Tectonic Shortening Research Articles

Structures and mechanisms associated with development of a fold in the Cantabrian Zone thrust belt, NW Spain

A detailed history of progressive fold development based on mesostructural analysis is provided for a metre-scale fold in the Cantabrian fold and thrust belt of northwestern Spain. The well-exposed asymmetric syncline displays a wide variety of structures including extensional faults, thrusts, veins, tectonic stylolites, bedding plane faults, drag folds, and a main limb thrust that transects the entire fold. Important constraints on fold development are provided by the spatial distribution and style of deformation, angular relations, and cross-cutting relations among the structures. Fold mechanisms varied as a function of structural position; while the northwest limb was subjected to pure shear shortening in the form of contractional faults, the southeast limb was bent under tangential longitudinal strain, resulting in a combination of outer-arc extensional faulting in dolostones and simple shear along interbedded shales. Sequential development of the Bárzana fold consisted of; (i) initial flexural slip folding until the southeast limb attained a dip of 60°; (ii) a phase of pressure solution upon locking of bedding plane faults; (iii) shortening accommodated by bed-confined mesofaulting and the propagation of a limb thrust; (iv) unlocking of bedding plane faults and resumption of flexural slip; and (v) propagation of a new limb thrust due to offset of the main thrust by flexural slip. This study demonstrates that a number of very different mechanisms can operate during the course of fold development, and that folding processes and geometries are strongly dependent upon structural position, mechanical stratigraphy, and magnitude of tectonic shortening.

Opposing gradients of permanent strain in the aseismic zone and elastic strain across the seismogenic zone of the Kodiak shelf and slope, Alaska

An industry‐acquired seismic reflection line from the Aleutian Arc to the Alaska Trench was prestack depth‐migrated and further constrained with wide‐angle ocean bottom hydrophone seismic velocity data and swath mapping. From the seismic reflection image, individual thrust slices, the backstop, and the forearc basin were resolved sufficiently well to balance the section. Tectonic shortening and age estimates provided rates of permanent contraction in five discrete segments over ∼150 km during the past 3 Myr. About 70% of the plate convergence is measured as contraction across the ∼40‐km‐wide Neogene accretionary prism which composes less than 10% of the forearc. Permanent contraction is minimal above the 1964 Alaska earthquake aftershock zone indicating dominently elastic strain. Since elastic strain is poorly recorded in the geological record, it was approximated from the historical record of the 1964 Alaska earthquake. The 1964 coseismic vertical deformation displays an inverse pattern to the permanent strain, consistent with the earthquake cycle concept. Elastic deformation is dominant beneath the shelf and upper slope, rapid permanent deformation is concentrated beneath the lower slope, and a transition between them occurs beneath the middle slope.

Structural framework of the Bashkirian anticlinorium, SW Urals

The Bashkirian anticlinorium of the southwestern Urals shows a much more complex structural architecture and tectonic evolution than previously known. Pre-Uralian Proterozoic extensional and compressional structures controlled significantly the Uralian tectonic convergence. A long-lasting Proterozoic rift process created extensional basement structures and a Riphean basin topography which influenced the formation of the western fold-and-thrust-belt with inversion structures during the Uralian deformation. A complete orogenic cycle during Cadomian times, including terrane accretion at the eastern margin of the East European platform, resulted in a high-level Cadomian basement complex, which controlled the onset of Uralian deformation, and resulted in intense imbrication and tectonic stacking in the subjacent footwall of the Main Uralian fault. The Uralian orogenic evolution can be subdivided into three deformation stages with differently oriented stress regimes. Tectonic convergence started in the Late Devonian with ophiolite obduction, tectonic accretion of basin and slope units and early flysch deposits (Zilair flysch). The accretionary complex prograded from the SE to the NW. Continuous NW/SE-directed convergence resulted finally in the formation of an early orogenic wedge thrusting the Cadomian basement complex onto the East European platform. The main tectonic shortening was connected with these two stages and, although not well constrained, appears to be of Late Devonian to Carboniferous age. In the Permian a final stage of E–W compression is observed throughout the SW Urals. In the west the fold-and-thrust-belt prograded to the west with reactivation of former extensional structures and minor shortening. In the east this phase was related to intense back thrusting. The East European platform was subducted beneath the Magnitogorsk magmatic arc during the Late Paleozoic collision. The thick and cold East European platform reacted as a stable rigid block which resulted in a narrow zone of intense crustal shortening, tectonic stacking and high strain at its eastern margin. Whereas the first orogenic wedge is of thick-skinned type with the involvement of crystalline basement, even the later west-directed wedge is not typically thin-skinned as the depth of the basal detachment appears below 15 km and the involvement of Archean basement can be assumed.

