FEBRUARY 1993
ROBERT M. REED, B.S., UNIVERSITY OF TEXAS AT AUSTIN
M.S., UNIVERSITY OF MASSACHUSETTS
Directed by: Professor Michael L. Williams
A microfabric study conducted in the northern part of the Pelham gneiss dome, central Massachusetts, has shown that the dome fabric is dominated by a relatively homogeneous, pervasive top-to-the-south shearing. Evidence consists of numerous kinematic indicators including: S-C fabrics, asymmetric winged porphyroclasts, asymmetric strain shadows, asymmetrically boudinaged pegmatites, and asymmetric folds. Shear-related fabrics are present throughout the exposed section, from the lowest exposed gneissic rocks to the uppermost units in the cover section. Some textural evidence exists to suggest that this shearing diminishes slightly in intensity with structural depth. Reconnaissance indicates that shear fabrics are present throughout the Pelham dome and perhaps in adjacent areas of the Bronson Hill anticlinorium.
Porphyroblasts within pelitic units contain inclusion trails which preserve evidence for at least one, and probably two, previous foliations occurring at a high angle to the shear fabric. Both the shear fabric and the one certain previous fabric seem to have formed at similar amphibolite-grade metamorphic conditions. Kyanite, the peak metamorphic mineral present in these assemblages, seems to have formed both before and after the shearing, with the majority overgrowing the shear-related foliation.
Fabrics within the Pelham dome are inconsistent with the Pelham dome having formed through solid-state diapirism. The concentric, oval foliation and shallow N-S lineation are clearly associated with top-to-the-south subhorizontal shearing. Several alternative mechanisms for dome formation have been examined, and none has proven completely satisfactory. The most likely mechanism is that the Pelham dome is a low-strain pod within a very large ductile shear regime.
Interpretations of the tectonics of the deformation seen here are complicated by uncertainty over the age of deformation. It is probable that some component of the deformation is Pennsylvanian in age (Gromet and Robinson, 1990), but other elements are probably Acadian. Serious questions about timing, style of deformation, and direction of shear undermine the direct correlation of shearing in the Pelham dome and shearing in the Willimantic dome suggested by Getty (1990).
CONCLUSIONS
1) The northern part of the Pelham dome is pervasively deformed by relatively homogeneous, subhorizontal shear with a top-to-the-south shear sense. The direction of shear is indicated by both microstructural and outcrop-scale kinematic indicators including: asymmetric winged porphyroclasts, asymmetric pressure shadows, S-C fabrics, asymmetric folds, and asymmetrically boudinaged pegmatites.
2) The deformation seen in the Pelham dome has several distinctive characteristics. The shear is parallel to a well-developed stretching lineation and shear is distributed homogeneously, only in rare instances forming discrete shear zones. Easily visible variations in shear intensity are associated with changes in rock type and are probably the result of strain partitioning due to differences in strength between the rock types. The shear plane is reoriented along the margins of the dome, staying perpendicular to compositional layering.
3) Based on reconnaissance geology by the author and others (V. DelloRusso, personal communication, 1989; P. Robinson, personal communication, 1992), and the presence of a strong stretching lineation throughout the Pelham dome (Figure 2.1), it appears that this top-to-the-south shearing is present throughout the central and southern part of the Pelham dome as well. Evidence for this shearing may also exist farther south in the Belchertown intrusive complex (Figure 1.1). Evidence for this includes; lineation data (Figure 2.16), and the presence of shear fabrics (Ashwal, 1974). There are other areas within the Bronson Hill anticlinorium with a N-S stretching lineation that also seem to have a significant component of ductile shear (Peterson, 1992). However, recent geochronologic work (Tucker and Robinson, 1991) suggests that the eastern part of the Bronson Hill anticlinorium was deformed at an earlier time than the western parts. However, it remains possible that many areas within the western Bronson Hill anticlinorium may have been deformed in a manner similar to the Pelham dome. All of the domes of the Bronson Hill anticlinorium should be examined to see if this top-to-the-south shearing is present.
4) Scarcity of strain markers has made it difficult to measure quantitatively the amount of strain seen in the Pelham dome. A sheared pegmatite from the Dry Hill Formation gives a 34 deg. angular shear parallel to the lineation and perpendicular to the foliation (Figure 2.17). If this is accepted as a valid minimum for the entire exposed section, it suggests a minimum transport of 1.25 kilometers across the shear zone. Reconstruction of objects pulled apart by shearing has given extensions ranging from 0.08 to 1.32. The minimum angular shear necessary to produce the maximum measured extension in the Pelham dome converts to 2.6 kilometers of transport across the shear zone.
5) Textural criteria in samples from the upper and lower exposures of the Poplar Mountain Gneiss suggest that the shear strain decreases slightly with depth in the dome. An attempt to compare textural criteria between quartzites from the cover rocks and from the core gneisses was inconclusive, partially due to the presence of primary recrystallization. However, extremely similar textures are seen in quartzites from the core and cover suggesting similar magnitudes of shear. Robinson (personal communication, 1992) has suggested that this conclusion is supported by an increased variation in the trend of fold axes with depth, implying that pre-shearing folds have been less reoriented by shearing with depth.
