Vertical Axis Block Rotations Research Articles

Neogene structural evolution of the Sierra San Felipe, Baja California: Evidence for proto-gulf transtension in the Gulf Extensional Province?

The Sierra San Felipe, located in the Gulf Extensional Province of northeastern Baja California, experienced a complex deformation history of integrated normal and strike-slip faulting, block rotations and extension-parallel folding as a result of Neogene transtensional plate margin shear between the Pacific and North American plates. The eastern range-front of the Sierra San Felipe is defined by three left-stepping, en-echelon detachment faults that are linked by dextral and sinistral transfer faults and accommodation zones. The Las Cuevitas, Santa Rosa and Huatamote detachments comprise NE- to SE-dipping, moderate- to low-angle normal faults that accommodate between ~ 1.5 and 9 km of broadly E- to SE-directed extension and show strong along-strike displacement gradients. Hanging wall half-grabens are infilled with northwest tilted Miocene–Pliocene volcanic and sedimentary rocks that have been deposited nonconformably onto the batholithic basement. Stratigraphic relationships indicate that faulting on the Las Cuevitas and Santa Rosa detachment faults initiated synchronously as a kinematically linked fault system before ~ 7 Ma (~ 9–8 Ma based on the timing of footwall exhumation), during the so-called ‘proto-gulf’ phase of rifting. Paleostress calculations suggest a transtensional stress regime with NE- to SE-directed extension and permutating vertical to N–S trending, subhorizontal contraction. Fault kinematics and paleostress orientations of the fault array do not vary through time, but reflect the spatial distribution of fault planes with respect to a transtensional stress regime that lasted throughout the entire slip history of the detachments. Our data indicate that clockwise vertical-axis block rotations and constrictional folding were an integral part of the deformation history in the Sierra San Felipe since rifting began in the late Miocene, and may have played an important role in facilitating the transfer of deformation between the Main Gulf Escarpment and offshore faults in the Gulf of California. These observations support the hypothesis that middle Miocene to present oblique-divergent plate motion was accommodated by a single phase of integrated transtensional shearing in the Gulf Extensional Province.

Paleomagnetism of Tertiary intrusive and volcaniclastic rocks of the Cerrillos Hills and surrounding region, Española Basin, New Mexico, U.S.A.: Assessment and implications of vertical-axis rotations associated with extension of the Rio Grande rift

Paleomagnetic data from parts of the northern Rio Grande rift provide evidence for clockwise (CW) and counterclockwise (CCW) vertical-axis block rotations associated with strike-slip deformation along basin-bounding faults during rift evolution. Despite the spatial consistency of the results, the quality and statistical significance of data sets are difficult to evaluate because of small sample size and potential failure to average secular variation. To understand the extent, importance, and origin of such rotations, we report paleomagnetic data from Tertiary intrusive and volcaniclastic rocks in the Cerrillos Hills and surrounding areas in the Espanola Basin. Paleomagnetic data from in situ Tertiary intrusions and tilt-corrected volcaniclastic strata of the Oligocene Espinaso Formation sampled at four localities yield a grand mean of declination = 342.9° and inclination = 58.3° (α 95 = 3.5°; N = 32 sites of normal polarity/21 reverse). Correction for minor postemplacement tilt of the Cerrillos Hills and La Cienega data sets yields a grand mean (declination = 349.5°, inclination = 55.3°, α 95 = 3.4°) that is indistinguishable from the 30 Ma reference direction for the study area, and there is no evidence of rotation (R = 1.8° ± 6.4°). However, if an alternative reference direction is used, minor CCW rotation (−6.6° ± 5.8°) is possible. Our data suggest that the magnitude of rotation in the Espanola Basin is significantly less than previously estimated and may be negligible. Regardless, paleomagnetic data from elsewhere in the basin suggest that CCW rotations may be an important component of recent rift extension and deformation.