Crustal thickening processes in the Central Andes and the different natures of the Moho-discontinuity

New seismic data from the Central Andes allow us to clarify the crustal structure of this mountain chain and to address the problem of crustal thickening. Evidence for the deep crustal root can be observed in both gravimetric and seismological data. Crustal structure and composition change significantly from east to west. In the eastern part of the backarc the Moho discontinuity is clearly recognisable. However only poor Moho arrivals are observed by active seismic measurements beneath the Altiplano and the Western Cordillera where broad-band seismology data indicate such a discontinuity. In the Precordillera, a pronounced discontinuity is detected at a depth of 70 km. Along the coast, the oceanic Moho is developed at a depth of 40 km. There are several processes which can change the petrological and petrophysical properties of the rocks forming the crust. Variations of the classical Moho discontinuity are presented which do not correspond to the petrological crust/mantle boundary. Tectonic shortening in the backarc is the dominant process contributing to at least 50–55% to the root formation along 21°S. In the forearc and arc, hydration of the mantle wedge produced ≈15–20% of crustal thickening. Magmatic thickening and tectonic erosion contributed only ≈5%. The other ≈25% is not yet explained.

Incorporation of the Paleogene foreland into the Neogene Puna plateau: The Salar de Antofalla area, NW Argentina

This paper presents a detailed tectonosedimentary and volcanic description of the Salar de Antofalla area, along with an evolutionary model of the Southern Puna during the Cenozoic. Cenozoic sedimentation started during the Late Eocene at the climax of the Incaic phase in the Chilean Precordillera. Both playa mud and sandflat as well as fluvial sediments (Quiñoas Formation) of this time are interpreted as deposits of an uniform sedimentation area which was part of a wide retroarc foreland basin east of the Precordillera. Tectonic activity started during the Late Oligocene and is documented by thrust and reverse faulting. Due to thick-skinned deformation, the former coherent part of the foreland basin was subdivided into broken foreland basins filled with alluvial and eolian sediments (Chacras Formation). Their buildup could have been accompanied by the initial uplift and crustal thickening of the Southern Puna. As a consequence of continuing contractional tectonism during Early Miocene, the Salar de Antofalla area was further subdivided into a number of small intra-arc depocenters in which alluvial fan and fluvial sediments (Potrero Grande Formation) were deposited. Several intermediate stratovolcanic centers began to erupt between 18 Ma and 14 Ma. Geochemically, these magmas display an arc signature. They were generated by interaction of basaltic magmas with Andean continental crust thickened to at least 40 km. Reverse faulting and thrusting during the Middle to Late Miocene led to an intra-arc basin filled with thick alluvial fan and evaporite sediments (Juncalito Formation). Since the Late Miocene/Early Pliocene, mainly basaltic andesitic magmas erupted at monogenetic centers located east of the volcanic arc. During the Pliocene, tectonic shortening continued with local strike-slip components. These relief-forming processes reduced the Salar de Antofalla basin to its present narrow and elongated shape and triggered the final alluvial fan sedimentation (Escondida Formation).

