John P. Platt

Dynamics of orogenic wedges and the uplift of high-pressure metamorphic rocks

Subduction-accretion complexes can be approximated as wedge-shaped continua with a rigid buttress behind and a subducting litho-spheric slab beneath. Thick wedges undergoing prograde metamorphism have a negligible long-term yield strength and are likely to exhibit a complex nonlinear viscous rheology. Such a wedge will tend to deform internally until it reaches a stable configuration, in which the gravitational forces generated by the wedge geometry balance the traction exerted on its underside by the subducting slab. Accretion of material at the wedge front will lengthen the wedge and cause it to shorten internally to regain the stable geometry. This shortening will be expressed as late (out-of-sequence) thrusting, backthrusting, and folding. Conversely, underplating of sediment or crustal slices will thicken the wedge, which may need to extend internally to regain stability. Extension will cause listric normal faults that may merge downward into zones of ductile extension. Continued underplating at depth and compensating extension above provides a mechanism for bringing high-P/low-T metamorphic rocks to upper levels in the rear of the wedge, where they are commonly observed. Many major tectonic boundaries in convergent orogens (such as the Coast Range thrust in the Franciscan Complex, major nappe contacts in the Alps, and the contact between the Nevado-Filabride and Higher Betic nappe complexes in the Betic Cordillera) show abrupt increases in metamorphic grade downward across them. This is consistent with their origin or reactivation as uplift-related, extensional structures.

Extensional collapse of thickened continental lithosphere: A working hypothesis for the Alboran Sea and Gibraltar arc

Several features of the Alboran Sea suggest that it may have been a high collisional ridge in Paleogene time that subsequently underwent extensional-collapse, driving radial thrusting around the Gibraltar arc. (1) The basin is underlain by thin (13-20 km) continental crust, has an east-west-trending horst and graben morphology, was the locus of Neogene volcanism, and has subsided 2-4 km since the middle Miocene. (2) Extension and subsidence in the basin coincided in time with outwardly directed thrusting in the surrounding mountain chains. (3) Africa and Europe were converging slowly during this period, so extension must have been driven by internally generated forces. (4) Onshore, rocks metamorphosed at 40 km depth are exposed beneath major low-angle normal faults that separate them from low-grade rocks above. (5) Emplacement of solid bodies of Iherzolite at asthenospheric temperature into the base of the collisional edifice in late Oligocene time suggests detachment of the lithospheric root beneath the collision zone. This would have increased the surface elevation and the potential energy of the system and would have favored extensional collapse of the ridge.

Extensional structures in anisotropic rocks

A distinct class of structures can form as a result of extension along a plane of anisotropy (foliation). The effect of the foliation is to decrease the ductility of the material in this orientation so that brittle fractures or shear-bands develop. Foliation boudinage is caused by brittle failure; extensional fractures cause symmetric boudinage, and shear fractures cause asymmetric boudinage. Extensional crenulation cleavage is defined by sets of small-scale ductile shear-bands along the limbs of very open microfolds in the foliation. The sense of movement on the shear-bands is such as to cause a component of extension along the older foliation. Conjugate cleavage sets indicate coaxial shortening normal to the foliation; the shortening axis bisects the obtuse angle between the sets. A single set indicates oblique or non-coaxial deformation. Extensional crenulation cleavage is microstructurally and genetically distinct from other types of cleavage. It does not occur as an axial plane structure in folds, and has no fixed relationship to the finite strain axes. It is common in mylonite zones, and may be favoured by crystal-plastic and cataclastic deformational mechanisms. These cause grain-size reduction, and hence softening, which favour the development of shear-bands.

Late orogenic extension of the Betic Cordillera and the Alboran Domain: A lithospheric view

The Betic Cordillera of southern Spain provides a clear example of a collisional orogen that has undergone large-scale extensional collapse while convergent motion of the bounding plates continued. Extension was accommodated by coeval shortening in thin-skinned fold and thrust belts around the periphery of the system, and much of the region has now subsided to form a large marine basin. The thermal and deformational record of these processes is preserved in rocks from the upper mantle, crystalline crust, and sedimentary cover. Upper mantle peridotites record evidence for exhumation in several stages from asthenospheric depths to the surface. Early stages of exhumation probably occurred during Mesozoic rifting. Cooling at midlithospheric depths reflects continental convergence, and subsequent heating indicates loss of most of the underlying lithosphere and ascent of asthenosphere, whilst the final stages of exhumation in early Miocene time reflect extensional collapse. Crustal rocks in the internal zone of the Betic Cordillera were metamorphosed down to 50 km depth and are now exposed beneath major low-angle normal detachment zones that separate them from heavily faulted low-grade rocks above. Cooling ages of associated mylonites indicate that these detachments were active during the early to middle Miocene. Fault-bounded intramontane basins, developed during the early to middle Miocene, contain coarse continental sediments heavily affected by normal fault systems, followed by a less deformed late Miocene marine succession. All of these phenomena can be explained by convective removal of the lithospheric root beneath a Paleogene collisional orogen, leading to large-scale extension followed by thermal subsidence of the center of the system.