Optimization of apparent polar wander paths: An example from the South China plate

Paleomagnetically derived apparent polar wander paths form the foundation of quantitative plate tectonic reconstructions. However, deformation leading to vertical axis block rotations displaces paleomagnetic poles away from their original positions, leading to an ambiguity as to which pole, or group of poles, best approximates the “true” reference pole position for a given time. Here we show that the best estimate of the “true” reference pole will match the observed paleolatitude (pλ) for each point on the plate. This means that the expected pλ from the “true” reference pole minus the observed pλ, derived from each individual study, will average to zero. Histogram plots and associated parameters help further discriminate between candidate reference poles when more than one of them fulfills the zero-average requirement within prescribed uncertainty limits. Our analysis of 44 Late Permian to Middle Triassic paleomagnetic poles from the South China plate corroborates previous assumptions that the poles from Sichuan Province best represent the “true” reference for the South China plate. To better understand the age of the rotations, we studied the paleomagnetism of Lower and Upper Triassic rocks from the Shiwandashan region in Guangxi Province. Early Triassic paleomagnetic directions isolated at high temperature demagnetization steps are of dual polarity and pass the fold test. This magnetization component is indistinguishable at 95% confidence limits from Middle Triassic paleomagnetic directions from other parts of Guangxi Province. The corresponding pole for this component lies within the swath of Early to Middle Triassic paleomagnetic poles from the South China plate, confirming that Guangxi has been a part of the South China plate since at least the Early Triassic. Late Triassic paleomagnetic data require further study before their complex magnetizations can be interpreted.

A new paleogeographic configuration of the Eurasian landmass resolves a paleomagnetic paradox of the Tarim Basin (China)

New paleomagnetic data from Permian red beds and Middle Jurassic limestones from the Tarim Basin pose a paradox. Their declinations are similar to Upper Carboniferous to Neogene rocks collected from the same sections, and their inclinations parallel present values. When assuming that lower than expected inclinations in continental sedimentary rocks arise from inclination shallowing effects, then the paleolatitudes of all Upper Carboniferous to Present rocks from Tarim are indistinguishable. Local vertical axis block rotations occurring in the last 20 million years explain why declinations vary at different localities in the basin. Our Middle Jurassic data positions Tarim 23.6 ± 8.4° farther south than that predicted from the coeval reference pole for Eurasia; however, no geologic argument exists to support the closure of a large ocean basin between Tarim and Siberia since the Middle Jurassic. Thus the paradox: are the rocks from Tarim totally overprinted, or is the middle Mesozoic part of the reference Eurasian apparent polar wander path erroneous? Several lines of evidence suggest the Tarim rocks are not remagnetized. We conclude that Tarim has experienced little or no apparent polar wander since the Carboniferous. Moreover, our Middle Mesozoic reconstruction of Eurasia using the new Middle Jurassic pole from Tarim results in a more geologically compatible solution for the eastern Asian blocks over previous reconstructions.

Strain partitioning of active transpression within the Lebanese restraining bend of the Dead Sea Fault (Lebanon and SW Syria)

Abstract Recent neotectonic, palaeoseismic and GPS results along the central Dead Sea fault system elucidate the spatial distribution of crustal deformation within a large ( c .180-km-long) restraining bend along this major continental transform. Within the ‘Lebanese’ restraining bend, the Dead Sea fault system splays into several key branches, and we suggest herein that active deformation is partitioned between NNE–SSW strike-slip faults and WNW–ESE crustal shortening. When plate motion is resolved into strike-slip parallel to the two prominent NNE–SSW strike-slip faults (the Yammouneh and Serghaya faults) and orthogonal motion, their slip rates are sufficient to account for all expected strike-slip motion. Shortening of the Mount Lebanon Range is inferred from the geometry and kinematics of the Roum Fault, as well as preliminary quantification of coastal uplift. The results do not account for all expected crustal shortening, suggesting that some contraction is probably accommodated in the Anti-Lebanon Range. It also seems unlikely that the present kinematic configuration characterizes the entire Cenozoic history of the restraining bend. Present-day strain partitioning contrasts with published observations on finite deformation in Lebanon, demonstrating distributed shear and vertical-axis block rotations. Furthermore, the present-day proportions of strike-slip displacement and crustal shortening are inconsistent with the total strike-slip offset and the lack of a significantly thickened crust. This suggests that the present rate of crustal shortening has not persisted for the longer life of the transform. Hence, we suggest that the Lebanese restraining bend evolved in a polyphase manner, involving an earlier episode of wrench-faulting and block rotation, followed by a later period of strain partitioning.

