are not the same even though they both have the same units, area per time.
30 km
38 km x 150 km = 5700 km2
if thickness ~ 25 km- 143,000 km3
MARMAR
The most desirable way of calculating fluxes, but requires good knowledge of the 3D geology, especially if done in small time increments (1-5 Ma);
30 km3/km My is informally known as an AU (Armstrong Unit).
The most desirable way of calculating fluxes, but requires good knowledge of the 3D geology, especially if done in small time increments (1-5 Ma);
30 km3/km My is informally known as an AU (Armstrong Unit).
MORB MAR for 1 My increments
MORB MARMORB MAR
Pacific - 10 cm/yr 10 km/My, 1km x 6km x 10 km = 60 km3/km My; 2 AU
Atlantic 1 cm/yr = 0.6 km3/km My; Average around 1 AU for oceans;
Pacific - 10 cm/yr 10 km/My, 1km x 6km x 10 km = 60 km3/km My; 2 AU
Atlantic 1 cm/yr = 0.6 km3/km My; Average around 1 AU for oceans;
ArcsArcs
Island arcs are built at 1 AU at the large scale - e.g. the entire Aleutian chair;
Steady state? If yes, it points to magmatic-dominated processes, if not, tectonic processes are influential.
Island arcs are built at 1 AU at the large scale - e.g. the entire Aleutian chair;
Steady state? If yes, it points to magmatic-dominated processes, if not, tectonic processes are influential.
Complicating the issue…Complicating the issue…
What is the area is not linear? E.g. the Arabian Nubian shield, the Mongolian terrane collage;
Choose reasonable arc widths and divide it into X arc wide area;
What is the area is not linear? E.g. the Arabian Nubian shield, the Mongolian terrane collage;
Choose reasonable arc widths and divide it into X arc wide area;
Steady state?Steady state?
Combine age with geology- AIFs
0
100
200
300
400
500
600
700
800
900
40 60 80 100 120 140 160 180
Time (Ma)
Apparent intrusive flux (km
2/My)
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Depth constraintsDepth constraints
Depth of emplacement; depth bias? Depth of emplacement; depth bias?
0
2
4
6
8
10
12
0 50 100 150 200 250 300 350 400 450
Age (Ma)
Depth (km)
Regional age-depth
Volcano-plutonic ratios?Volcano-plutonic ratios?
Anywhere between 1/4 to 1/20; Use 1/10 if you have no info and quote me
on that; There is also a 1/1 plutonic evolved to
plutonic residual ratio (actually anywhere between 2 to 1/3), so make it 1/1;
Anywhere between 1/4 to 1/20; Use 1/10 if you have no info and quote me
on that; There is also a 1/1 plutonic evolved to
plutonic residual ratio (actually anywhere between 2 to 1/3), so make it 1/1;
IDB - Age vs. Sr(i)
0.7
0.71
0.72
0.73
0.74
0 20 40 60 80 100 120 140 160 180
Age (Ma)
Sr(i)
Idaho
Montana
Idaho Batholith - Strontium
Apparent intrusive flux vs. time and plate motions
Short, high flux events separated by lulls Baseline fluxes coincide with steady state
island arcs (10-30 km3/km Ma). Flare-ups generate 10 times more magma within short (5-15 My) periods. Most of the continental arcs are made in flare-ups.
Magmatic flare-ups
HW9HW9
Calculate the average addition rate (km3/km My) of the BC arc between 52 and 54 lat N, knowing the arc was active for 150 My, and has an average width of 160 km and thickness of 25 km (see Figure X below);
Compare that to the average calculated by Gehrels et al., 2009 in the table Y below (or the pdf of his GSAB paper available on the class page).
Calculate the average addition rate (km3/km My) of the BC arc between 52 and 54 lat N, knowing the arc was active for 150 My, and has an average width of 160 km and thickness of 25 km (see Figure X below);
Compare that to the average calculated by Gehrels et al., 2009 in the table Y below (or the pdf of his GSAB paper available on the class page).
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Fig X
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Table Y
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