Laser probe argon‐40/argon‐39 dating of pseudotachylyte from the Sudbury Structure: Evidence for postimpact thermal overprinting in the North Range

Abstract— Ten pseudotachylyte samples from the North Range of the 1850 Ma Sudbury impact structure have been analyzed by the 40Ar/39Ar laser spot fusion method. Field and petrological evidence indicate that the pseudotachylytes were formed at 1850 Ma by comminution and frictional melting due to impact‐induced faulting. The cryptocrystalline to microcrystalline grain size (<30 μm) of the pseudotachylyte matrices and the predominance of orthoclase as the main K‐bearing phase, have rendered the rocks particularly susceptible to Ar loss. The age determinations range from ∼1850 to ∼1000 Ma, with some samples yielding multiple ages that cannot be correlated with known geological events in the area. However, if the finite‐difference algorithm of Wheeler (1996) is used to calculate combined Ar loss and the accumulation of radiogenic Ar for the K‐bearing phases, it is possible to reproduce the range of observed ages. The model infers that the long‐term volume diffusion of Ar has occurred and that, as a result, the Ar system cannot be treated with a conventional closure temperature approach. The algorithm requires burial of the impact structure to 5–6 km depth and 160–180 °C at 1850 Ma, followed by exhumation at ∼1000 Ma. These ages may be equated with two events: Penokean thin‐skinned overthrusting in the North Range, immediately following impact, and exhumation ∼850 Ma later, coincident with the Grenville orogeny to the southeast. The results suggest that, contrary to previously accepted paradigms, the North Range has been affected by a protracted period of postimpact, low‐grade thermal metamorphism. If these events also involved tectonic shortening within the North Range (as has been documented for the South Range), then the original size of the Sudbury impact structure has been underestimated.

Tectonic shortening and crustal thickness in the Central Andes: How good is the correlation?

Exceptional thickening of continental crust beneath the Central Andes is believed to be mainly a result of tectonic shortening of the South American plate in Neogene time. This shortening has been estimated to have contributed as much as 70%–80% of the present crustal volume. A compilation of published shortening values and our own estimates based on balanced cross sections from the Central Andes between lat 3°S and 40°S suggest that 70%–80% is a maximum rather than an average value for this part of the Andes. Tectonic shortening and the crustal cross-section area are only loosely correlated. Variations in tectonic shortening are more abrupt than those of crustal areas, particularly near the northern and southern ends of the Altiplano-Puna high plateau, where thick crust is associated with relatively small amounts of shortening. Shortening there may account for no more than about 30% of the present crustal cross-section area. The processes that created the remaining crustal area are not clear, but are likely to involve poorly constrained pre-Neogene tectonic shortening, moderate magmatic additions to the crust, tectonic underplating of material derived from the forearc, and possible flow of ductile lower crust along strike.

Latest Cretaceous-Tertiary Tectonic Evolution of Northern Yukon and Adjacent Arctic Alaska

Regional crustal shortening in northern Yukon and northeastern Alaska occurred episodically from the latest Cretaceous to the late Miocene, with a major culmination occurring in the Paleocene to middle Eocene, and a secondary culmination in the late Miocene. Structural trends are predominantly north to northeastward in northern Yukon and adjacent east-central Alaska, and generally east-west along the Brooks Range. The early Tertiary trend is strongly arcuate in the Beaufort fold belt and adjacent onshore areas. The continuity of structures southward from the Beaufort Sea region through northern Yukon and east-central Alaska supports the interpretation that the structures north of 65°N form a single orogenic entity. The Beaufort Sea region is 1000 km from the nearest plate margin. Northward shortening across Arctic Alaska and the northern Canadian cordillera reflects the convergent component of Kula (later, Pacific)-North America interactions throughout the Tertiary. North- to northeast-trending structures of latest Cretaceous to early Tertiary age in northern Yukon and east-central Alaska accommodate shortening of 180-240 km, and reflect Eurasia-North America convergence. The strongly arcuate offshore Beaufort fold belt and similar structures in adjacent northernmost Yukon and northeast Alaska were formed by the complex interplay of three factors: shortening of northern Yukon between Arctic Alaska and the craton to produce a north-trending orogenic welt; northward displacements propagated from the Kula plate margin; and local boundary conditions imposed by lithology and crustal structure, which aided lateral escape of the deforming supracrustal succession northward into the Beaufort Sea. In central Alaska, any kinematic linkage between the Kaltag fault in the west and the Tintina fault of the northern cordillera is more complex than was previously assumed. A new regional tectonic reconstruction of northern Yukon-Alaska quantifies the tectonic shortening in central Alaska south of the Kaltag and Tintina faults in an area where tectonic shortening is difficult to quantify due to complex geology. Complex deformation accommodated by folding, thrust and strike-slip faulting, and/or tectonic rotations accounts for an estimated 460 km of crustal shortening, approximately equivalent in magnitude to the total Tintina fault displacement of at least 450 km. The foreland of the Brooks Range, the Beaufort fold belt, and the northern cordillera contain proven petroleum basins. This regional synthesis validates a model of orogenic shortening for the Beaufort fold belt and provides a unifying tectonic setting for oil and gas plays throughout the region. The latest Cretaceous-Tertiary structural evolution of the Brooks Range, Beaufort, and northern Yukon fold belts is a case study of the temporally and kinematically complex far-field deformation arising from the convergence of four major tectonic plates.