New Early Permian paleomagnetic results from the Brive basin (French Massif Central) and their implications for Late Variscan tectonics

In order to assess the structural evolution of the Brive basin and the Paleozoic activity of surrounding major faults in the French Massif Central, we carried out a paleomagnetic study on Early Permian rocks from this basin. Positive-fold tests and solely reversed polarities indicate that the characteristic remanent magnetization is likely to be primary. Early Permian tilt-corrected site mean declinations vary from 207°–167° indicating that the Brive basin experienced internal vertical-axis rotations. On the contrary, Late Permian paleomagnetic site means exhibit a circular Fisherian distribution showing no relative rotations. Detailed analyses of Permian paleomagnetic data from five contemporaneous basins of the French Massif Central reveal that these basins share the same equatorial paleolatitude with stable Europe throughout the Permian. However, in Early Permian, three of the five basins experienced differential rotations. The Saint-Affrique basin not only suffered internal deformation during the Early Permian, but the basin as a whole underwent a full-scale counterclockwise vertical-axis block rotation with respect to stable Europe. As a consequence, paleomagnetic data from similar late orogenic basins have to be thus carefully considered for establishment of Apparent Polar Wander paths.

Insight into the Neogene tectonic history of the northern Bolivian Orocline from new paleomagnetic and geochronologic data

A paleomagnetic study of 36 sites (315 samples) of Neogene strata from the Peruvian Altiplano and adjacent sub‐Andean belt (Pilcopata area), combined with 40Ar/39Ar dating of 13 lava flows, provide new constraints on the Cenozoic deformation history of the northern Bolivian Orocline. In the Peruvian Altiplano, 40Ar/39Ar dating brackets the age of the strata sampled for paleomagnetism between 26.5 and 12.1 Ma for the Huaccochullo basin, 18.6 and 12.1 Ma for the Descanso‐Yauri basin, and 16.8 and 15.5 Ma for the Ayaviri basin. Sub‐Andean sediments are paleontologically constrained as Miocene, yet pre‐9 Ma. Prefolding magnetizations isolated in the Huaccochullo, Descanso‐Yauri, and Pilcopata localities indicate significant counterclockwise vertical axis block rotations of 11.3° ± 5.4°, 31.0° ± 10.2° and 7.8° ± 4.8°, respectively. The pattern of rotations together with mapped structures suggest that deformation of the northern Altiplano was partitioned into large regions experiencing relatively minor rotation versus smaller, isolated basins exhibiting high‐amplitude counterclockwise rotations that lie in a major left‐lateral shear zone. Our new results imply significant transpressional deformation occurred throughout the Peruvian Altiplano since circa 12 Ma and are integrated in a model detailing the tectonic evolution of the northern Bolivian Orocline since 25 Ma.

Vertical coupling and decoupling in the lithosphere

Recent advancements in the understanding of mountain belts have focused on vertical coupling of the lithospheric layers. This volume describes the extent to which observed or inferred sub-horizontal coupling or attachment zones provide vertical kinematic linkage between rheologically distinct layers in the continental lithosphere. A common theme is whether the deformation, which partitioned differently in each layer, is linked kinematically across attachment zones and driven by flow in the deeper crust and mantle lithosphere. The papers are divided into six sections. The first analyses the extent to which mantle flow controls deformation of the overlying layers. The Vertical axis block rotations section uses geological and palaeomagnetic data to constrain the role and magnitude of basal shear across mid-crustal attachment zones. The Lower crustal flow and topography section addresses the time-dependent development of orogenic plateaux and their role in the orogenic cycle. Multiple examples of the spatial and temporal development of lithospheric coupling are given in both the Orogenic examples and Subduction examples sections. Finally, rheological constraints of vertical coupling in the lithosphere are investigated.

About this title

Recent advancements in the understanding of mountain belts have focused on vertical coupling of the lithospheric layers. This volume describes the extent to which observed or inferred sub-horizontal coupling or attachment zones provide vertical kinematic linkage between rheologically distinct layers in the continental lithosphere. A common theme is whether the deformation, which partitioned differently in each layer, is linked kinematically across attachment zones and driven by flow in the deeper crust and mantle lithosphere. The papers are divided into six sections. The first analyses the extent to which mantle flow controls deformation of the overlying layers. The Vertical axis block rotations section uses geological and palaeomagnetic data to constrain the role and magnitude of basal shear across mid-crustal attachment zones. The Lower crustal flow and topography section addresses the time-dependent development of orogenic plateaux and their role in the orogenic cycle. Multiple examples of the spatial and temporal development of lithospheric coupling are given in both the Orogenic examples and Subduction examples sections. Finally, rheological constraints of vertical coupling in the lithosphere are investigated.

