Orogenic Contraction Research Articles

Coast Plutonic Complex: A mid-Cretaceous contractional orogen

Early Late Cretaceous east-vergent thrusts deform much of the eastern margin of the Coast Plutonic Complex in western British Columbia. Similarities in timing and style suggest that the faults represent a system of backthrusts to a west- vergent thrust belt on the west side of the complex. This geometry and the rarity of mid-Cretaceous strike-slip faults indicate that the Coast Plutonic Complex was a strongly contractional orogen in mid-Cretaceous time.

Transpression, orogenic float, and lithospheric balance

In contractional orogens, a throughgoing basal-detachment system at middle to lower crustal levels or conceivably in the upper mantle detaches the entire crustal section from the underlying lithosphere. The decollement system connects transcurrent faults and coeval thrust belts and allows their motions to be viewed as the resolved components of a regional displacement field largely driven by relative plate motions. The formation of the regional decollement system during contraction may result in subdecollement volume problems and the formation of a lithospheric root in regions where crustal volume is conserved. In regions characterized by large-scale terrane accretion, addition of mass above the basal decollement may result in an essentially balanced system and may not require formation of a lithospheric root.

Thrust sequences

The sequential evolution of arrays of thrusts can occur by a number of geometric models: these models form the subject of this contribution. The most common of these is a simple foreland-directed (piggy-back) series which develops into either an emergent imbricate fan or, if the faults converge upwards, a duplex structure. Depending on the geometry and distribution of thrust ramps and the relative magnitude of displacements on the faults, a range of duplex geometries can result where the imbricate slices have either hinterland-dipping, antiformal stack or foreland-dipping attitudes. A frequently recognized complexity occurs when a later fault climbs across from the footwall into the hanging wall of an earlier one leading to a breached geometry. A less common generation of imbricate fans occurs by the hinterland-directed progression of thrusts in a break-back sequence. This can lead to an overstep geometry where earlier structures are truncated in the footwall of a later fault. The evolution of hinterland-directed, back-thrusts is also considered. Both internally piggy-back (regionally hinterland-directed) and break-back (regionally foreland-directed) sequences can develop, depending on the ability of the active displacement front to propagate forwards. Simple imbricate geometries can develop with an internally break-back sequence but piggy-back propagation will generally require substantial strains within the back-thrust slices or the generation of multiple detachment levels. The range of structures which can result from these fore-thrust and back-thrust sequences is illustrated using hypothetical cross-sections and natural examples from the Scottish Moine thrust belt and the western Alps. The implications of each geometry for estimates of orogenic contraction and the construction of balanced cross-sections are also emphasized.

Structural inversion and its controls : examples from the Alpine foreland and the French Alps

Positive structural inversion involves the uplift of rocks on the hanging-walls of faults, by dip slip or oblique slip movements. Controlling factors include the strike and dip of the earlier normal faults, the type of normal faults — whether they were listric or rotated blocks, the time lapsed since extension and the amount of contraction relative to extension. Steeply dipping faults are difficult to invert by dip slip movements; they form buttresses to displacement on both cover detachments and on deeper level but gently inclined basement faults. The decrease in displacement on the hanging-walls of such steep buttresses leads to the generation of layer parallel shortening, gentle to tight folds — depending on the amount of contractional displacement, back-folds and back-thrust systems, and short-cut thrust geometries — where the contractional fault slices across the footwall of the earlier normal fault to enclose a “floating horse”. However, early steeply dipping normal faults readily form oblique to strike slip inversion structures and often tramline the subsequent shortening into particular directions.Examples are given from the strongly inverted structures of the western Alps and the weakly inverted structures of the Alpine foreland. Extensional faulting developed during the Triassic to Jurassic, during the initial opening of the central Atlantic, while the main phases of inversion date from the end Cretaceous when spreading began in the north Atlantic and there was a change of relative motion between Europe and Africa. During the mid-Tertiary well over 100 km of Alpine shortening took place; Alpine thrusts, often detached along, or close to, the basement-cover interface, stacking the late Jurassic to Cretaceous sediments of the post-extensional subsidence phase. These high level detachments were joined and breached by lower level faults in the basement which, in the external zones of the western Alps, generally reactivated and rotated the earlier east dipping half-graben bounding faults. The external massifs are essentially uplifted half-graben blocks. There was more reactivation and stacking of basement sheets in the eastern part of this external zone, where the faults had been rotated into more gentle dips above a shallower extensional detachment than on the steeper faults to the west.There is no direct relationship between the weaker inversion of the Alpine foreland and the major orogenic contraction of the western Alps; the inversion structures of southern Britain and the Channel were separated from the Alps by a zone of rifting from late Eocene to Miocene which affected the Rhone, Bresse and Rhine regions. Though they relate to the same plate movements which formed the Alps, the weaker inversion structures must have been generated by within plate stresses, or from those emanating from the Atlantic rather than the Tethyan margin.