Accretion and exhumation at a Variscan active margin, recorded in the Saxothuringian flysch

The Saxothuringian flysch basin, on the north flank of the Central European Variscides, was fed and eventually overthrust by the northwestern, active margin of the Tepla-Barrandian terrane. Clast spectra, mineral composition and isotopic ages of detrital mica and zircon have been analyzed in order to constrain accretion and exhumation of rocks in the orogenic wedge. The earliest clastic sediments preserved are of early Famennian age (ca. 370 Ma). They are exposed immediately to the NW of the suture, and belong to the par-autochthon of the foreland. Besides ultramafic (?ophiolite) material, these rocks contain clasts derived from Early Paleozoic continental slope sediments, originally deposited at the NW margin of the Saxothuringian basin. These findings, together with the paleogeographic position of the Famennian clastics debris on the northwestern passive margin, indicate that the Saxothuringian narrow ocean had been closed by that time. Microprobe analyses of detrital hornblendes suggest derivation from the “Randamphibolit” unit, now present in the middle part of the Saxothuringian allochthon (Munchberg nappes). Detrital zircons of metamorphic rocks formed a little earlier (ca. 380 Ma) indicate rapid recycling at the tectonic front. The middle part of the flysch sequence (ca. early to middle Visean), both in the par-autochthon and in the allochthon, contains abundant clasts of Paleozoic rocks derived from the northwestern slope and rise, together with debris of Cadomian basement, 500-Ma granitoids and 380 Ma (early Variscan) crystalline rocks. All of these source rocks were still available in the youngest part of the flysch (c. middle to late Visean), but some clasts record, in addition, accretion of the northwestern shelf. Our findings permit deduction of minimum rates of tectonic shortening well in excess of 10–30 mm per year, and rates of exhumation of ca. 3 mm/a, and possibly more.

Stratigraphic contrasts and tectonic relationships between Carboniferous successions in the Trans‐Alaska Crustal Transect corridor and adjacent areas, northern Alaska

The Carboniferous succession along the Trans‐Alaska Crustal Transect (TACT) corridor in the Atigun Gorge area of the central Brooks Range consists of the Kayak Shale (Kinderhookian) and the Lisburne Group (Kinderhookian through Chesterian). The Kayak Shale is at least 210 m thick; it is chiefly black, noncalcareous shale with several limestone beds of pelmatozoan‐bryozoan packstone and formed in an open‐marine setting. The Lisburne Group is a carbonate rock succession about 650 m thick and consists mainly of skeletal packstone, wackestone, and milestone which contain locally abundant calcispheres, ostracodes, algae, and sponge spicules; it accumulated largely in a shallow water platform environment with restricted circulation. This restriction was probably produced by a coeval belt of skeletal sand shoals recognized 70 km to the west in the Shainin Lake area. Significant and apparently abrupt shifts in the age and lithofacies of Carboniferous strata occur across the central and eastern Brooks Range. These shifts are most marked in a zone roughly coincident with what is interpreted by many workers to be the leading edge of the Endicott Mountains allochthon. Notable lithologie contrasts are also observed, however, between sections in the northern and southern parts of the Endicott Mountains allochthon. This suggests that considerable tectonic shortening has taken place within the allochthon, as well as between it and parautochthonous rocks to the northeast. The Carboniferous section near Mount Doonerak is more similar in age and lithofacies to coeval sections in the central Brooks Range that are considered allochthonous than to parautochthonous sections to the northeast.

