Clockwise Block Rotation Research Articles

Topo-Iberia CGPS network: a new 3D crustal velocity field in the Iberian Peninsula and Morocco based on 11 years (2008–2019)

The Topo-Iberia network covering the Spanish part of the Iberian Peninsula and Morocco was established in 2008. Following the first publication of the horizontal velocity field based on an analysis of the first 4 years (2008–2012) in 2015, a new 3D velocity field based on 11 years (2008–2019) is presented. The double-differencing technique is applied with GAMIT software, and the precise point positioning (PPP) technique is applied with GipsyX software for GPS processing to obtain loosely constrained solutions. Then, a kalman filter is used to merge both solutions using GLOBK software and produce a velocity solution. The combined velocity solution is presented as a new horizontal velocity field and the first vertical velocity field of the Topo-Iberia CGPS network. A global view of the data quality from the Topo-Iberia stations is also presented for the first time, as well as a program specifically created to convert GipsyX gdcov files to SINEX files, gdcov2sinex.sh. The results in the IGb14 reference frame are in good agreement with previous publications, except in the Betic Cordillera, where the new velocities with respect to Eurasia are generally lower and present a higher southwestward component rather than a purely westward motion. The main discrepancy is observed at the ALJI station, where the maximum velocity is reached (3 ± 0.1 mm/yr), showing west-northwestward motion. The smallest and highest residual displacements with respect to Eurasia occur in northern Iberia and Morocco, respectively. The results support the conclusions of other studies that a clockwise block rotation occurs at present in the northern branch of the Gibraltar Arc. The velocities indicate a northwest-southeast convergence rate of 3.5 ± 0.1 mm/yr with respect to Nubia. A general low subsidence (mean value of 0.5 ± 0.1 mm/yr) is observed in the Iberian Peninsula except for some isolated stations presenting uplift.

Steady Long‐Term Slip Rate on the Blue Cut Fault: Implications for Strain Transfer Between the San Andreas Fault and Eastern California Shear Zone

AbstractThe Eastern Transverse Ranges (ETR) province of California contains a system of E‐W‐trending left‐lateral faults accommodating clockwise block rotation between the San Andreas Fault (SAF) system in the Coachella Valley and the Eastern California Shear Zone (ECSZ) in the Mojave Desert. Building upon established geometric relationships, we estimate that this rotation across the ETR may transfer right‐lateral strain from the SAF to the ECSZ at a time‐averaged rate of ∼4.3–7.7 mm/yr. Through geomorphic mapping and the analysis of 38 10Be surface exposure samples, we derive a long‐term slip rate of 1.26 ± 0.50 (2σ) mm/yr over the late Pleistocene, yet analysis of the displacement record indicates a rate decrease ∼71 kya. While a rate change on this fault could have implications for possible plate boundary reorganization in this area, we argue that this slip rate variability more likely reflects routine fluctuation about a steady lifetime slip rate.

Sigmoidal normal faults and evidence for vertical-axis block rotation in an oblique convergent margin: A 3D seismic example from offshore Colombia

Sigmoidal fold and fault geometries are typical kinematic indicators of strike-slip fault zones. We document kilometer-scale, normal faults with sigmoidal plan-view geometries within the dextral pull-apart Bahia Basin, at the rear of the obliquely convergent South Caribbean Deformed Belt, offshore Colombia. Using 3D seismic reflection data calibrated to wells, closely spaced, low-displacement, planar normal faults are mapped within the Miocene strata. A series of seismic horizontal (time) slices and computed seismic attributes are used to interpret the 3D configuration of these faults. The closely spaced faults display an east–west trend with a progressive rotation into a northwest–southeast trend. In map view, the fault traces curve toward their tips, describing a sigmoidal-Z geometry that terminates at discrete northeast–southwest-trending fault zones. The structures observed may correspond to either tension fractures, which form theoretically at 45°, or antithetic shear fractures with normal displacement formed at 50°–70° to the boundaries of a dextral shear zone. These scenarios lead to a clockwise block rotation of between 20° and 40° within the shear zone. This study shows the first example of vertical-axis block rotations observed offshore in the western end of the South Caribbean margin and is an important example of the use of 3D seismic data to identify rotations where paleomagnetic studies are not available.

