AV俱乐部

Folds are commonly accompanied by thrusts in deformed low- to medium-grade sedimentary belts. The Acadian fold belt (AFB) of the Meguma Group (comprising the slatey Halifax and sandy Goldenville formations) in east mainland Nova Scotia lacks thrusts. To address the question of why folds predominate in the AFB, an applied geographic information systems-geophysical approach was employed to establish and constrain the basic geometry of the folds by the construction of balanced cross-sections.

The construction of balanced cross-sections requires a minimum of two bed (marker) horizons. Although there is only one mapped marker in the Meguma Group, the Halifax-Goldenville formations boundary, the presence of a consistent pattern of vertical gradient magnetic (VG) anomalies within the sandy Goldenville Formation suggested the possibility of two Avirtual@ marker units could be identified and used. Surprisingly, these discrete VG anomalies correlated with ornamented intervals on cross-sections that suggested the presence of lithologies within the Goldenville Formation not mentioned on map legends of E.R. Faribault. Geophysical modeling of Faribault=s geological cross-section found that the magnetic susceptibilities determined for lithologies Marker 1 and Marker 2 agree with drill core measurements. Two additional Avirtual@ markers were then established and, together with the Halifax-Goldenville boundary, allowed for the construction of four balanced cross-sections.

The balanced cross-sections reveal two orders of folding. The 1st order folds are large-scale, 11 to 18 km wavelength, steep limbed anticlinoria and synclinoria with strong VG anomalies highlight the boundaries between 1st order folds. The 2nd order folds are 4 to 6 km wavelength anticlines and synclines that include cylindrical folds with periclinal fold tips. Periclinal fold tips range in plunge from 1 to 22o. Buckle shortening is 35%. Calculated detachment depths increase from south to north, approximately 6 km along the coast to between 10 and 14 km inland.

The folds are interpreted to have developed in a thick competent multilayer (Goldenville Formation) embedded in an incompetent medium with wavelengths determined both by gravity and mechanical properties of the multilayer. The incompetent embedding medium comprises the Halifax Formation above and a weak unit, perhaps less competent Goldenville Formation below, the competent, multilayered Goldenville Formation.

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Pages: 203
Supervisor: Nicholas Culshaw