Thermal evolution, rate of exhumation, and tectonic significance of metamorphic rocks from the floor of the Alboran extensional basin, western Mediterranean

High-grade metamorphic rocks drilled at Ocean Drilling Program Site 976 in the Alboran Sea show a PT path characterized by decompression from about 1050 MPa (40 km depth) to 350 MPa (13 km depth) accompanied by an increase in temperature from about 550°±50°C to 675°±25°C. The Ar/Ar dating on muscovite and apatite fission track analysis indicate that the final stage of exhumation and cooling occurred very rapidly in the interval 20.5–18 Ma, which coincides with the initiation of sedimentation in the Alboran Sea basin. The Alboran Sea formed by Miocene extension on the site of a Late Cretaceous? to Paleogene contractional orogen, and extension coincided with thrusting in the peripheral parts of the Betic-Rif arc, which surrounds the basin on three sides. Thermal modeling of the PT path was carried out with the aim of constraining geodynamic models for the formation of the basin. Variables considered in the modeling included (1) the thickness and thermal gradient of the postorogenic lithosphere; (2) the radiogenic heat production in the thickened crust; (3) the time gap (pause) between the end of contractional tectonics and the start of extension; (4) removal of lithospheric mantle below 125, 75, or 62.5 km; and (5) the rate of extension. The only combinations of variables that produce modeled PT paths with the observed characteristics involve high radiogenic heat production combined with a significant postcontractional pause (to produce high temperatures in rocks initially at 40 km depth), removal of lithosphere below 62.5 km (to produce further heating during decompression), extension by a factor of 3 in 6 m.y. (to delay the attainment of the maximum temperature until the rocks reached shallow depths), and final exhumation and cooling in about 3.3 m.y. (to satisfy radiometric and petrological constraints). This gives a maximum of about 9 m.y. for exhumation from 40 km depth to the surface. Lithospheric stretching in response to plate-boundary forces such as trench rollback, without removal of lithosphere, cannot explain the late onset of heating and the high temperatures reached by these rocks. Removal of lithosphere at depths significantly greater than 62.5 km cannot explain the combination of high temperatures reached by these rocks and the shallow depth at which they attained the maximum temperature. Only a combination of significant postcollisional radiogenic heating, then wholesale removal of lithospheric mantle below the orogenic crust, followed by rapid stretching can explain the observed PT path. These results appear to support some form of lithospheric delamination as the primary cause for the formation of the Alboran Sea basin.

Secondary cleavages in ductile shear zones

Abstract Secondary cleavages developed at late stages in ductile shear zones show several features that are inconsistent with progressive simple shear in the zone. These are: the orientation of a single secondary cleavage oblique to the shear zone boundaries; conjugate sets with opposite senses of shear, and multiple sets with the same sense of shear. These features can be explained if the bulk flow is partitioned into slip along discrete failure planes parallel to the primary foliation (S), coaxial stretching along the foliation, and spin.

Simultaneous extensional exhumation across the Alboran Basin: Implications for the causes of late orogenic extension

Large-scale crustal thinning of the Alpine orogen in the westernmost Mediterranean (Alboran Sea and surrounding regions) was rapid, simultaneous over an area of 60,000 km(2), and took place similar to25 m.y. after the main crustal thickening event. Crustal thinning, which locally exposed subcontinental mantle, was accompanied by high-grade metamorphism and partial melting. Both U-Pb dating of zircon and thermal modeling of cooling histories indicate that the thermal peak was reached between 23 and 21 Ma over the entire region. Final exhumation and cooling followed immediately. Possible explanations for this dramatic late orogenic extension include subduction rollback, slab detachment, lithospheric delamination, and convective removal of subcontinental lithosphere. Of these processes, only convective removal of subcontinental lithosphere predicts a significant time lapse between crustal thickening and the onset of extension, the simultaneity of extension over the whole region, and heating of rock during rapid exhumation to produce the high-grade metamorphic event.