The Caledonian deformation of the Brabant Massif and the Early Palaeozoic in northeast Germany: compared

The Caledonian deformation of the Brabant Massif and the Rügen Early Palaeozoic has been compared by structural analysis including strain determinations, X-ray texture analysis, illite crystallinity and vitrinite reflectivity. Both areas have experienced weak internal deformation under anchimetamorphic to very low-grade metamorphic conditions. The deformation of the Brabant Massif is characterized by (a) a pronounced diagenetic foliation which resulted from compaction and weak extensional tectonics; (b) tilting or weak folding of the Cambrian strata around the Cambrian/Ordovician boundary without internal deformation; (c) a main phase of Caledonian deformation in the Lower Devonian which was associated with open folding and an axial-plane pressure solution cleavage; (d) bending of Caledonian structures from northwest–southeast to northeast–southwest. The internal deformation appears homogeneous with decreasing strain values towards upper structural levels. The strain ellipsoid is predominantly prolate with the main axis of extension parallel to the bedding–cleavage intersection lineation. It is interpreted as the result of superposition of compaction, tectonic shortening and volume loss through pressure solution. The Rügen Early Palaeozoic is less deformed and less metamorphosed under anchizonal conditions. While the upper structural level is almost undeformed, towards the deeper structural levels a spaced and then a continuous slaty cleavage is developed which is successively crenulated by a fracture cleavage. The internal deformation appears inhomogeneous due to several thrust zones with somewhat higher strain values. Outside the thrust zones strain values increase towards lower structural levels, but not significantly. The strain ellipsoid is prolate with a strongly varying orientation of the X-axis. The structural inventory of both areas is comparable with other fold-and-thrust belts. While the Rügen Palaeozoic represents an imbricated thrust-fan of an upper structural level with discrete thrusts and inhomogeneous strain, the Brabant Massif represents a deeper structural level dominated by folding and more homogeneous internal deformation.

The geometry of the central Andean backarc crust: Joint interpretation of cross-section balancing and seismic refraction data

Detailed cross section balancing and the interpretation of seismic refraction data in the Andean backarc at 21°S latitude allow the development of a crustal balancing model for the backarc region. The thrust belt developed on the eastern flank of the Andean orogen can be divided into three major units: the Subandean fold and thrust belt incorporating Paleozoic to Cenozoic sediments, the Interandean with tightly folded Silurian to Triassic strata, and the eastern margin of the Eastern Cordillera with predominantly Ordovician anchimetamorphic sediments, structured by large open folds. The structure of the thrust belt is dominated by two large “blind” thrusts. High seismic velocities in the upper to middle crust of the Eastern Cordillera with an underlying thick low velocity zone, derived from seismic refraction investigations, are interpreted as lower crustal material overthrust to the east over the crust of the Brazilian shield. The minimum shortening between the Eastern Cordillera and the foreland since Upper Cretaceous is about 145 km, as derived from cross section balancing. The crustal thickening, derived from the seismic refraction data, can be explained only from the foreland to the Eastern Cordillera by tectonic shortening. At least two main thrusts, cutting the whole backarc crust, are necessary to fit the high velocity bodies in the Eastern Cordillera to the geological observations. Further to the west, the thick continental crust, as derived from seismic refraction and gravimetric data, cannot be explained by tectonic shortening alone.