The Pattern and Kinematics of Deep Deformation of 2012 Ahar-Varzaghan Earthquake Doublet (MW 6.4 and 6.2), a New Seismotectonic Interpretation

The 11th August 2012 Ahar-Varzaghan earthquake doublet Mw 6.4 and 6.2 occurred near the city of Ahar, northwest Iran, in a region where there was no major mapped fault or any well-documented historical seismicity. To investigate the active tectonics and the state of pre and post-seismic stress distribution of the source region, we applied a combination of Coulomb stress change, b-value mapping, and the Fry method. Inferred Coulomb stress field reveals the E–W-striking (dextral) fault responsible for the first event and the NNE–SSW-striking (sinistral reverse) fault for the second event. The high slip stress-released regions in the obtained b-value map and the dominant anisotropies of aftershocks on regional stress-parallel cross-sections achieved by the Fry method, together with the distribution of aftershocks mechanisms, merely highlight the particular wedge-shaped structures namely the rhombic structures. The clockwise block rotation about the vertical axis under the right-lateral regional shear between the Kura basin to the north and the Central Iranian Block to the south and NW-oriented coeval shortening leads to the formation of rhombic structures. The results of this study improve our understanding of the kinematics of active deformation in NW Iran and have important implications for seismic hazard assessment of the region and potential future failure areas.

New paleomagnetic constraints for the large-scale displacement of the Hronic nappe system of the Central Western Carpathians

The thin-skinned Hronic nappe system represents the structurally highest tectonic unit in the Late Cretaceous thrust-stack of the Central Western Carpathians. It mostly comprises a Permian volcano-sedimentary sequence and Triassic carbonate sediments which crop out in different parts of the Central Western Carpathians. We carried out a systematic paleomagnetic study on 24 Permian and 20 Triassic localities geographically distributed over 300 km in W-E direction. Several samples from each locality were drilled and oriented in-situ and specimens cut from them subjected to standard paleomagnetic and magnetic mineralogy experiments. The results were evaluated using principal component analysis, statistical evaluation of the characteristic remanences, and applying inclination-only and tilt tests. We documented the pre-tilting age of remanences for the majority of both the Permian and Triassic age groups. However, the latter was interpreted as remagnetized during the Cretaceous Normal Super-Chron in the course of nappe stacking between 90−80 Ma. The Permian group is exhibiting about 70°, the Triassic about 34° clockwise vertical axis rotations with respect to the present north. There is no indication in our data set for oroclinal bending of the Hronic Unit. We interpret the difference in clockwise rotations (about 36°) between Permian and 90−80 Ma as a clockwise block rotation taking place during major extensional and/or compressive events between stable Europe and Africa. Taking into consideration the well-documented counterclockwise rotation observed for the overstep sequences in the Central Western Carpathians and in the Pieniny Klippen Belt, the remagnetization of the Triassic sediments was closely followed by about 94° clockwise rotation. Research in progress will serve to decide if this large clockwise rotation involved the whole Central Carpathian nappe stack or part of this was due to the thin-skinned nappe emplacement of the Hronic Unit.

Contemporary crustal deformation of Northeast Tibet from geodetic investigations and a comparison between the seismic and geodetic moment release rates

We processed the data recorded by 32 continuous GPS stations from the Crustal Movement Observation Network of China and combined them with published GPS observations in a common geodetic reference frame to better depict the ongoing crustal velocities and deformation field of Northeast Tibet. The derived velocity field indicates that the deformation of Northeast Tibet is characterized mainly by crustal shortening coupled with the eastward extrusion of crustal material and clockwise block rotation. We further used the GPS velocity field to evaluate the strain rate tensors by the method of Sandwell and Wessel (2016). The strain rate field shows an obvious transition of the contraction rate from NE-SW to NEE-SWW, and a decrease in the maximum shear strain rate from west to east was observed along the East Kunlun fault and Haiyuan fault, indicating that the block rotation and lateral extrusion therein may be partitioned by NE-SW-oriented compression to some extent. The inhomogeneity of the rotation strain rate implies that Northeast Tibet does not behave like a single rigid block but instead resembles a left-lateral shear zone, supporting the hypothesis that the northward transfer of motion along the East Kunlun fault may play a significant role in the deformation style of this area. The dilatation rate verified that most of Northeast Tibet experiences crustal thickening, which coincides with the average uplift rate of ~1 mm/y revealed by GPS velocities and channel steepness indices. We also estimated the geodetic moment rates and compared them with the seismic moment rates estimated using the truncated Gutenberg-Richter earthquake distribution. The ratio of the seismic strain rate to the geodetic strain rate in the regions near the Altyn Tagh fault, the Qaidam Basin adjoining the Qilian Mountains, the Haiyuan fault and the central part of the East Kunlun fault are near unity, indicating that the deformation in these areas is dominated by seismic release and may represent fully coupled seismogenic zones. The occurrence of large earthquakes in the past (estimated magnitudes M ≥ 6) coupled with a high geodetic deformation rate in these regions could indicate overdue M ≥ 6 earthquakes. However, other factors, including the occurrence of creep and the long recurrence time of Mw ≥ 8.0 earthquakes, should be taken into consideration along the Haiyuan fault. By contrast, in other regions, including the southern and eastern parts of the Haiyuan fault, along the Elashan fault, and the northern Qilian Mountains, the geodetic moment rate is higher than the seismic moment rate by a factor between 3 and 9. We propose that this phenomenon is related to the undersampling of long-term earthquake rates within the seismic catalogue, long-term aseismic crustal deformation, or a combination of these factors. Our study has implications for better comprehending the current seismotectonic pattern of Northeast Tibet.