Structures and fabrics in a crustal-scale shear zone, Betic Cordillera, SE Spain

Abstract A broad zone of dominantly ductile high-strain deformation lies beneath the Aguilon nappe in the Sierra Alhamilla, southern Spain. It forms part of a crustal-scale movement zone, traceable through much of the Betic Cordillera, which separates the Higher Betic Nappes from the underlying Nevado-Filabride Complex. The zone is characterized in outcrop by a distinctive platy foliation and a strong NNE-trending stretching lineation. Microstructural characteristics include quartz ribbons, mica fish, augen of feldspar and other minerals in a matrix of dynamically recrystallized quartz, and extensional crenulation cleavages. Narrow bands of ultramylonite and cataclasite occur within and on the margins of the movement zone. Deformation occurred under lower greenschist-facies conditions and was accompanied by retrogression of earlier higher-grade mineral assemblages. Structures in the movement zone developed in a temporal sequence, beginning with isoclinal folding and transposition of older foliations. This was followed by the formation of extensional crenulation cleavages, and the progressive localization of strain into the ultramylonite bands. Mylonitic foliation in these bands is deformed by syn-mylonite folds restricted to the bands. All these structures were then deformed by S- to SE-vergent small-scale folds restricted to the movement zone as a whole. Cataclasis, associated with alteration, is localized along the ultramylonite bands and indicates a transition to late-stage brittle deformation. The lower boundary of the movement zone is gradational: strain decreases, recrystallized grainsize and the degree of recrystallization of quartz increases, and pressure solution becomes the dominant deformation mechanism in mica-schist. Asymmetric quartz fabrics in the movement zone indicate a NNE sense of shear; but variations in the degree of asymmetry suggest that flow was partitioned, with the ultramylonite bands taking up much of the shear strain, and the intervening rocks deforming more slowly and with a lower degree of non-coaxiality. Diffuse fabrics in the fine-grained ultramylonite bands may indicate a switch to a grainsize-sensitive deformation mechanism, and an overall downward increase in the opening angle of crossed-girdle fabrics may reflect increased water activity at depth.

Large-scale sediment underplating in the Makran accretionary prism, southwest Pakistan

Field relations in the emergent part of the Makran accretionary prism show that a mid-Miocene to early Pliocene slope and shelf sedimentary sequence was deposited directly on abyssal-plain turbidites without any detectable stratigraphic or structural discordance. Sedimentological evidence for rapid shoaling, however, indicates that the underlying sediment column was tectonically thickened by a factor of between two and three during this period. This can be explained by large-scale underthrusting and underplating of sediment—a mode of accretion that is also favored by mass-balance considerations. The visible deformation in the coastal Makran occurred when the region was 70–100 km north of the contemporary prism front.

Metamorphic and deformational processes in the Franciscan Complex, California: Some insights from the Catalina Schist terrane

On Santa Catalina Island, blueschist is structurally overlain by glaucophanic greenschist, which is overlain in turn by a unit of amphibolite and ultramafic rock. These three units are juxtaposed along sub-horizontal postmetamorphic thrusts; tectonic blocks of amphibolite are distributed along the thrust between the greenschist and the blueschist. Physical conditions of metamorphism are estimated to be approximately 300°C and 9 kb for blueschist, 450°C and 8 kb for greenschist, and 600°C and 10 kb for amphibolite. I suggest that metamorphism occurred in a newly started subduction zone, where an inverted thermal gradient developed below the hot hanging-wall peridotite. Postmetamorphic eastward underthrusting along surfaces of varying dip can explain the present structural relationships. Tectonic blocks of glaucophane-epidote schist, amphibolite, and eclogite elsewhere in the Franciscan Complex may be disrupted remnants of similar metamorphic zones. The inverted thermal gradient will only exist in the early stages of subduction, which explains why the blocks are the oldest rocks in the Franciscan Complex. The gross decrease in age and metamorphic grade westward across the Franciscan results from successive underthrusting and accretion of progressively younger slices of supercrustal material, concurrent with uplift and erosion. Pressure-temperature (P-T) conditions of metamorphism in each east-dipping tectonic slice will increase down-dip. At any given time, older, more easterly slices will have been uplifted further, hence metamorphic grade in the exposed edges will increase eastward and structurally upward. If erosion is faster than accretion for a time, younger slices will be metamorphosed at lower pressures than were the older higher ones. Simple reverse faulting can then produce the observed interleaving of rocks of different metamorphic grade.