solution. In other words, they are not the product of provides a record of the original size of the grain. The local replacement of existing detrital grains by central idea behind the pds method is that principal recrystallziation. (4) There are many descriptions in directions with S<1. SMt deformation has reduced the literature of a"fine-grained matrix "in low-grade the average dimension of the detrital grains by a factor immature sandstones. The matrix problem"is at the equivalent to the principal stretch. In contrast, the centre of the old debate about the distinction of average initial dimension of a detrital grain should be greywacke from arkose(see Dickinson, 1970, for a preserved in the X direction because the grains lack review). More recently, some of our colleagues have any significant internal deformation and because the suggested that the matrix of the rock may account for original grain surface is mantled by fibre overgrowth the volume loss that we have measured. Deformed Therefore, a contractive principal stretch can be lithic grains can appear like matrix, but the outlines of determined by finding the average grain dimension such grains are usually easily recognized in plane light arallel to a contractive principal direction and (pseudomatrix of Dickinson, 1970). Our experience is dividing it by the average grain dimension parallel to that the rest of the"matrix " is fibre overgrowth. The X Dimensions are measured in a two-dimensional overgrowths appear as a fine-grained matrix"when thin section, so a correction is needed to get the viewed in sections oblique to the X direction. XY and appropriate three-dimensional result(see Feehan and XZ sections are needed to see the overgrowth texture Brandon, 1999 for details) with XZ sections providing the best view. Diagnostic Our measurements were made using a petrographic features include the elongated habit of the fibre microscope with a camera lucida tube and digitising minerals(commonly quartz and mica), the consistent tablet. Measurements are precise to better than #3 um orientation of the fibres across the section and the The dimension of each grain is represented by its generally uniform composition of the overgrowths. (5) caliper dimensions(or projected dimensions)in the We found no petrographic evidence for syntaxial principal directions lying in the section. The caliper overgrowths, but cathodoluminescence work is needed dimensions of the grains are not affected in any to fully test this conclusion significant way by grain rotations associated with compaction. For instance, PDS measurements on 4.2. Methods undeformed sandstones gave undeformed results(i.e Our study employs the pds(projected Dimension S-1)(Ring and Brandon, 1999 Strain), Mode, and Fiber methods for measuring The Mode method is used to determine the strains and internal rotations in sandstones deformed extensional strain recorded by the fibre overgrowths by the SMT mechanism. a brief summary is provided The modal percentage of fibres in a rock is directly here. Further details can be found in Feehan and related to the absolute extensional stretch in the rock Brandon(1999)and ring and brandon (1999) Fibre modes are most easily measured in the XZ Relevant computer programs are available at section. For unidirectional fibres, S,=(1-m where www.geologyyaleedu/-brandon m is the modal fraction of fibre Traditional methods, such as the r/ method, are Given absolute strains, the volume stretch Sr not suitable because the grains did not deform as final volume/initial volume) is equal to the product of passive markers but rather by truncation and the principal stretches(S,. Sy.S,). Because our precipitation along grain boundaries. In our discussion methods focus entirely on the loss of mass from grains here, the principal stretches are designated as Sr> Sy> and the amount of mass locally precipitated, our S, where S= final length/initial length estimates of Sp, only represent the mass-transfe Measurements were made using XZ and Xr thin omponent of the volume strain. Other sources of sections. Samples in this study have unidirectional volume strain include changes in porosity and mineral fibres, which means that Sx>I and Sy and s,< 1 density. Porosity is thought to have been small at the Thus, S, and S, were determined by the PDs method start of smt deformation and is thus ignored and Sy by the Mode method Changes in mineral density are insignificant at the low The Pds method is used to metamorphic grade in our study shortening produced by dissolution of grain The geometric relationship between the fibre boundaries. The method exploits the fact that for SMt overgrowths and the trace of cleavage was used to deformation, the dimensions of the detrital quartz and estimate the internal rotation associated with Smt feldspar grains remain unchanged in the X direction deformation. This method is described in Ring and (i.e, deformation is intergranular, not intragranular) Brandon(1999). The basic idea is that the fibre Therefore, the grain diameter in the X direction overgrowths track the incremental extension direction 66 solution. In other words, they are not the