Strong Neotectonic Block Rotations, Related to the Africa‐Eurasia Convergence in Northern Algeria: Paleomagnetic Evidence From the Mitidja Basin

AbstractSignificant tectonic clockwise rotations were evidenced in the Tell Atlas (Neogene Algerian Cheliff basin) by previous paleomagnetic studies. For northwestern Africa and in the context of the Africa‐Eurasia convergence, they provided a new argument validating a kinematic model, based on transpression with shortening accommodated by clockwise block rotations. To corroborate this deformation pattern at a larger scale, new paleomagnetic studies were performed on 349 cores in the Mitidja basin, of Mio‐Plio‐Quaternary age, in the Algerian Tell Atlas. This intramontaneous basin is structured by two regional major E‐W to WNW‐ESE dextral shear zones. Primary magnetization data were obtained in 43 out of the 49 sampled sites. This magnetization is carried by Ti‐poor titanomagnetite. Its direction shows that significant tectonic block rotations affected this basin since 16 Ma. Zones located between the E‐W and WNW‐ESE major structures are affected by coherent clockwise rotations (average magnitude of 48°) of large blocks, compartmented by presently associated sinistral NE‐SW faults. Along the shearing structures, smaller blocks, resulting from the fragmentation of the large blocks, show various rotations, many of which are of large magnitude. These rotations, similar to those highlighted previously in the Cheliff basin, are interpreted as resulting from bookshelf, consequence of the Africa‐Eurasia plates convergence in the Tell Atlas.

Coupled Crust‐Mantle Response to Slab Tearing, Bending, and Rollback Along the Dinaride‐Hellenide Orogen

AbstractWe integrate structural, geophysical, and geodetic studies showing that the Dinarides‐Hellenides orogen along the Adria‐Europe plate boundary in the Western Balkan peninsula has experienced clockwise oroclinal bending since Eocene‐Oligocene time. Rotation of the Hellenic segment of this orogen has accelerated since the middle Miocene and is associated with a north‐to‐south increase in shortening along the orogenic front. Within the Paleogene nappe pile, bending was accommodated by orogen‐parallel extension, clockwise block rotation, and thrusting in the hanging wall of the Skhoder‐Peja Normal Fault (SPNF). The SPNF and related faults cut the older Skhoder‐Peja Transfer Zone with its pre‐Neogene dextral offset of the West Vardar ophiolite nappe. Rotation of the SPNF hanging wall involved Miocene‐to‐recent, out‐of‐sequence thrusting that was transferred to the Hellenic orogenic front via lateral ramps on dextral transfer zones. Along strike of the Dinarides‐Hellenides and coincident with the southward increase in Neogene shortening, the depth of the Adriatic slab increases from ~160 km north of the SPNF to ~200 km just to the south thereof, to several hundreds of kilometers to the south of the Kefalonia Transfer Zone. The geodynamic driver of tectonics since the early Miocene has been enhanced rollback of the Hellenic segment of the Adriatic slab in the aftermath of Eocene‐Oligocene slab tearing and breakoff beneath the Dinarides, which focused slab pull in the south. The SW‐retreating Hellenic slab segment induced clockwise bending of the southern Dinarides and northern Hellenides, including their Adriatic foreland, about a rotation pole in the vicinity of the Mid‐Adriatic Ridge.

