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We used illite Ar/Ar dating to obtain absolute ages of folds and shear zones formed within the Mexican Fold–Thrust Belt (MFTB). The methodology takes advantage of illite dating in folded, clay-bearing layers and the ability to obtain accurate ages from small-size fractions of illite using encapsulated Ar analysis. We applied our approach to a cross-section that involves folded Aptian–Cenomanian shale-bentonitic layers interbedded with carbonates of the Zimapan (ZB) and Tampico–Misantla (TMB) Cretaceous basins in central-eastern Mexico. Basinal carbonates were buried by syn-tectonic turbidites and inverted during the formation of the MFTB in the Late Cretaceous. Results from folds and shear zones record different pulses of deformation within this thin-skinned orogenic wedge. Mineralogical compositions, variations in illite polytypes, illite crystallite size (CS), and Ar/Ar ages were obtained from several size fractions in limbs and hinges of the folds and in the shear zones. 1Md-illite polytype (with CS of 6–9 nm) dominates in two folds in the TMB while 2M1-illlite (with CS of 14–30 nm) dominates in the third fold, in the ZB, and in the fold/shear zone. From west (higher grade) to east (lower grade): Ar retention ages indicate shearing occurred at ~84 Ma in the westernmost shear zone, folding at ~82 Ma in the ZB with subsequent localized shearing at ~77 Ma, and Ar total gas ages constrain the time of folding at ~64 Ma on the west side of the TMB and ~44 Ma on the eastern edge. These results are consistent with the age and distribution of syn-tectonic turbidites and indicate episodic progression of deformation from west to east.

In the traditional geology curriculum, developed before plate tectonics, the emphasis was on rocks, minerals, fossils, and structures, which were largely studied separately. This approach was appropriate for the descriptive/cataloging phase of the earth sciences. The development of plate tectonics has changed the zeitgeist within the geological community so that processes are now viewed as being at least as important as materials. We have restructured our undergraduate curriculum in this spirit. The new curriculum begins with two courses in geodynamics, one emphasizing the solid mechanics of Earth, the other the fluid aspects. A key element of these courses has been to bring together basic physical topics, such as stress and strain, that are of fundamental importance in a variety of earth-science applications and present them in a unified way, emphasizing geologic applications. We also added courses in basic geochemistry, geologic time, and a “capstone” course in Earth systems. We retained several of the ...

The Proterozoic iron ores of the Kiruna district, northern Sweden, occur as discontinous lenses of magnetite and hematite up to 4 kilometers in length within a sequence of felsic porphyries. The association of low-grade felsic volcanics and sedimentary rocks with basaltic pillow lavas and granitic rocks shows a strong similarity to typical Archean greenstone belts. Primary younging indicators, bedding/cleavage relationships, and minor fold symmetries through the district suggest that the area occupies the eastern limb of a major antiform, cored by granitic rocks to the west. A steep regional cleavage, inhomogeneously developed through the district, suggests one major episode of compressional deformation. The cleavage and associated flattened clasts indicate a compression direction plunging 10°WNW. Elongate clasts, boudins, and fibrous veins indicate a strong extension plunging 60°SSE, parallel to fold hinge lines. Boudinage of the major ore bodies to produce their present distribution is compatib...

Folds in rocks form by three processes: buckling, in which an instability arises when rocks are compressed parallel to the layering; bending, in which differential transverse forces are applied to the layering; and passive folding, in which the layers have no mechanical significance. Geometrical properties and strain distribution help us identify which processes produced natural folds, and also provide information about the mechanical properties and relative competence of the rocks, and the sense of shear in shear zones. For single competent layers and matrix of given compositions, the characteristic features of buckle folds are: (1) a preferred arclength/thickness (L/h) ratio; (2) a tendency for the competent layer to maintain constant thickness; (3) rapid decay of folds away from the competent layer; and (4) cuspate interfaces between layers, with cusps pointing toward the competent layer. The average value of L/h provides a measure of viscosity contrast and, if the strain in the competent layer is know...