Foreland crustal structure of the New York recess, northeastern United States

A new structural model for the northeast part of the Central Appalachian foreland and fold-and-thrust belt is based on detailed field mapping, geophysical data, and balanced cross-section analysis. The model demonstrates that the region contains a multiply deformed, parautochthonous fold-and-thrust system of Paleozoic age. Our interpretations differ from previous ones in which the entire region north of the Newark basin was considered to be allochthonous. The new interpretation requires a substantial decrease in Paleozoic tectonic shortening northeastward from adjacent parts of the Central Appalachian foreland and illustrates the common occurrence of backthrusting within the region. During early Paleozoic time northern New Jersey consisted of a Taconic orogenic foreland in which cover folds (F1) involved lower Paleozoic carbonate and flysch overlying Middle Proterozoic basement. F1 folds are open and upright in the foreland and more gently inclined to recumbent southeastward toward the trace of the Taconic allochthons. F1 structures were cut and transported by a fold-and-thrust system of the Alleghany orogeny. This thrust system mostly involves synthetic faults originating from a master decollement rooted in Proterozoic basement. Antithetic faults locally modify early synthetic overthrusts and S1 cleavage in lower Paleozoic cover and show out-of-sequence structural development. The synthetic parts of the regional thrust system are bounded in the northwestern foreland by blind antithetic faults interpreted from seismic-reflection data. This antithetic faulting probably represents Paleozoic reactivation of Late Proterozoic basement faults. Tectonic contraction in overlying cover occurred by wedge faulting where synthetic and antithetic components of the foreland fault system overlap. S2 cleavage in the Paleozoic cover stems from Alleghanian shortening and flattening and commonly occurs in the footwall of large overthrust sheets. Paleozoic structures in Proterozoic basement include fault blocks bounded by high-angle faults and low- to moderate-angle shear zones that locally produce overlying cover folds. Broad and open folds in basement probably reflect shear-zone displacement of subhorizontal foliation. Our cross-section interpretations require limited involvement of lower Paleozoic cover folds in the footwalls of major overthrust faults. Palinspastic restoration of F1 folds produces an arched passive-margin sequence. The tectonic contraction for the Valley and Ridge province and southeastern Pocono Plateau is about 25 km, and tectonic wedge angles are 8°–11°.

Pre-Andean and Andean-age deformation in the Eastern Cordillera of southern Bolivia

The Eastern Cordillera of southern Bolivia is situated between the Altiplano high plateau in the west and the thin-skinned Subandean thrust belt in the east. It is mainly built of Ordovician anchimetamorphic sedimentary rocks with a discontinuous, unconformable cover of Cretaceous to Neogene, mostly continental sediments. The eastern margin of the Eastern Cordillera exhibits E-verging basement-involved thrusts linked to the Subandean belt. Its central and western parts, discussed in this paper, represent a province of dominantly W-verging, basement-involved thrusts of Tertiary age superimposed on a deeply eroded pre-Cretaceous fold belt. Several well-preserved remnants of the pre-Cretaceous fold belt show kilometer-scale, W-verging folds with axial plane slaty cleavage. The main activity of Andean (post-Cretaceous) thrusts is bracketed between about 25 and 12 Ma. However, some Andean contractional deformation occurred earlier. The exposed major Andean thrusts dip steep to intermediately at the surface. Cross-section balancing suggests that they merge into a detachment at maximum depths of 9 to 17 km. Neogene shortening is concentrated in an imbricated thrust system at the border of the Eastern Cordillera with the Altiplano in the west, but is relatively moderate (about 10–15%) within the Eastern Cordillera itself. W-verging thrusts in the Eastern Cordillera predate the E-verging thrust belt which underlies the eastern flank of the Andes. Strain rates are comparatively low during these first stages of deformation, but increase markedly at the onset of thin-skinned thrusting farther east. Balancing of a regional cross-section of 150 km present length indicates that the western and central Eastern Cordillera has shortened by about 55–80 km since Cretaceous time. Overall Tertiary tectonic shortening between the deformation front and the magmatic arc is about 215–250 km. This is insufficient to build the entire crustal root of the Andes by thrust thickening alone.

The Ansó transect of the southern Pyrenees: basement and cover thrust geometries

A balanced geological section through the western central Pyrenees allows the Alpine geometry of Hercynian basement to be determined. This shows marked differences with other transects of the southern part of the chain. Basement-involved thrust sheets do not form an antiformal stack, but are imbricated and define steps separated by frontal culmination limbs. Basement thrusts propagate upwards and have formed distinct thrust systems in the cover. In this transect, the North Pyrenean Zone is bounded to the south by gently dipping thrusts of large displacement, and the Axial Zone massif, at its western termination area, is a flat-topped culmination caused by a single thrust fault, the Gavarnie thrust. This thrust emerges to the south in the interior of the syntectonic Jaca Basin, the geometry of which is also controlled by underlying basement thrusts. Various generations of structures can be correlated with distinct episodes of syntectonic sedimentation. A sequential restoration of the section illustrates how successive foreland sedimentary wedges were generated and progressively involved in deformation. Tectonic shortening is 48 km, almost half of that estimated further to the east for the southern Pyrenees. The structure deduced at the ECORS-Pyrenees seismic section cannot be translated to this transect.