Structure of the Marathon basin (NE Attica, Greece) based on gravity measurements

This work presents the results of a high resolution gravity survey conducted in the area of Marathon Basin to asses the structure of the basement. A total of 120 gravity stations were measured. Standard data reduction procedures were followed, albeit based on modern, high precision techniques. The inversion of the Bouguer anomaly map provided a rather detailed image of the basement topography completing the information available from surface geological and morphological surveys. The major structural features of the Marathon Basin appear to be controlled by a system of NE-SW neotectonic faults causing stepwise NW-SE deepening of the basement to a maximum detectable depth of~450m. These faults, together with a system of NW-SE major, and a host of minor faulting features, lend to the Basin characteristics of a Riedel structure formed by a predominantly N-S extensional field effecting clockwise block rotation. This mode of deformation may also have facilitated the formation of local anticlinal/ synclinal structures, which endow the area with its rather complicated morphology. Finally, the multiple intersecting faults appear to form a permeable network, presumably responsible for the salination of the local aquifer system; the NE-SW major normal faults may the most significant contributor to this effect.

Accommodation of missing shear strain in the Central Walker Lane, western North America: Constraints from dense GPS measurements

We present 264 new interseismic GPS velocities from the Mobile Array of GPS for Nevada Transtension (MAGNET) and continuous GPS networks that measure Pacific–North American plate boundary deformation in the Central Walker Lane. Relative to a North America-fixed reference frame, northwestward velocities increase smoothly from ∼4 mm/yr in the Basin and Range province to 12.2 mm/yr in the central Sierra Nevada resulting in a Central Walker Lane deformation budget of ∼8 mm/yr. We use an elastic block model to estimate fault slip and block rotation rates and patterns of deformation from the GPS velocities. Right-lateral shear is distributed throughout the Central Walker Lane with strike-slip rates generally <1.5 mm/yr predicted by the block model, but extension rates are highest near north-striking normal faults found along the Sierra Nevada frontal fault system and in a left-stepping, en-echelon series of asymmetric basins that extend from Walker Lake to Lake Tahoe. Neotectonic studies in the western Central Walker Lane find little evidence of strike-slip or oblique faulting in the asymmetric basins, prompting the suggestion that dextral deformation in this region is accommodated through clockwise block rotations. We test this hypothesis and show that a model relying solely on the combination of clockwise block rotations and normal faulting to accommodate dextral transtensional strain accumulation systematically misfits the GPS data in comparison with our preferred model. This suggests that some component of oblique or partitioned right-lateral fault slip is needed to accommodate shear in the asymmetric basins of the western Central Walker Lane. Present-day clockwise vertical axis rotation rates in the Bodie Hills, Carson Domain, and Mina Deflection are between 1–4°/Myr, lower than published paleomagnetic rotation rates, suggesting that block rotation rates have decreased since the Late to Middle Miocene.

Palaeomagnetic data from the Precordillera fold and thrust belt constraining Neogene foreland evolution of the Pampean flat-slab segment (Central Andes, Argentina)

Abstract Available and new palaeomagnetic data reveal transpressional deformation in the Argentine Precordillera fold and thrust belt contemporaneous with Juan Fernández ridge subduction. Localized changes in the orientation of palaeomagnetic directions indicate a vertical axis rotation pattern linked to local, oblique brittle-ductile shear zones that overprint the regional structure. The nearly homogeneous clockwise block rotation pattern from Western-Central Precordillera shows localized rotation nulls along NNW-trending left-lateral oblique belts, revealing that overprinting anticlockwise tectonic rotations could have balanced previous clockwise rotations. Conversely, clockwise rotations in Eastern Precordillera are only localized in the vicinity of these NNW-trending structures, and are controlled by rigid block rotations linked to basement-involved deformation.These results combined with available regional palaeomagnetic data in the forearc would indicate a regional bending of the upper plate margin between 27° S and 33° S that seems to be related to the subduction of the Juan Fernández aseismic ridge.

Stratigraphy and structural development of the southwest Isla Tiburón marine basin: Implications for latest Miocene tectonic opening and flooding of the northern Gulf of California

