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GSA Bulletin; January 2009; v. 121; no. 1-2; p. 80-99; DOI: 10.1130/B26202.1
© 2009 Geological Society of America
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Microcracks in New England granitoids: A record of thermoelastic relaxation during exhumation of intracontinental crust

Brett J. Nadan1 and Terry Engelder1,{dagger}

1 Department of Geosciences, The Pennsylvania State University, University Park, Pennsylvania 16827, USA


Figure 01
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  		Figure 1. A Brown-Hoek stress profile (BHSP) to a depth of 6 km in sedimentary basins (adapted from Plumb, 1994). Data are categorized by lithology: sandstones (open circles), shales (filled circles), and carbonates (squares). Upper inset is an earlier BHSP including data from both crystalline and sedimentary rocks to a depth of 3 km where dashed line indicates R = 1 (adapted from Brown and Hoek, 1978). Data are categorized by continent: Scandinavia (open circle), Australia (solid circle), Canada (upright triangle), USA (overturned triangle), South Africa (square), and other regions (star). Lower inset shows a compilation of all 58 focal mechanisms from the United States east of longitude 104° (taken from the World Stress Map database). Data divided into three tiers as indicated. TF—thrust-fault mechanisms (R >1), TS—thrust-strike slip mechanisms (R {approx}1), SS—strike-slip mechanisms (R <1), NS—normal-strike-slip mechanisms (R <1), NF—normal fault mechanisms (R <1).

 

Figure 02
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  		Figure 2. Simplified geological map of New England showing sample locations and the strike of all microcracks in the horizontal thin sections. FIP are the dominant microcrack.

 

Figure 03
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  		Figure 3. The Milford granite at the Mason quarry, Milford, New Hampshire. The view is to the east with a rough, flame-cut wall to the left (i.e., the grain wall) and a smooth, wire-sawed wall to the right (the hardway wall). Rose diagrams show microcrack distribution of all micro-cracks as seen in vertical thin sections cut parallel to the grain and hardway walls. The horizontal microcrack population is responsible for the horizontal rift in this quarry.

 

Figure 04
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  		Figure 4. (A)–(C) Samples of Milford granite from the Mason quarry, Milford, New Hampshire. Thin sections of quartz grains cut in the vertical orientation parallel to the hardway wall of the Mason quarry (crossed nicols). Inset shows a rose diagram of all microcracks (474 data) measured in quartz grains cut parallel to the hardway (refer back to Fig. 3 for a view of the Mason quarry). Each thin section is oriented in the same manner as the rose diagram so that FIP are the common vertical microcrack and open cracks are the common horizontal microcrack. (D) Example of open networks of microcracks in the Barre granite, Barre, Vermont. These are rift cracks in a thin section cut parallel to the hardway.

 

Figure 05
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  		Figure 5. Fabric of microcracks within quartz grains of the Concord granite (Swenson quarry) near Concord, New Hampshire. Orientation data displayed in the form of rose diagrams for microcracks measured in a thin section cut in the orientation indicated by azimuthal arrows. Orientation data are binned into 10° intervals. First row: Data cube with microcrack orientations presented in the form of three mutually perpendicular rose diagrams. Labels on the faces of the data cube indicate planes in the working quarry known as rift (horizontal), grain, and hardway. Second row: Orthographic projection of the three sides of the microcrack data cube. Mutually perpendicular data are scaled so that the outer ring has a data count indicated by the circled number. Each type of microcrack has a different maximum data count. The data count for the open microcracks includes (open networks)/(open single microcracks).

 

Figure 06
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  		Figure 6. Fabric of microcracks within Milford granite (Mason quarry) southwest of Milford, New Hampshire. See Figure 5 caption for further explanation. Note: Working directions in the Mason quarry follow Dale's original designation and are so labeled.

 

Figure 07
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  		Figure 7. Fabric of microcracks within Chelmsford granite (Fletcher quarry) near Chelmsford, Massachusetts. Three orthogonal thin sections cut along arbitrary directions of azimuth 331°, 061°, and horizontal. See Figure 5 caption for additional explanation. Note: Working directions in the Fletcher quarry are called flame cut, wire saw, and horizontal. Google Earth image of the Fletcher quarry shows the clockwise rotation of the directions in the working quarry from the older portions (west) to the newer portions (east).

 

Figure 08
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  		Figure 8. Fabric of microcracks within the Bethlehem granite (outcrop) near Grantham, Vermont. See Figure 5 caption for further explanation. Because this sample comes from an outcrop where quarry directions have not been established, labels on the faces of the data cube are outcrop orientations with no connection to rift, grain, or hardway.

 

Figure 09
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  		Figure 9. Fabric of microcracks within the Milford Granite (Kittredge quarry) in Milford, New Hampshire. See Figure 5 caption for further explanation. Labels on the faces of the data cube indicate planes in the working quarry known as grain (horizontal), rift, and hardway.

 

Figure 10
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  		Figure 10. Fabric of microcracks within the Barre granite (Pierre quarry) near Barre, Vermont. See Figure 5 caption for further explanation to parts (A) and (B). Labels on the faces of the data cube indicate planes in the working quarry known as grain (horizontal), rift, and hardway. (C) Dip and dip directions for 19 quarries in Barre granite (Dale, 1923). Three quarries have horizontal sheet fractures, and four quarries (box) are without well-developed sheet fractures. (D) Directions of vertical rift planes in 14 quarries in Barre granite (Dale, 1923).

 

Figure 11
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  		Figure 11. (A) Proposed loading for crack propagation within quartz grains of granite assuming each grain is cylindrical and subject of loading conditions found in Brazilian tests. (B) Flaw size necessary for initiation of vertical microcracks in quartz grains under vertical load, assuming the quartz grains are loaded in a manner resembling a laboratory Brazilian test. Curves represent initiation for quartz and assuming static failure at KIcsubcritical crack propagation at <50% KIc.

 

Figure 12
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  		Figure 12. Stress-depth curve showing the lithostatic stress. Dark arrows are stress paths for isothermal decompression and isobaric cooling. Three cubes are hypothetical models of stress in granite for lithostatic stress at the solidus, after isobaric cooling, and after isothermal decompression. Double arrows on the cubes show the relative magnitudes of the horizontal and vertical principal stresses.

 

Figure 13
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  		Figure 13. Hypothetical Brown-Hoek stress profiles for thermoelastic relaxation: (A) for three gradients, {Delta}Shmin/{Delta}z (= 0.6 {Delta}Sv/{Delta}z; = 0.7 {Delta}Sv/{Delta}z; = 0.77 {Delta}Sv/{Delta}z) starting at 4 km and R = 0.73. (B) Hypothetical Brown-Hoek stress profiles for thermoelastic relaxation starting at three depths (2 km, 3 km, 4 km) with R = 0.73 and {Delta}Shmin/{Delta}z = 0.6 {Delta} Sv/{Delta}z.

 

Figure 14
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  		Figure 14. Normalized rate of thermoelastic relaxation, R* = {Delta}Shmin/{Delta}Sv, as a function of geothermal gradient in an upper crust for a granite with various effective Poisson's ratios, {nu}. (A) E = 40 MPa, {alpha} = 0.000008 °C–1; (B) E = 30 MPa, {alpha} = 0.000008 °C–1; (C) E = 40 MPa, {alpha} = 0.000005 °C–1. The horizontal shading defines the range of R* that produces a BHSP consistent with Figure 1. Fields favoring horizontal and vertical microcracking are indicated.

 




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