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The AAPG/Datapages Combined Publications Database
CSPG Bulletin
Abstract
Low-taper Triangle-zone Geometry: An Interpretation for the Rocky Mountain Foothills, Pine Pass - Peace River Area, British Columbia
ABSTRACT
Three structural
packages with distinctive
structural
geometries have been recognized in the Pine Pass - Peace River segment of the northern (British Columbia) Foothills. The upper package, of Upper Triassic to Cretaceous strata, is characteristically folded into broad synclines and narrow, complex anticlines. The middle package, of Upper Devonian to Middle Triassic strata, is deformed by numerous faults and disharmonic folds. Shale formations at the base and top of this package appear to be major zones of detachment. The lower package, of Middle Devonian and older strata, is probably deformed by large, blind (nonsurfacing) thrusts under the western Foothills. Relationships portrayed on a regional structure section across the Foothills suggest that strata of the lower and middle
structural
packages were shortened in excess of 10 km more than strata of the upper, while all
structural
packages were being displaced eastward relative to the underlying autochthonous basement. Equivalent strata east of the Foothills were not deformed and are autochthonous. Therefore, strata of the more-shortened lower and middle packages must have moved eastward between basal and upper detachments relative to the basement and the less-shortened upper
structural
package. The geometry is that of a classical triangle zone, but on a larger scale with a very low taper.
The exposed fold belt of upper package strata, interpreted as the upper level of the triangle zone, conceals important fold and thrust structures in the middle and lower structural
packages. Published structure
sections
suggest that similar relationships may exist throughout the southern part of the fold-dominated British Columbia Foothills.
GEOMETRIE D'UNE ZONE TRIANGULAIRE A FAIBLE RELIEF: INTERPRETATION DES FOOTHILLS DES MONTAGNES ROCHEUSES, REGION DE PINE PASS - PEACE RIVER, COLOMBIE BRITANNIQUE
RESUME
Trois ensembles structuraux a geometries distinctes ont ete reconnus dans le segment Pine Pass - Peace River des Foothills des Montagnes Rocheuses. L'ensemble superieur, d'age Trias superieur a Cretace, est plisse en de larges synclinaux separes par des anticlinaux etroits et complexes. L'ensemble intermediaire, d'age Devonien superieur a Trias moyen, est deforme par de nombreuses failles et plis disharmoniques. Deux zones principales de decollement se seraient formees au sein des couches argilleuses a la base et au sommet de l'ensemble intermediaire. Un grand chevauchement aveugle (sans affleurement) semble deformer l'ensemble inferieur, d'age Devonien moyen et plus ancien, sous les Foothills occidentaux. Les relations decrites dans la coupe regionale au travers des Foothills suggere que des strates des ensembles inferieur et intermediaire furent raccourcies de plus de dix kilometres par rapport aux strates de l'ensemble superieur, cependant que les trois ensembles furent transportes vers l'Est par rapport au socle autochtone sous-jacent. Les strates equivalentes vers l'Est n'ont pas ete deformees et sont autochtones. En consequence, une zone de detachement inferieure et superieure separe les ensembles inferieur et intermediaire, qui ont du chevaucher plus a l'Est que le socle (fixe) et l'ensemble superieur (a moindre raccourcissement). La geometrie est celle d'une zone triangulaire classique, mais a plus grande echelle et a plus faible relief structural
.
L'affleurement des couches plissees de l'ensemble superieur, interprete comme etant la partie superieure d'une zone triangulaire, dissimule d'importantes structures plissees et chevauchees au sein des ensembles inferieur et intermediaire. Les coupes structurales deja publiees indiquent que des relations semblables existeraient dans la region meridionale, a plissement predominant des Foothills de Colombie Britannique.
Traduit par A. Zolnai
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