Accurate information on the timing of earliest marine incursion into the Gulf of California (northwestern Mexico) is critical for paleogeographic models and for understanding the spatial and temporal evolution of strain accommodation across the obliquely divergent Pacific–North America plate boundary. Marine strata exposed on southwest Isla Tiburon (SWIT) have been cited as evidence for a middle Miocene marine incursion into the Gulf of California at least 7 m.y. prior to plate boundary localization ca. 6 Ma. A middle Miocene interpretation for SWIT marine deposits has played a large role in subsequent interpretations of regional tectonics and rift evolution, the ages of marine basins containing similar fossil assemblages along ∼1300 km of the plate boundary, and the timing of marine incursion into the Gulf of California. We report new detailed geologic mapping and geochronologic data from the SWIT basin, an elongate sedimentary basin associated with deformation along the dextral-oblique La Cruz fault. We integrate these results with previously published biostratigraphic and geochronologic data to bracket the age of marine deposits in the SWIT basin and show that they have a total maximum thickness of ∼300 m. The 6.44 ± 0.05 Ma (Ar/Ar) tuff of Hast Pitzcal is an ash-flow tuff stratigraphically below the oldest marine strata, and the 6.01 ± 0.20 Ma (U/Pb) tuff of Oyster Amphitheater, also an ash-flow tuff, is interbedded with marine conglomerate near the base of the marine section. A dike-fed rhyodacite lava flow that caps all marine strata yields ages of 3.51 ± 0.05 Ma (Ar/Ar) and 4.13 ± 0.09 Ma (U/Pb) from the base of the flow, consistent with previously reported ages of 4.16 ± 1.81 Ma (K-Ar) from the flow top and (K-Ar) 3.7 ± 0.9 Ma from the feeder dike. Our new results confirm a latest Miocene to early Pliocene age for the SWIT marine basin, consistent with previously documented latest Miocene to early Pliocene (ca. 6.2–4.3 Ma) planktonic and benthic foraminifera from this section. Results from biostratigraphy and geochronology thus constrain earliest marine deposition on SWIT to ca. 6.2 ± 0.2 Ma, coincident with a regional-scale latest Miocene marine incursion into the northern proto–Gulf of California. This regional marine incursion flooded the northernmost, >500-km-long portion of the Gulf of California shear zone, a narrow belt of localized strike-slip faulting, clockwise block rotation, and subsiding pull-apart basins. Oblique Pacific–North America relative plate motion gradually localized in the >1000-km-long Gulf of California shear zone ca. 9–6 Ma, subsequently permitting the punctuated south to north flooding of the incipient Gulf of California seaway.

Strong clockwise block rotation of the Ali-Sabieh/Aïsha Block: evidence for opening of the Afar Depression by a ‘saloon-door’ mechanism

Abstract Palaeomagnetic investigations were carried out on tilted Jessoma sandstones in the Aïsha block. The average natural remanent magnetization intensity was 0.026A m −1 with good behaviour on demagnetization both by thermal and alternating fields. The normalized magnetization intensity decay curves indicated that fine-grained, Ti-poor titanomagnetites were the carriers of magnetization. A well-defined single-component magnetization with a poorly defined secondary component was found. The latter was removed at temperatures below 300°C or at 10 mT. After these steps, a straight-line segment directed towards the origin was identified, which was interpreted as the characteristic remanent magnetization. Site mean direction coordinates of Dec g =227.8°, Inc g =–33.9° ( n =7, α 95 =2.8°, K =478) ( in situ ) and Dec s =225.6°, Inc s =0.4° ( n =7, α 95 =2.8°, K =478) (tilt-corrected) were obtained. Corresponding mean virtual geomagnetic pole coordinates of Lon g =117.5° E, Lat g =43.3° N ( A 95 =3.2°, n =7) ( in situ ) and Lon s =143.5° E, Lat s =43.3° N ( A 95 =2.8°, n =7) (tilt-corrected) were obtained. The corrected in situ mean pole position was compared with the African apparent polar wander path curve and a deposition age for the sandstone of 60±5 Ma was determined, which is consistent with previous reports. A vertical axis rotation ( R ±Δ R ) of the Aïsha block of 29.1±4.06° was obtained, which is equivalent to an inferred 30° counterclockwise rotation of the Danakil block, in agreement with a ‘saloon-door’ mechanism for the opening of the Afar Depression.

New constraints on the evolution of the Gibraltar Arc from palaeomagnetic data of the Ceuta and Beni Bousera peridotites (Rif, northern Africa)