Palaeozoic geography of the Alpine realm, new palaeomagnetic data from the Northern Greywacke Zone, Eastern Alps

The Palaeozoic units of Alpine Europe are generally accepted as having been part of the northern margin of Gondwana throughout the Cambro-Ordovician. By Permo-Carboniferous times, northern Gondwana had collided with Laurasia, forming Pangaea, but the palaeogeographic affinities of the Alpine units in intervening times, and whether they remained an integral part of Gondwana or formed part of the Armorican terrane assemblage (ATA), remains unclear. In order to resolve this problem, a palaeomagnetic study of Late Silurian and Mid Devonian rocks from the Kitzbuhel Alps of the Northern Greywacke Zone, Eastern Alps (part of the Norian-Bosnian terrane, or Proto-Alpine terrane), has been carried out. Three ancient directions of magnetisation have been identified in this study. The youngest (component B) is identified in the most deformed rocks, and represents a remagnetisation event of Alpine age and indicates rotation of the region with respect to stable Europe since the remagnetisation event. The other two directions are interpreted to be primary in origin, and Late Silurian (component C) and Mid Devonian (component D) in age. Both components are identified as high unblocking temperature directions of magnetisation, and stereographic projection of site mean directions from each age shows a clear, small circle distribution after bedding correction, which is related to local and small-scale vertical axis block rotations. Both data sets pass the inclination-only fold test, demonstrating the primary nature of these magnetisations. The resulting mean inclination values obtained for the Silurian and Devonian rocks are 65° (α95=1.7°) and 42° (α95=4.6°). Using the negative polarity option, these yield palaeolatitudes for the southern Proto-Alpine terrane of 47°S (+2.3° and –2.1°, upper and lower 95% confidence level) during the Silurian, and 24°S (+3.7° and –3.4°) during Mid Devonian times. In terms of palaeogeography, therefore, these new results indicate that in Mid Palaeozoic times, the Proto-Alps were located significantly further south than, and were independent of, the ATA. The relationship with northern Gondwana is more complex, given the uncertainty of the APW path for Gondwana. Palaeobiogeographic indicators, however, support the interpretation that the Proto-Alps formed an independent microplate, rather than having remained part of the north Gondwana margin.

Fault strength and transpressional tectonics along the Castle Mountain strike-slip fault, southern Alaska

Analyses of structures along the Castle Mountain fault reveal the mechanical relations and relative timing of orogen-parallel strike-slip faulting to distributed wrenching and shortening and to plate motion in the southern Alaskan transpressive margin. The Castle Mountain fault is a >200-km- long, orogen-parallel, right-lateral seismogenic fault in the southern Alaskan plate margin. Exposures of the fault in the study area have been exhumed from 3−4 km depth and ∼80 °C. Moment tensor summations and stress inversions of slip data from fault networks within 400 m of the Castle Mountain fault and analyses of structures in forearc deposits 2−4 km from the fault yield maximum incremental shortening and maximum compressive stress (σ 1 ) axes that are subhorizontal and trend ∼325°. This trend is close to the 340° North American–Pacific plate convergence direction. The inferred σ 1 makes a 70°−80° angle with the ∼070°-striking Castle Mountain fault, indicating that the fault slips at a lower shear stress than predicted by laboratory rock friction experiments if hydrostatic pore pressure exists in the fault (i.e., coefficient of friction ∼0.85, Byerlee9s Law). The forearc structures record coaxial strain and apparently remain in their formative orientations, showing that previously documented distributed shearing in the form of vertical-axis block rotations in the forearc ceased prior to formation of the structures in late Oligocene time. The mechanical weakness of the Castle Mountain fault probably results in part from its clay- rich gouge, which averages about 43 wt% clay phases. However, the gouge composition does not account for slip in response to σ 1 at 70°−80° to the fault. Another mechanism, such as elevated pore pressure, further weakens the fault and enables it to slip right laterally in response to the small component of oblique motion across the plate boundary. Upon its inception in early Tertiary time, the fault could not have formed a >200-km-long planar zone of low-friction clay-rich gouge or elevated pore pressure; thus, the mechanisms that weaken it must have developed over time. The distributed shearing in the forearc may have ended in response to the weakening and localization of dextral slip onto the Castle Mountain fault.

