Timing Of Deformational Events Research Articles

Clues for a Tortonian reconstruction of the Gibraltar Arc: Structural pattern, deformation diachronism and block rotations

We proposed a reconstruction of one of the tightest orogenic arcs on Earth: the Gibraltar Arc System. This reconstruction, which includes onshore and offshore data, is completed for approximately 9Ma. The clues that lead us to draw it are based on a review in terms of structures and age of the superposed deformational events that took place during Miocene, with special attention to the external zones. This review and new structural data presented in this paper permit us to constrain the timing of vertical axis-rotations evidenced by previously published paleomagnetic data, and to identify homogeneous domains in terms of relationships between timing of deformation events, (re)magnetization and rotations. In particular, remagnetization in the Betics took place after the main shortening which produced the external fold-and-thrust belts (pre-upper Miocene), but was mostly previous to a contractive reorganization that affected the whole area; it should have occurred during lower Tortonian (between 9.9 and 11Ma). From Tortonian to Present, block-rotations as high as 53° took place. Together with plate convergence, they accommodated a tightening and lengthening of the Gibraltar Arc System and drastically altered its geometry. As the orientation and position of any pre-9Ma kinematic indicator or structural element is also modified, our reconstruction should be used as starting point for any pre-Tortonian model of the westernmost orogenic segment of the Alpine-Mediterranean system.

U–Pb Zircon (SHRIMP) ages of granite sheets and timing of deformational events in the Natal Metamorphic Belt, southeastern Africa: Evidence for deformation partitioning and implications for Rodinia reconstructions

This study provides constraints on the ages of deformation events and fabric development in deformed rocks of the Margate Terrane of the Natal Metamorphic Province. The Margate Terrane forms the southernmost of three terranes considered to represent multiple arc accretion onto the southern margin of the Kalahari Craton, and geochronological data indicate that the Margate Terrane has a long history of sporadic magmatism from ∼1180 to ∼1025Ma. Two granite sheets of differing structural age, as revealed by deformational fabric and cross-cutting relationships, were sampled for U–Pb (SHRIMP) dating from coastal outcrop at Southbroom (30°54′43.61″S, 30°20′1.61″E). The older sheet contains fabrics related to both D1 and D2 events, whereas the younger shows evidence for syn-D2 emplacement and contains an S2 fabric. For both intrusive sheets, zircon core domains showing magmatic zoning yielded ages that are statistically identical at 1075±6Ma. This is interpreted to represent the intrusion age of the older sheet, and to sampling of a xenocrystic population in the cross-cutting sheet due to assimilation. Zircons from both sheets show metamorphic rim zones with a mean age of 1042±10Ma, which is attributed to rim growth during development of the S2 foliation, close to the intrusion age of the younger sheet. The 1075±6Ma age is comparable to the ages of other granitic units in the Margate Terrane that intruded between ∼1091 and 1070Ma and implies that all of these predated the D1 deformation, which is considered to record the accretion of the terrane onto the Kalahari Craton. The D2 event is characterized by northward-verging folds with a pervasive southward-dipping axial planar fabric S2, which largely overprinted the S1 fabric. Previously, the pure-shear D2 deformation was considered to be older than the narrow sinistral shear zones occurring within the Mzumbe and Margate Terranes. However, dating of the D2 event at 1042±10Ma indicates that these deformations are coeval, and represent deformation partitioning during a transpressional event at ∼1050–1025Ma that may have been related to collision with Laurentia during the amalgamation of Rodinia.

Timing of deformational events in the Río San Juan complex: Implications for the tectonic controls on the exhumation of high-P rocks in the northern Caribbean subduction–accretionary prism