The Betic Cordillera and the Moroccan Rif together form one of the smallest and tightest orogenic arcs on Earth and almost completely close the Mediterranean to the west. For the explanation of the geodynamic evolution of the mountain belt, palaeomagnetic data that generally found clockwise block rotations in the Iberian and anticlockwise rotations in the Moroccan part of the mountain belt, have played a key role in recent works. This palaeomagnetic study has found new constraints on the rotations and timing of the peridotitic bodies outcropping in the key position at the westernmost margin of the mountain belt, in Ceuta and Beni Bousera (Rif, northern Africa).Detailed thermal demagnetization of 115 individually oriented samples from 14 sites was combined with rock magnetic and scanning electron microscopic experiments to analyze the magnetic mineralogy responsible for the remanences and the mechanisms and relative times of their acquisition. In Ceuta, up to three magnetic components, and in Beni Bousera, up to two magnetic components have been found, that are all to be interpreted as chemical remanent magnetizations (CRM). The data suggests the following succession of geodynamic events affecting the peridotites until recent times: (1) after their exhumation and subsequent cooling about 20Ma ago, they recorded a characteristic remanent magnetization of both normal and reversed polarities, carried by (pseudo-)single-domain magnetite grains; (2) after their dismembering, the Ceuta peridotites were tilted southward by 22–34° about a horizontal or tilted axis (up to plunge 50°) with an azimuth of 72–145° and the Beni Bousera peridotites were rotated anticlockwise by 72.3±12.1° about a vertical axis and (3) both recorded another magnetic signal of normal polarity only, carried by multi-domain magnetite grains; and finally (4) the Ceuta peridotites rotated anticlockwise by 19.7±5.9° about a vertical axis.This study provides the first palaeomagnetic data for the Ceuta peridotites that, with their tilt and recent small net rotation, had a distinct geodynamic evolution from the large net rotations about vertical axes in Beni Bousera and Ronda (Betic Cordillera). Moreover, earlier palaemagnetic data for Beni Bousera is improved, as mixed polarities have been found in the older of the remanences for the first time, and its interpretation as a CRM changes the rotation timing that was proposed previously. The sequence of events exposed in this work are important constraints that need to be incorporated in any geodynamic model of the evolution of the Betic–Rifean mountain belt.

Transition from the Farallon Plate subduction to the collision between South and Central America: Geological evolution of the Panama Isthmus

This paper presents new geological constraints on the collision of southern Central America with South America, and the resulting deformational episodes that have affected the Panama Isthmus since the Late Cretaceous. The Panama Isthmus is located in southwestern Central America, and it represents the zone of contact between the two land masses: Central America and South America. This collision event is still active today. It has resulted in regional uplift since the Late Miocene/Pliocene and is responsible for the Great American Biotic Interchange between South and North America. Depending on the methods of investigation used, and due to the lack of data available, the time when this collision began is still widely debated and poorly constrained. To better constrain this age, we have studied the rock formations and the tectonic deformations in central and eastern Panama that have occurred since the Late Cretaceous. This study presents new rock ages, field-work documentation and analyses, and seismic-line interpretations, and it is complemented by spatial images for the eastern Panama area. During the Middle Eocene, a number of changes suddenly appeared in the geological records that were synchronous with the break-up of southern Central America into two smaller blocks: Chorotega and Choco. Our main results identify the prevalence of an extensional tectonic regime from the Middle Eocene to the Middle Miocene that caused the formation of horst and graben structures with thick sedimentary basin fills, and a synchronous clockwise block rotation. Here, we propose that these geologic events are associated with the initiation of the oblique collision of southern Central America with South America. The first contact of the southeastern extremity of Central America occurred around 40 Ma to 38 Ma, and then propagated northwestwards. We describe here this long-term collision episode in relation to the history of the Panama Isthmus.

Transtensional rifting in the proto-Gulf of California near Bahia Kino, Sonora, Mexico

Continental rifts require focused strain to rupture and form an ocean basin. In oblique rifts, such as the Gulf of California, focused transtensional strain associated with strike-slip faulting may serve as a catalyst for rupture. To test this hypothesis, we analyzed structural mapping, geochronology, paleomagnetism, and fault kinematics of pre- and synrift rocks exposed in an ∼200 km 2 coastal mountain belt flanking the eastern rift margin of the northern Gulf of California. This coastal Sonora region hosts the onshore portion of the transform boundary between the Upper Tiburon and Adair-Tepoca marine basins—two early-formed oblique rift segments. Extension commenced here between 11.5 Ma and 7 Ma, resulting in 25°–40° of E-NE tilting, initiation of clockwise vertical-axis rotation of fault-bounded blocks, and minor basin sedimentation. Rates of deformation prior to 7 Ma are unconstrained due to a lack of exposed syntectonic deposits. Deformation after 7 Ma was associated with rapid tilting and the majority of observed clockwise vertical-axis rotation and strike-slip faulting. Nonmarine sedimentary basins accumulated coarse sediments above an unconformity eroded across older, tilted strata. By 5–6 Ma, deformation in coastal Sonora must have largely ceased and migrated westward into the Upper Tiburon marine basin. We document up to 120% total extension and total clockwise block rotations up to 53°. In portions of the study area, extension and rotation were supplanted by strike-slip faulting as deformation proceeded. We develop a tectonic model for this Coastal Sonora fault zone, which is bounded by major NW-striking transform faults with >10 km of displacement. Internal to the Coastal Sonora fault zone, the majority of an estimated 6.2 ± 1.1 km of dextral deformation, associated with up to 5.7 km of WNW-directed extension, occurred over the final 1–2 m.y. of its life span, at a strain rate approaching 10 –14 s –1 . This activity occurred as the plate boundary localized along nascent pull-apart basins in the northern Gulf of California, consistent with the hypothesis that late proto–Gulf of California dextral shear zones, such as the Coastal Sonora fault zone, acted to focus lithospheric-scale strain and promoted continental rupture in the wide-rift setting of the Mexican Basin and Range.