Cleavage strain in the Variscan fold belt, County Cork, Ireland, estimated from stretched arsenopyrite rosettes

In south-west Ireland, hydrothermally formed arsenopyrite crystals in a Devonian mudstone have responded to Variscan deformation by brittle extension fracture and fragment separation. The interfragment gaps and terminal extension zones of each crystal are infilled with fibrous quartz. Stretches within the cleavage plane have been calculated by the various methods available, most of which can be modified to incorporate terminal extension zones. The Strain Reversal Method is the most accurate currently available but still gives a minimum estimate of the overall strain. The more direct Hossain method, which gives only slightly lower estimates with this data, is more practical for field use. A strain ellipse can be estimated from each crystal rosette composed of three laths (assuming the original interlimb angles were all 60°) and, because actual rather than relative stretches are estimated, this provides a lower bound to the area increase in the plane of cleavage. Based on the average of our calculated strain ellipses this area increase is at least 114% and implies an average shortening across the cleavage of at least 53%. However, several lines of evidence suggest that the cleavage deformation was more intense and more oblate than that calculated, and we argue that a 300% area increase in the cleavage plane and 75% shortening across the cleavage are more realistic estimates of the true strain. Furthermore, the along-strike elongation indicated is at least 80%, which may be regionally significant. Estimates of orogenic contraction derived from balanced section construction should therefore take into account the possibility of a substantial strike elongation, and tectonic models that can accommodate such elongations need to be developed.

Discussion on the use of balanced cross-sections in the calculation of orogenic contraction: a review

Dr D. H. K upper writes:In geology we are constantly trying to reconstruct the past from the present, and in the process we use all the help we can get. Thus the ‘balanced cross-section’(Hossack 1979) is a valuable tool, but we must keep firmly in mind the assumptions that are involved.Hossack has pointed up a few of these, but many geologists still hold the end result in too great faith. In essence, the balanced section is just an improvement on the old process of unfolding a geological section as described, for example, by Chamberlin (1910). When the unfolding process was first used it received much attention; less so in recent years, because it is now realized that most folding is not a simple case of contraction. Carey (1962) showed that a fold can form even if a section is expanding. In the balanced section, the attempt is made to correct for this potential error by measuring the ‘original thickness’ of a bed in the undisturbed foreland. The area of the present, observed geologic cross-section is then ‘balanced’ against this to calculate the ‘original length’, instead of finding it by unfolding ( L = A/T ). As pointed out by the father of the geosynclinal theory, James Hall (1859, pp. 67–73), beds in the orogenic belt are commonly10-20 times thicker than those in the foreland. Much more important, Cooper (1957) demonstrated that geosynclinal beds change thickness rapidly fromp lace to place, being many times thicker in the depositional lows (depo-centers), than in the adjacent

The use of balanced cross-sections in the calculation of orogenic contraction: A review

Balanced section calculations assume that the section has been deformed by a plane strain. If the section is underlain by a décollement, it can be restored to its predeformational length by dividing the area of the section by its original stratigraphic thickness. The thickness can be measured in undisturbed foreland beds or reconstructed by unstraining deformed beds. The restoration of the balanced section does not depend on any particular mechanism of deformation or folding, but in the special case of flexural slip folding the section can be unstrained by straightening out the sinuous bed lengths of the folds. However, in nature the area of the section may have decreased by 15–45%, but the assumption of plane strain always leads to a minimum estimate of shortening. Balanced section calculations suggest that the margins of orogenic belts have contracted by 35–54%.