Chronostratigraphic framework and evolution of the Fortuna basin (Eastern Betics) since the Late Miocene

ABSTRACT A Tortonian to Pliocene magnetostratigraphy of the Fortuna basin supports a new chronostratigraphic framework, which is significant for the palaeogeographical and geodynamic evolution of the Eastern Betics in SE Spain.The Neogene Fortuna basin is an elongated trough which formed over a left‐lateral strike‐slip zone in the Eastern Betics in the context of the convergence between the African and Iberian plates. Coeval with other basins in the Alicante–Cartagena area (Eastern Betics), rapid initial subsidence in the Fortuna basin started in the Tortonian as a result of WNW–ESE stretching. This led to transgression and deposition of marine sediments over extensive areas in open connection with the neighbouring basins. Since the late Tortonian, N–S to NW–SE compression led to inversion of older extensional structures. The transpressional tectonics along the NE–SW‐trending Alhama de Murcia Fault is related to the rising of a structural high which isolated the Fortuna basin from the open Mediterranean basin. The progression of basin confinement is indicated by the development of restricted marine environments and deposition of evaporites (7.8–7.6 Ma). The new basin configuration favoured rapid sediment accumulation and marine regression. The basin subsided rapidly during the Messinian, leading to the accumulation of thick continental sequences. During the Pliocene, left‐lateral shear along the Alhama de Murcia Fault caused synsedimentary folding, vertical axis block rotations and uplift of both the basin and its margins. The overall sedimentary evolution of the Fortuna basin can be regarded as a developing pull‐apart basin controlled by NE–SW strike‐slip faults. This resembles the evolution that has taken place in some areas of the Eastern Alboran basin since the late Tortonian.

Ductile and brittle shortening, extension‐parallel folds and maintenance of crustal thickness in the central Aegean (Cyclades, Greece)

In contrast to previous studies that concentrated on the two‐dimensional crustal strain in the central Aegean region (Cyclades), it is shown that NNE‐SSW extension via ductile and brittle stretching and low‐angle detachments was accompanied and/or alternated with horizontal shortening perpendicular to the stretching direction since at least the early Miocene. Roughly E‐W directed ductile shortening produced large‐scale overturned and upright folds having axes parallel or slightly oblique to the stretching lineation. Upright folding and arching of low‐angle normal faults occurred above the brittle‐ductile transition and brittle E‐W compression culminated with vertical axis block rotations, strike‐slip faults and minor thrusts. Since at least the early Miocene, the structure of the Cycladic massif evolved through alternating and/or coeval increments of horizontal shortening and vertical thinning associated with an approximately constant NNE‐SSW stretching. Exact magnitudes of the vertical and horizontal deformational components are difficult to assess. Nevertheless, we note that extensional tectonics that affected the Cyclades during the last 15–20 m.y. have produced no net crustal thinning. We suggest that crustal thickness was maintained by extension‐parallel folds, which represent true contractional structures, and that crust was fed into the extended region from its margins.

Timing of the North‐South China collision from new middle to late Mesozoic paleomagnetic data from the North China Block

We collected paleomagnetic samples from 43 sites of Middle and Upper Jurassic and Cretaceous rocks along a 250 km wide east‐west trending belt located north of the suture between the North and South China Blocks. In each case, laboratory treatment isolates dual polarity, antipodal magnetizations that are demonstrably pre‐folding. The paleomagnetic poles show very good coherence with coeval poles from other localities from the North China Block (NCB), suggesting the apparent polar wander path (APWP) from the NCB is not distorted by post‐Middle Jurassic local vertical axis block rotations. A comparison of the NCB APWP with that from the South China Block suggests that collision continued after the Early Jurassic and ended no later than the Late Jurassic. A cusp in the NCB APWP coincides with a marked angular unconformity that separates Middle Jurassic from Upper Jurassic and younger sediments. Together, these observations indicate that the scissor‐like collision and associated deformation ceased at the Middle to Late Jurassic boundary (∼159 Ma). The time of rapid (1°/Ma) relative angular velocity between the two plates (225 to 190 Ma) coincides with a peak in U‐Pb and Ar‐Ar dates obtained from metamorphic rocks in the suture. The combined Late Jurassic and Cretaceous paleomagnetic data from both blocks suggest that the united China plate traveled 2000 km north relative to Eurasia during the Late Jurassic before docking in the Cretaceous. This younger collision event is also marked by a cusp on the Cretaceous part of the Eurasian APWP.