Structural controls on a geothermal system in the Tarutung Basin, north central Sumatra

The Sumatra Fault System provides a unique geologic setting to evaluate the influence of structural controls on geothermal activity. Whereas most of the geothermal systems in Indonesia are controlled by volcanic activity, geothermal systems at the Sumatra Fault System might be controlled by faults and fractures. Exploration strategies for these geothermal systems need to be verified because the typical pattern of heat source and alteration clays are missing so that conventional exploration with magnetotelluric surveys might not provide sufficient data to delineate favorable settings for drilling. We present field geological, structural and geomorphological evidence combined with mapping of geothermal manifestations to allow constraints between fault dynamics and geothermal activity in the Tarutung Basin in north central Sumatra. Our results indicate that the fault pattern in the Tarutung Basin is generated by a compressional stress direction acting at a high angle to the right-lateral Sumatra Fault System. NW–SE striking normal faults possibly related to negative flower structures and NNW-SSE to NNE-SSW oriented dilative Riedel shears are preferential fluid pathways whereas ENE–WSW striking faults act as barriers in this system. The dominant of geothermal manifestations at the eastern part of the basin indicates local extension due to clockwise block rotation in the Sumatra Fault System. Our results support the effort to integrate detailed field geological surveys to refined exploration strategies even in tropical areas where outcrops are limited.

Pliocene sinistral slip across the Adobe Hills, eastern California–western Nevada: Kinematics of fault slip transfer across the Mina deflection

The Adobe Hills region (California and Nevada, USA) is a faulted volcanic field located within the western Mina deflection, a right-stepping zone of faults that connects the northern Eastern California shear zone (ECSZ) to the south with the Walker Lane belt (WLB) to the north. New detailed geologic mapping, structural studies, and 40 Ar/ 39 Ar geochronology in the Adobe Hills allow us to calculate fault slip rates and test predictions for the kinematics of fault slip transfer into the Mina deflection. The Adobe Hills are dominated by Pliocene tuffaceous sandstone, basaltic lavas that yield 40 Ar/ 39 Ar ages between 3.13 ± 0.02 and 3.43 ± 0.01 Ma, and basaltic cinder cones. These Pliocene units unconformably overlie Middle Miocene latite ignimbrite that yields an 40 Ar/ 39 Ar age of 11.17 ± 0.04 Ma, and Quaternary tuffaceous sands, alluvium, and lacustrine deposits cap the sequence. Northwest-striking normal faults, west-northwest–striking dextral faults, and northeast-striking sinistral faults cut all units; the northeast-striking sinistral faults are the youngest and most well developed fault set. We calculate ∼0.1 mm/yr of approximately east-west horizontal extension and northwest dextral shear since the Pliocene. The prominent northeast-striking sinistral faults offset basalt ridgelines, normal fault–hanging-wall intersections, a channelized basalt flow, a basalt flow edge, and a basalt flow contact a net minimum of 921 ± 184 to 1318 ± 264 m across the Adobe Hills. These measured sinistral offsets yield a minimum Pliocene sinistral fault slip rate of 0.2–0.5 mm/yr; our preferred minimum slip rate is 0.4–0.5 mm/yr. The geometry and orientation of the prominent sinistral faults are consistent with simple shear/couple clockwise block rotation within a broad dextral shear zone. Vertical axis block rotation data are needed to test this interpretation. We propose that a set of faults subparallel to Sierra Nevada–North America motion and associated releasing steps, located west of the White Mountains fault zone and east of the Long Valley Caldera, transfer a portion of dextral Owens Valley fault slip northwestward onto the sinistral faults in the Adobe Hills. Dextral slip distributed across faults between the White Mountains fault zone and the Sierra Nevada and east of the Fish Lake Valley fault zone may account for the apparent discrepancy between summed long-term geologic slip rates and present-day geodetic rates across the northern ECSZ. Fault slip in the Adobe Hills is part of a regional pattern of initiation and renewal of dextral, sinistral, and normal fault slip during the Pliocene that extends from lat ∼40°N to ∼36°N within the ECSZ-WLB and along the western margin of the Basin and Range Province. This regional deformation episode may be related to changes in gravitational potential energy.