An integrated structural, petrological and geochronological study was undertaken to constrain the tectonic history and controls on the exhumation of the high-P rocks of the Río San Juan complex in the northern Caribbean subduction–accretionary wedge. In the main structural units of the complex, microtextural analyses were performed to identify the fabrics formed at peak of metamorphism in eclogite-facies conditions and during the main retrogressive event toward the low-P amphibolite or blueschist/greenschist-facies conditions.U–Pb SHRIMP dating on zircon rims (71.3±0.7Ma) coupled with 40Ar–39Ar analyses on phengite (~70–69Ma) in felsic sills placed temporal constraints on the exhumation of the Jagua Clara serpentinite-matrix mélange during the blueschist-facies stage at the early Maastrichtian. In the Cuaba unit, U–Pb TIMS zircon ages of 89.7±0.1Ma and 90.1±0.2Ma obtained for the crystallization of tonalitic/trondhjemitic melts in the lower Guaconejo and upper Jobito subunits, respectively, are similar. These ages coupled with a U–Pb SHRIMP zircon age of 87±1.8Ma obtained in a garnet amphibolite and a group of older 40Ar–39Ar cooling ages on calcic amphibole constrain the exhumation of the Guaconejo subunit from the high-P stage to the low-P stage at the ~90–83Ma time interval. Further, the age data collectively supports a genetic relationship between the distributed extensional ductile shearing, the related decompression and the local partial anatexis in the subunit, at least from the Turonian–Coniacian boundary to the early Campanian. A group of younger 40Ar–39Ar ages obtained in the mylonitized amphibolites of the basal Jobito detachment zone indicates late ductile deformation and exhumation/cooling in the late Campanian to Maastrichtian (~75–70Ma). Therefore, structural and age data established deformation partitioning and reworking of retrograde fabrics during ~20Ma in the Cuaba unit.The different exhumation rates obtained for the Jagua Clara mélange can be explained by uplift in two contrasting tectonic settings: a first stage of slow exhumation (1.4mm/yr) in the subduction channel, largely lower than plate velocities, and a second stage of relatively fast exhumation (7.6mm/yr) up to the surface. Therefore, the exhumation was temporally discontinuous and the velocity increase at ~70Ma probably was triggered in response to the entrance of buoyant material in the subduction zone, such as a Caribeana continental ribbon or the distal part of Yucatán–Bahamas continental margin. In contrast, the exhumation path of the Guaconejo subunit is composed of a first segment from the baric peak to the low-P amphibolite stage (at 84–83Ma) with an exhumation rate of 7.2mm/yr, a second segment around the closure temperature of calcic-amphibole (at 82–70Ma) with a rate of 0.4mm/yr, and a third segment to the surface exposure (at 60Ma) with a rate of 1.8mm/yr. These velocity differences can be correlated with the P–T path proposed for the exhumation of the subunit, with an initial isothermal decompression from the peak in the high-P amphibolite to the eclogite-facies produced by distributed extensional shearing, followed by a relatively slow cooling at low-P in the newly acquired structural position, and the final tectonics mainly partitioned in the late Jobito detachment zone. The initial fast exhumation can be related to a major modification in the convergence conditions across the intra-oceanic subduction zone as a jump of the basal subduction thrust beyond a previously accreted arc terrane.

Investigating gravitational grabens related to lateral spreading and evaporite dissolution subsidence by means of detailed mapping, trenching, and electrical resistivity tomography (Spanish Pyrenees)

The active lateral spread of the Peracalc Range (Spanish Pyrenees) has developed on a Cretaceous limestone sequence around 250 m thick, underlain by tectonically thickened (∼2.5 km) Triassic halite-bearing evaporites and clays. Outward expansion of the Triassic sequence by ductile deformation and probably halokinesis toward the debuttressed and unloaded front of the range has been accommodated in the overlying cap rock through the development of a striking horst and graben morphostructure. Fault scarps show anomalously high height to length ratios (aspect ratio; H max / L ) compared to the values reported for tectonic faults. This retrogressive gravitational deformation has aborted a paleodrainage, expressed as wind gaps, hanging valleys, and defeated streams. The significant vertical displacement component in this rock spread is attributed to subsidence caused by interstratal evaporite dissolution, as supported by the dissolution-induced collapse and graben structures mapped at the foot of the range. To our knowledge, the rock spread of Peracalc, covering around 4.5 km 2 and with a minimum volume of 0.9 km 3 , is the largest documented landslide of the Pyrenees. The excavation of trenches and the acquisition of electrical resistivity tomography profiles provided information on the thickness and subsurface structure of the graben fills, the age of the lateral spread (older than 45 ka), an unexpected episodic kinematic behavior of the gravitational faults, and the timing of deformation events, including slumping of lake deposits.

Deformation history of the southwestern Taiwan foreland thrust belt: insights from tectono-sedimentary analyses and balanced cross-sections