Rapid magma production rates, underplating and remelting in the Andes: isotopic evidence from northern-central Peru (9–11 °S)

Combined strontium and neodymium isotope data on the Miocene-Pliocene Cordillera Blanca batholith (9–11 °S) are compared with acid plutonic and volcanic rocks of similar age and composition from the central Andes (ca. 16–23 °S). Although intruded through 50–60 km of continental crust the batholith rocks, which range in composition from quartz diorite to high silica leucogranodiorite, show little sign of contamination by mature continental basement. Initial 87Sr 86Sr ratios define a range of 0.7041 to 0.7057, with average ϵ Nd values close to bulk earth (−0.5), over a relatively large range in SiO 2. It is difficult to reconcile the isotopic composition of the batholith rocks with simple AFC models involving fractionation of either clinopyroxene (deep-level) or plagioclase (high-level) dominated assemblages, and the isotopic variation in these rocks is instead considered to be inherited from a primary subcontinental lithosphere source through a two-stage process of crustal underplating and subsequent partial melting. Estimated (mantle) magma production rates during underplating are 0.1–0.3 km 3 yr −1. Comparisons with Miocene plutonic rocks from the western and eastern Cordillera at ~ 11 °S show no clear trends in isotopic compositions in time or space. Furthermore, the isotopic compositions of Nd and Sr in the batholith rocks contrast strongly with basaltic to high silica volcanic and plutonic rocks of similar age exposed in the central Andes, where 143Nd 144Nd and 87Sr 86Sr are thought to reflect contamination of mantle-derived magmas and/or tectonic reworking of old basement material during crustal thickening. The lack of significant involvement of basement material in the petrogenesis of the batholith magmas may be due to differences in the mechanism of crustal thickening along the Andean chain during the Late Miocene, with thickening in the central Andes being caused predominantly by tectonic shortening, while in northern-central Peru thickening occurred mostly through crustal underplating of mantle-derived basalt. These two regions thus represent first-order crustal domains in the central Andean Cordilleras. The boundary between them is marked by the Abancay Deflection.

Possible cryptic tectono-magnetic fabrics in ‘post-tectonic’ granitoid plutons of the Canadian Shield

Two late Archean granitic plutons forcefully intruded and thermally metamorphosed Archean schists. Feldspar magacrysts aligned concentrically with the margins during inflation. Substantial tectonic deformation never affected the plutons. Nevertheless, both plutons show the same, consistently oriented, cryptic magnetic fabric revealed by anisotropy of low field susceptibility (AMS) from 134 samples. They have an NE-SW vertical magnetic foliation and a magnetic lineation trending 060/15. Eigenvalues of the orientation distribution of principal susceptibilities show that the magnetic fabrics have almost identical strengths, orientations and symmetry in both plutons. We propose a post-magmatic reactivation of the earlier regional tectonic shortening. This imparted a ‘cryptic tectonic’ magnetic fabric on the plutons that overprinted the magmatic AMS fabric in most outcrops. The plutons' AMS fabrics are subparallel to the much older schistosity of the country rocks. Because multidomain magnetite provides > 99% of the low field susceptibility of the rocks, it controls the AMS. However, we see no alignment of the magnetite grains. Thus, magnetic anisotropy may be due to stress alignment of intragranular domain walls and not controlled by grain shape, as usually assumed for magnetite.