Segmentation of the Hellenides recorded by Pliocene initiation of clockwise block rotation in Central Greece

New paleomagnetic data from Early Miocene to Pliocene terrestrial sedimentary and volcanic rocks in Central Greece constrain the history of vertical-axis rotation along the central part of the western limb of the Aegean arc. The present-day pattern of rapid block rotation within a broad zone of distributed deformation linking the right-lateral North Anatolian and Kephalonia continental transform faults initiated after Early Pliocene time, resulting in a uniform clockwise rotation of 24.3±6.5° over a region >250km long and >150km wide encompassing Central Greece and the western Cycladic archipelago. Because the published paleomagnetic dataset requires clockwise rotations of >50° in Western Greece after ∼17Ma, while our measurements resolve no vertical-axis rotation of Central Greece between ∼15Ma and post-Early Pliocene time, a large part of the clockwise rotation of Western Greece must have occurred during the main period of contraction within the external thrust belt of the Ionian Zone between ∼17 and ∼15Ma. Pliocene initiation of rapid clockwise rotation in Central and Western Greece reflects the development of the North Anatolia–Kephalonia Fault system within the previously extended Aegean Sea region, possibly in response to entry of dense oceanic lithosphere of the Ionian Sea into the Hellenic subduction zone and consequent accelerated slab rollback. The development of the Aegean geometric arc therefore occurred in two short-duration pulses characterized by rapid rotation and strong regional deformation.

Earthquake mechanisms and active tectonics of the Hellenic subduction zone

We use improved focal mechanisms and centroid depth estimates of earthquakes, combined with GPS velocities, to examine the tectonics of the Hellenic subduction zone, and in particular the processes occurring at both ends of the Hellenic Arc. Nubia-Aegean convergence is accommodated by shallowly dipping thrust-faulting along the subduction–zone interface, as well as by steeper splay faults in the overriding material. From a comparison of observed and expected seismic moment release over the last 100 yr, combined with existing knowledge of the longer-term documented historical record, we confirm earlier suggestions that most (80 per cent) of this convergence is accommodated aseismically, that is, that the subduction zone is uncoupled. This conclusion is robust, even allowing for rare very large earthquakes on splay faults, such as that of AD 365, and also allowing for the contribution of small earthquakes. The downgoing Nubian plate deforms by arc-parallel contraction at all depths, from 200 km seaward of Crete to at least 100 km within the subducting slab. Extensional (T) axes of earthquakes are aligned downdip within the descending slab suggesting that, even if the aseismic prolongation of the slab has reached the 670 km mantle discontinuity, it does not transmit stresses to shallower depths. Shallow thrust-faulting earthquakes on the subduction interface show a divergence of slip vectors round the arc, and GPS measurements show that this is accommodated mainly by E–W extension on normal faults in the overriding Aegean material. The eastern end of the subduction zone, south of Rhodes, displays distributed deformation in the overriding material, including a mixture of strike-slip and splay-thrust faulting, and probably involves rotations about a vertical axes. Here slip on the interface itself is by thrust faulting with slip vectors oblique to the arc but parallel to the overall Nubia-Aegean convergence: there is no evidence for slip-partitioning in the traditional sense. In the west, the subduction zone terminates in a distributed zone of parallel NE–SW strike-slip faults, of which the most prominent is the Kefalonia Transform Fault (KTF). A flexural gravity anomaly confirms that the deep bathymetric escarpment of the KTF is a lateral ramp, formed as the Ionian islands are emplaced SW onto the Apulian lithosphere, and enhanced by minor thrust faulting with slip vectors perpendicular to the scarp. Distributed parallel strike-slip faults both SW and NE of mainland central Greece terminate in E–W graben in central Greece, which accommodate the overall NE–SW shear by clockwise block rotation. Central Greece therefore acts as a relay zone between the strike-slip faulting of the NE Aegean and the Ionian Islands–western Peloponnese.