6) Some previously interpreted "late shears of reversed sense" (Onasch, 1973) are probably related to the interaction of shearing and folding (Figure 2.21). Other evidence given for supposed top-to-the-north shearing (Onasch, 1973) is a misinterpretation of the kinematic development of feldspar pods with asymmetric tails (Figure 2.19). Whether the "late-flattening and extension" proposed by Onasch (1973) is also related to shearing is less clear, but also likely. All the boudins seen in this study seem to be related to disaggregation of pegmatites during shearing, and not a separate phase of "late flattening and extension".
7) Evidence for two phases of deformation and probably a third has been found in porphyroblast inclusion fabrics from the cover rocks of the Pelham dome. One of these fabrics is clearly related to the dominant shear-related fabric present in the matrix. Some porphyroblasts preserve an included foliation at a high angle to the shear foliation with very steep dips to the north or to the south. The strike of this foliation is oblique to the stretching lineation because the fabric appears in thin sections oriented both parallel and perpendicular to the lineation. The orientation and timing of this fabric suggest that it is probably related to a pre-shearing deformation, possibly the back-fold stage of Acadian deformation (Figure 1.5) and not an early phase of shearing. Previous studies (Ashenden, 1973, Onasch, 1973; Laird, 1974) have shown evidence for folding that probably relates to this relict foliation. Unfortunately, porphyroblasts from the core gneisses typically lack inclusion fabrics and therefore show microfabric evidence only of the shearing. The only fabric evidence for earlier events is an example of a possible pre-shearing retrograde reaction (Figure 3.17).
8) Both deformations represented by microfabrics in the rocks took place at roughly the same metamorphic grades. The peak metamorphic assemblage in pelitic rocks (staurolite-kyanite-biotite-garnet) was stable during and after shearing, and possibly during the preceding phase of deformation. Most of the kyanite seen in the pelitic rocks formed after shearing (Figure 3.15).
9) Two highly localized phases of retrograde metamorphism occurred after the end of shearing. The first phase was controlled by fluid access and may be related to the New Salem retrograde zone (Figure 1.2). The second phase was typically very minor and seems to be related to brittle faults that cut the region in the early Mesozoic.
10) Despite an extensive examination, no evidence has been found in the cover rocks of the relict high-grade metamorphism seen in recently recognized Paleozoic rocks (Robinson and Tucker, 1991) infolded into the core rocks of the southern part of the Pelham dome (Roll, 1987). Possible evidence for this metamorphic event in the basement rocks of the northern part of the Pelham dome is confined to one instance of a garnet in a calc-silicate pod undergoing a retrograde reaction to epidote (Figure 3.17). The lack of evidence in the basement gneisses is not surprising in that the mineral assemblages of the gneisses, quartzites, and semi-pelitic rocks that make up the Proterozoic units are not particularly sensitive to metamorphic conditions.
11) The previously suggested (Robinson, 1963; Thompson and others, 1968; Dixon, 1974) density-driven solid-state buoyancy mechanism proposed for formation of domes in the Bronson Hill anticlinorium fails to account for the stretching lineation and the shear seen within the Pelham dome. Formation of the Pelham dome by superposition of orthogonal folds, thrust-fault processes, or symmetrical boudinage requires features not seen in the Pelham dome, and/or fails to explain all the microstructural features. It has been proposed (Robinson and others, 1989) that the Pelham dome may be the surface expression of a subhorizontally oriented sheath fold. This explanation could account for the shearing, the elongate oval shape, and the mineral lineation if the shape of the sheath fold is narrowly constrained. However, a sheath fold mechanism fails to account for the orientation of the foliation, and could require some radical revisions to the mapping done in the southern end of the dome. The mechanism that best fits the requirements is a newly proposed one, that the Pelham dome is the result of partitioning of strain around a large pod within a very large shear zone.
12) On the basis of evidence from this study, previous suggestions that deformation in the Pelham dome may be related to deformation in the Willimantic dome (Gromet, 1989; Getty, 1990) seem to be incorrect. Deformation in the Willimantic dome is subhorizontal shear with a top-to-the-northwest sense (Getty, 1990), while the shear seen in the Pelham dome has a top-to-the-south sense. Deformation in the Willimantic dome is in discrete shear zones which are concentrated at the boundary between Proterozoic rocks and Paleozoic pelitic cover rocks (Getty, 1990). Deformation in the Pelham dome is diffuse, with no such concentration of deformation either at the boundary between Proterozoic and younger rocks, or at the boundary between core gneisses and cover rocks. Deformation in the Willimantic dome is clearly of an extensional nature and shows a transition from ductile to brittle (Getty, 1990). Deformation in the Pelham dome probably has an E-W compressional component in addition to shearing. Brittle deformation seen in the Pelham dome does not appear to be related in any way to earlier ductile deformation. An isotopic technique that is supposed to yield radiometric ages for deformation fabrics (Gromet and Robinson, 1990; Getty, 1990), gives ages of approximately 290 Ma for deformation in the Pelham dome (Gromet and Robinson, 1990); while the same method in the Willimantic dome gives dominantly ages of approximately 270 Ma (Getty, 1990).
13) The shearing seen in the northern part of the Pelham dome does not clearly fit with models proposed for the Alleghanian orogeny in central New England (Hatcher and others, 1989; Getty, 1990). Much more will have to be known about microfabrics and ages of deformation in the Bronson Hill anticlinorium before the discrepancies can be resolved.
(Sorry, I didn't do the references)
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