Analysis of the basin-scale and local deformation history and estimates of shortening rates are performed in the southwestern foreland thrust belt of the Plio-Pleistocene Taiwan orogen. Timing of deformational events is constrained using both syntectonic unconformities and other markers of syndepositional tectonic activity. Three cross-sections are investigated and balanced. Intermediate shortening rates are estimated based on balanced and restored cross-sections. The total amount of shortening along studied balanced sections results from superimposed thin-skinned and thick-skinned styles. A shallow aseismic thick décollement is located to depth of 6–8km at the base of thick Late Miocene shales that behaves as ductile layer. The occurrence of a deep seismic décollement to depth of 12–15km is confirmed by seismicity, fault plane solutions of focal mechanisms and a seismic reflection profile providing evidence of reactivated and inverted inherited extensional features beneath the Coastal Plain. Three major tectonic stages are recognized within the southwestern Foothills, which are correlated with local thrusting events. The first stage begins at 5Ma with the onset of the Taiwan collision and ends at 2–1.6Ma. It reflects an initial submarine accretionary stage associated with moderate tectonic activity. The second major stage occurs during Late Pliocene–Early Pleistocene. This “intermediate” collision stage sealed the evolution of the southern foreland thrust wedge from submarine to continental environment in relation with exhumation and denudation of the Central Range. The last stage of deformation begins in the Middle Pleistocene and continues until present. It is associated with high rates of sedimentation and most of the thrusts are activated or reactivated at that time and out-of-sequence thrusting occurs. The collision started synchronously near 5Ma throughout the southwestern Taiwan. Total and intermediate shortening rates are more or less constant through time in studied sections; that may suggest that long-term deformation in the Taiwan foreland thrust belt occurred more or less at steady-state. Since 5Ma, the southward decrease of rates of shortening from 3.6 to 4.8mm/year in northern sections to 2.2mm/year in southern section might be related to both influence of indentation by the Peikang High to the north and southward transition, at the scale of the southwestern Foothills, from collision to subduction setting; the effect of the obliquity of the convergence resulting in regional southward migration of the collision cannot be definitely inferred from this analysis. Our results place new kinematic and time constraints, which must be taken into account in forthcoming geodynamic models of the Taiwan collision.

Structural and kinematic evolution of a Miocene to Recent sinistral restraining bend: the Montejunto massif, Portugal

The Montejunto massif lies in the apex of a large-scale restraining bend at the southern termination of a sinistral transpressive fault system, in the Lusitanian basin of Portugal. Cenozoic deformation within the Montejunto massif initiated with southerly directed thrusting along the southern boundary of the massif, in association with the development of the E–W oriented Montejunto anticline, probably during the Langhian. Deformation switched to the northern boundary of the massif, in association with a change to NW-directed thrusting and continued development of the Montejunto anticline. The youngest set of structures within the massif is related to the sinistral reactivation of the Arieiro fault system, and steeply inclined bedding. This late phase of deformation represents the accommodation of a component of sinistral displacement across the restraining bend along mechanical anisotropies formed during this progressive Cenozoic deformation event. Variation in the kinematic style of the Main Arieiro fault is related to the angle ( α) between the fault plane and the displacement vector. Where α≈20°, abrupt pene-contemporaneous switches in displacement direction are recorded along the fault, whereas strike-slip kinematics predominate where α<20°. The timing of deformation events in the Montejunto massif is uncertain. However, correlation with the established Cenozoic Africa/Europe plate convergence directions may provide potential temporal constraints.

Zircon ages constrain the timing of deformation events in the the Granites‐Tanami region, northwest Australia*

Dating of extensive granitoids in the The Granites‐Tanami Block of northern Australia has enabled time limitations on deformational events, including five Palaeoproterozoic events (D1‐D5) and one older, almost Neoarchaean event. D1 is present in the basal Tanami Group, D2 to D5 help define other stratigraphic groups in the region. The intrusive rocks have been dated by traditional zircon and stepwise lead evaporation techniques. A post‐D2/pre‐D3 foliated and boudinaged leucocratic tonalite gave 1870 ± 3 Ma. A syn‐D3 foliated and boudinaged tonalite 1858 ± 3 Ma and a post‐D3/pre‐D4 foliated tonalite 1840 ± 11 Ma. Two unmetamorphosed and post‐D5 granitoids gave 1797 ± 1 Ma and 1787 ± 3 Ma, respectively. Some deformation events are separated by as little as 10 to 20 million years. These times of intrusive igneous activity exactly replicate other igneous events, many of which are volcanic, in the Halls Creek Province 400 km to the northwest, which strengthens proposed geological relationships between the two regions. A granitic gneiss in a basement inlier gives 2487 ± 3 Ma, a Neoarchaean age. Tectonic uplift in the Carboniferous is deduced from concordia lower intercept data that is similar to the eastern Arunta Block. Further age definition of D1 D2 and D5 is under current investigation.

U-Pb geochronology of potassic syenites from Southwestern Nigeria and the timing of deformational events during the Pan-African Orogeny

Two syenite bodies from south-western Nigeria considered to be pre to syntectonic (Iwo) and late tectonic (Okeiho) with respect to the dominant N-S trending structures that characterize the Nigerian Percambrian basement were dated by the U-Pb zircon method. However, they yield Pan-African emplacement ages of 610 ± 7 Ma and 618 ± 4 Ma respectively which are indistinguishable within limits of the experimental errors. This suggests that the Pan-African deformation in the region was either migratory or heterogeneous in nature. Comparison of these results with data from other parts of the Pan-African domain east of the West African craton provides further evidence of the synchronous nature of the peak of Pan-African magmatism (630-600) in the area.