product of local replacement of existing detrital grains by recrystallziation. (4) There are many descriptions in the literature of a “fine-grained matrix” in low-grade immature sandstones. The “matrix problem” is at the centre of the old debate about the distinction of greywacke from arkose (see Dickinson, 1970, for a review). More recently, some of our colleagues have suggested that the matrix of the rock may account for the volume loss that we have measured. Deformed lithic grains can appear like matrix, but the outlines of such grains are usually easily recognized in plane light (pseudomatrix of Dickinson, 1970). Our experience is that the rest of the “matrix” is fibre overgrowth. The overgrowths appear as a “fine-grained matrix” when viewed in sections oblique to the X direction. XY and XZ sections are needed to see the overgrowth texture, with XZ sections providing the best view. Diagnostic features include the elongated habit of the fibre minerals (commonly quartz and mica), the consistent orientation of the fibres across the section, and the generally uniform composition of the overgrowths. (5) We found no petrographic evidence for syntaxial overgrowths, but cathodoluminscence work is needed to fully test this conclusion. 4.2. Methods Our study employs the PDS (Projected Dimension Strain), Mode, and Fiber methods for measuring strains and internal rotations in sandstones deformed by the SMT mechanism. A brief summary is provided here. Further details can be found in Feehan and Brandon (1999) and Ring and Brandon (1999). Relevant computer programs are available at www.geology.yale.edu/~brandon. Traditional methods, such as the Rf /φ method, are not suitable because the grains did not deform as passive markers but rather by truncation and precipitation along grain boundaries. In our discussion here, the principal stretches are designated as SX ≥ SY ≥ SZ, where S = final length/initial length. Measurements were made using XZ and XY thin sections. Samples in this study have unidirectional fibres, which means that SX > 1 and SY and SZ < 1. Thus, SY and SZ were determined by the PDS method, and SX by the Mode method. The PDS method is used to measure the average shortening produced by dissolution of grain boundaries. The method exploits the fact that for SMT deformation, the dimensions of the detrital quartz and feldspar grains remain unchanged in the X direction (i.e., deformation is intergranular, not intragranular). Therefore, the grain diameter in the X direction provides a record of the original size of the grain. The central idea behind the PDS method is that principal directions with S < 1, SMT deformation has reduced the average dimension of the detrital grains by a factor equivalent to the principal stretch. In contrast, the average initial dimension of a detrital grain should be preserved in the X direction because the grains lack any significant internal deformation and because the original grain surface is mantled by fibre overgrowths. Therefore, a contractive principal stretch can be determined by finding the average grain dimension parallel to a contractive principal direction and dividing it by the average grain dimension parallel to X. Dimensions are measured in a two–dimensional thin section, so a correction is needed to get the appropriate three–dimensional result (see Feehan and Brandon, 1999 for details). Our measurements were made using a petrographic microscope with a camera lucida tube and digitising tablet. Measurements are precise to better than ±3 µm. The dimension of each grain is represented by its caliper dimensions (or projected dimensions) in the principal directions lying in the section. The caliper dimensions of the grains are not affected in any significant way by grain rotations associated with compaction. For instance, PDS measurements on undeformed sandstones gave undeformed results (i.e. S ~ 1) (Ring and Brandon, 1999). The Mode method is used to determine the extensional strain recorded by the fibre overgrowths. The modal percentage of fibres in a rock is directly related to the absolute extensional stretch in the rock. Fibre modes are most easily measured in the XZ section. For unidirectional fibres, SX = (1 - m) -1, where m is the modal fraction of fibre. Given absolute strains, the volume stretch SV (= final volume/initial volume) is equal to the product of the principal stretches (SX ⋅ SY ⋅ SZ). Because our methods focus entirely on the loss of mass from grains and the amount of mass locally precipitated, our estimates of SV only represent the mass-transfer component of the volume strain. Other sources of volume strain include changes in porosity and mineral density. Porosity is thought to have been small at the start of SMT deformation and is thus ignored. Changes in mineral density are insignificant at the low metamorphic grade in our study area. The geometric relationship between the fibre overgrowths and the trace of cleavage was used to estimate the internal rotation associated with SMT deformation. This method is described in Ring and Brandon (1999). The basic idea is that the fibre overgrowths track the incremental extension direction