Relation of Anadarko Basin Movements to Theory of Contracting Continents: ABSTRACT

Part I--Orogenic Movements in Anadarko Basin This paper attempts to clarify controversial articles regarding the age and magnitude of deformation in the Anadarko basin, diastrophism which seems to have occurred at different times and in varying intensity across the Mid-Continent region. Evidence is cited and inferences defended to support the following sequence of events: (1) the appearance and rejuvenated arching of the Central Kansas and Ozark uplifts, upon which converged (northward with increasing interruption) deposits of Cambrian to pre-Mississippian age out of an original basin centering in southern Oklahoma: (2) an episode of regional tilting and broad gentle warping that successively beveled off northward the Hunton to Arbuckle sequence; (3) the advent of full-scale orogeny in post-Morrow pre-Des Moines (early Pennsylva ian time) which established the Amarillo, Wichita, Arbuckle, and Ouachita uplifts (increasing in deformation from west to east) and the adjoining Anadarko, Ardmore, and McAlester basins on their flanks; (4) rejuvenated faulting, warping, and over-thrusting, culminating in late Pennsylvanian and early Permian time, that re-elevated these features as well as northwest-trending adjustment axes in the adjoining basins to locally reexpose basement rocks which contributed a thick and widespread arkose facies to contemporaneous deposition. Thus, the Anadarko basin, defined in origin as post-Morrow pre-Des Moines in age and in outline as flanked by the Amarillo-Wichita-Arbuckle uplifts and Nemaha ridge on the east was involved in numerous Paleozoic movements on the north flank which have no counterpart on the south flank. This is the same contest of simultaneous, world-wide orogenic dogma that is more and more frequently raised by geologists. The pattern of differential crustal shortening with maximum compression to the east in the Ouachitas encourages a new concept: that contraction of the crust in continental areas bordering the Gulf of Mexico may explain the pronounced positive gravity anomaly of that area indicative of a basaltic, high-density floor and an absence of granitic crust, conceivably withdrawn during deformation of surrounding orogenic belts. Part II--Contracting Continents--A Theory Geologists have revived their search for a concept that will replace the extremes of permanent and drifting continents and ocean basins by means of foundering or the gravitational withdrawal of crust from the Pacific to create our satellite moon--a theory that will relate accumulating geophysical and geological facts bearing upon the derivation of continents, ocean basins and orogenic belts. Such a theory must explain (1) the localization of continents from the logically assumed crustal shell of granitic material originally covering the surface of the earth, (2) the presently existing contrast in physical properties and relief of continents and ocean basins, (3) the strategic position of major orogenic belts within and paralleling the margins of the continents and their relation to similar structural trends in the continental nuclei, or shield areas, (4) the distribution of critical organisms, dependent on climate and dry land, whose similarity of morphology requires land connections between widely separated continents. This theory of contracting continents on the face of a shrinking globe accounts for the great ocean basins by the withdrawal of the original granitic crust from these areas during the processes of orogenesis and crustal shortening in which successive belts of deposition and diastrophism were folded and thrust toward pre-existing shield areas which acted as buttresses to contraction from all directions because they had been stabilized in pre-Cambrian time by intense deformation, injection and re-crystallization. If Paleozoic to recent crustal shortening measurable in the mappable sediments and basement rocks could not explain a sufficient withdrawal of crustal material from the ocean basins, it is obvious that the creation of the pre-Cambrian continental nuclei must have already begun o differentiate the continents and ocean basins. On the North American continent, orogenic belts are peripheral to the shield, have counterparts in the lineation of shield structures, and border the continental margins which they evidently have established. If the continents contracted and withdrew crust during successive movements into late Cenozoic time, it is conceivable that suitable land connections for organic migration existed throughout much of Paleozoic to Recent time. This theory also seeks to explain specifically, (1) the Gulf of Mexico, a pronounced and unexplained positive gravity anomaly, as a typical ocean basin devoid of its granitic veneer, withdrawn by crustal shortening of Mid-Continent and Tertiary orogenic movements on the north and south, (2) the Mediterranean, a comparable anomaly, by crustal shortening most pronounced on the north in the Alpine structural system, (3) the other great ocean basins by the process of peripheral End_Page 618------------------------------ orogenic contraction of the adjoining continents, for example, the Cordilleran and eastern Asiatic (Himalayan) systems, resulting in tensional by-products of volcanic islands, earthquakes, and vulcanism bordering the Pacific basin. End_of_Article - Last_Page 619------------