Aseismio fault slip and block deformation in North China

In North China, the tectonic fault-block system enables us to use the Discontinuous Deformation Analysis (DDA) method to simulate the long-term cross-fault survey and other geodetic data related to aseismic tectonic deformation. By the simulation we have found that: (1) Slips on faults with different orientation are generally in agreement with the ENE-WSW tectonic stress field, but the slip pattern of faulting can vary from nearly orthogonal, to pure shear along the strike of the faults, this pattern cannot be explained by simple geometric relation between the strike of the fault and the direction of the tectonic shortening. This phenomenon has been observed at many sites of cross-fault geodetic surveys, and might be caused by the interactions between different blocks and faults. (2) According to the DDA model, if the average aseismic slip rate along major active faults is at the order of several tenths of millimeter per year as observed by the cross-fault geodetic surveys, the typical strain rate inside a block is at the order of 10−8 year−1 or less, so that the rate of 10−6 year−1, as reported by observations in smaller areas, cannot be the representative deformation rate in this region. (3) Between the slips caused by regional compression and block rotation, there is a possibility that the sense of slip caused by rigid body rotation in two adjacent blocks is opposite to the slip caused by the tectonic compression. But the magnitude of slip resulting from the tectonic compression is much larger than that due to the block rotation. Thus, in general, the slip pattern on faults as a whole agrees with the sense of tectonic compression in this region. That is to say, the slip caused by regional compression dominates the entire slip budget. (4) Based on (3), some observed slips in contradiction to ENE tectonic stress field may be caused by more localized sources, and have no tectonic significance.

High-frequency sequences deposited during Quaternary sea-level cycles on a deforming continental shelf, north Canterbury, New Zealand

A closely spaced grid of high-resolution seismic reflection profiles reveals twelve unconformity-bounded sedimentary units of Late Pliocene to Recent age beneath the inner to middle north Canterbury continental shelf, New Zealand. This part of the shelf lies in < 80 m deep water and is undergoing tectonic deformation at the edge of the Australian-Pacific plate boundary zone. Coastal uplift, folding and wave planation have exposed the units at the seabed. Nannofloral biostratigraphy of sediment cores from at least six units constrain the age of the succession. The oldest unit (unit 12) of Late Pliocene-Early Pleistocene age is exposed in a submerged coastal platform, 3–8 km wide. Units 11 to 1 of middle Pleistocene to Recent age each thin shoreward, wedge out against the outer edge of the platform, and onlap the underlying erosion surface. The units are interpreted to be predominantly highstand systems tract (HST) silty muds separated by thin (< 1 m), transgressive systems tract (TST) gravel layers that mantle the underlying ravinement surface. The HSTs, which were deposited at rates ranging from 0 to > 3 m/ka, have prograded along shelf, shorewards and offshore, so that they onlap the TSTs beneath the inner shelf and downlap the TSTs beneath the outer shelf. Isolated pockets of sediment that infill channels beneath the ravinement surfaces may represent the preserved remnants of lowstand systems tract (LST) fluvial sediments and Type 1 sequence boundaries, respectively. Seismic units 11 to 1 and intervening unconformities A to J are tentatively interpreted to represent nine sea-level cycle sequences, deposited during high-amplitude (> 100 m) glacio-eustatic fluctuations corresponding to 100 ka (eccentricity) cycles in Earth's orbit. The sequences are tentatively correlated with marine oxygen isotope stages 19 to 1 (most represent odd-numbered, interglacial stages of ca. 0.75 Ma to Recent age) and with a succession of alternating glacial and interglacial deposits recovered in an onland testbore at Bexley, near Christchurch City. The outer edge of the submerged coastal platform, exposing unit 12, is close to the axis of regional tilting. Landward of this axis, unit 12 has been progressively uplifted, deformed and repeatedly eroded, and coastal hills and marine terraces have been uplifted. Seaward of the axis, the middle to outer shelf has been subsiding at about 0.4 m/ka, which created accommodation space for mostly HST deposition. Large-scale growing folds beneath the inner shelf have progressively deformed and modified the geometry of the units as a result of contemporaneous sedimentation and tectonic shortening. Therefore, although the sequences and unconformities owe their origin mainly to glacio-eustatic fluctuations, regional tilting and tectonic deformation have played a role in modifying the geometry of the units.

Geologic Framework of a Transect of the Central Brooks Range: Regional Relations and an Alternative to the Endicott Mountains Allochthon

Geologic Framework of a Transect of the Central Brooks Range: Regional Relations and an Alternative to the Endicott Mountains Allochthon