Paleomagnetism and tectonics of the Southern Tarim Basin, northwestern China

We report Late Carboniferous, Permian, and early Tertiary paleomagnetic data from the southern Tarim basin. Prefolding magnetizations were isolated in each case. The Late Carboniferous–Permian and early Tertiary poles lie at 64.6°N, 166.5°E, A95 = 6.3° and 58.1°N, 202.0°E, A95 = 12.7°, respectively. The Late Jurassic to early Tertiary (J3–E1) paleolatitudes of Tarim and several basins throughout central Asia are similar, yet significantly (10° to 20°) shallower than those predicted by the Eurasian apparent polar wander path. Resolving this discrepancy remains a major problem in Asian paleomagnetism. Discordance of the late Paleozoic poles from Tarim and Siberia suggest that Tarim has rotated about 30° counterclockwise with respect to Siberia since the Permian. Where paleomagnetic samples of both Late Carboniferous to Early Triassic (C3–T1) and J3–E1 ages were collected from the same area of Tarim, a great circle passes through the means of the poles and the sampling locality. This suggests that (1) only a difference in inclination (and not declination) distinguishes the two data sets, and (2) vertical axis block rotations of the C3–E1 strata occurred after E1. Although based on data of lesser quality, the mean Early to Middle Jurassic (J1–2) pole from Tarim differs significantly from the Eurasian reference pole, requiring radical tectonic solutions to resolve them. The Tarim J1–2 pole is indistinguishable from both the mean J3–E1 and C3–T1 poles. The similarity of all the poles and the analogous tectonic setting of present‐day central Asia to that of the late Paleozoic in eastern North America raises the question whether all the data from Tarim are overprinted.

Paleomagnetic evidence of vertical axis block rotations from the Mesozoic of northern Chile

We present paleomagnetic results for three Mesozoic formations from northern Chile: La Ternera Formation (Upper Triassic), Quebrada Monardes Formation (Upper Jurassic); Cerrillos Formation (Upper Cretaceous). Results from the Cerrillos are divided into eastern (Cuesta El Gao (CEG)) and western (Elisa De Bordo (EBD)) localities. Most specimens from La Ternera volcanic and sedimentary rocks are magnetically stable, as shown by alternating field and thermal demagnetization. More complicated but still reliable results were obtained from Quebrada Monardes red beds. Normal and reverse polarities are present in both units; means of both populations are antiparallel at 95% confidence. The Quebrada Monardes Formation also yields positive conglomerate and fold tests. Paleomagnetic poles for La Ternera and Quebrada Monardes are 60.9S, 218.3E (A95, 7.8°), and 66.9S, 191.6E (A95, 12.7°), respectively. Comparison with appropriate reference poles shows that this region of Chile has undergone about 25° of clockwise rotation, with negligible latitudinal transport. Cerrillos CEG results are less reliable and possibly complicated by remagnetization during emplacement of early Tertiary intrusives. Most Cerrillos EDB specimens are stable, but marked increase in scatter upon unfolding suggests remagnetization. Results for the Cerrillos CEG locality, which is contiguous to the sampling area of the La Ternera and Quebrada Monardes formations, show only about half the rotation of those two units, suggesting that rotation commenced after deposition of the Quebrada Monardes rocks in the Late Jurassic and was approximately half complete by the time Cerrillos CEG rocks acquired their magnetization. Cerrillos EDB results come from an area approximately 40 km to the west; these show roughly 45° of clockwise rotation. Dispersion is very low between EDB sites, suggesting that secular variation may not be completely averaged. Nevertheless, the great difference in direction between the two Cerrillos localities suggests that they lie in different structural blocks.