OROCLINES: THICK AND THIN

An orocline is a thrust sheet or orogen that is curved in map-view due to it having been bent or buckled about a vertical axis of rotation. Two types of oroclines are recognized: Progressive and Secondary. Progressive oroclines occur at the scale of a thrust belt, are thin-skinned and develop during thrust sheet emplacement. Secondary oroclines occur at the scale of an orogen, and affect crust and lithospheric mantle. Unlike progressive oroclines, which develop during initial orogenesis, secondary oroclines are extra-orogenic, developing after initial orogenesis and in response to an orogen-parallel principal compressive stress that is oriented at a high angle to the stress responsible for orogen development. We present case studies of the Wyoming Salient, a progressive orocline that characterizes the Sevier thrust belt of the western USA; and of the coupled Cantabrian and Central Iberian secondary oroclines of the Variscan orogen of Iberia. The Wyoming Salient has a component of primary curvature attributable to the initial sedimentary prism; 60 to 80% of the 90 degrees of curvature is from progressive bending during thrust sheet emplacement. Bending is attributable to basement highs that characterize the footwall and which impeded eastward translation of the thrust sheets. The coupled Cantabrian and Central Iberian oroclines are isoclinal buckles of an originally linear, north-trending Variscan orogen. The oroclines post-date Variscan orogenesis, formed between 310 and 290 Ma, and developed in response to an orogen-parallel compressive stress oriented at a high angle to the Variscan principal compressive stress. Syn-oroclinal delamination of the mantle lithosphere explains significant post-Variscan magmatism, metamorphism and crustal-scale hydrothermal alteration, including epithermal gold mineralization and dolomitization. Buckling affected a 2300 km long segment of the Variscan orogen; was responsible for >1100 km of relative translation of the south-western end of the Central iberian orocline toward the northeastern limb of the Cantabrian orocline, and involved rates of translation of 5.5 to 11 cm/a. Such large-scale and tectonically rapid translations imply that orocline formation was driven, and accommodated, by subduction.