Abstract In a previous laborious study we showed that considerably more new area is formed in hackle sections than in en echelon sections per given nominal area of fringe. In the present study we resort to an alternative quantitative method that would enable a relatively rapid correlation between the fracture mode of secondary fractures on joint fringes and their change in fracture morphology. We introduce a set of seven criteria that enable a geologist to place every fringe on the 'index of hackle raggedness' (IHR) along a scale between 1 and 10. This method is applied to the 'Mrákotin joint set' from a granite quarry in the Bohemian Massif of the Czech Republic. The results show an IHR range between 1.4 ± 0.7 and 6. 8 ± 0.7. The gap between the top value of the scale, 10, and the value 6. 8 probably suggests that the joints in this study did not form under the most dynamic conditions. Experimental results carried out on polymers show that dynamic fracture is better correlated with the dynamic stress intensity and additional fracture mechanics parameters than with fracture velocity. The demonstration that en echelon segmentation may be obtained at maximum fracture velocity, if mechanical constraints are imposed on the system by crystal anisotropy, possibly suggests that similar results may be obtained under other constraints that need to be elucidated.

Abstract Fault segments belonging to a fault population can link and interact, eventually forming a single larger fault, and thus affecting the estimation of the maximum expected earthquake. We present throw distribution data along the Quaternary normal faults of the Colfiorito fault system (central Italy), which consists of four main fault segments and where a seismic sequence occurred in 1997–1998. Throw values along the two central overlapping, en-échelon segments (8.5–9.5 km long) were measured on a good stratigraphic marker, by constructing a set of closely spaced geological cross-sections, perpendicular to the fault strike. As these faults are commonly retained active and border Quaternary basins, we compare morphological and geological throws in order to verify the faults neotectonic activity. Geological and morphological throw distributions show good correlation, testifying that recent faulting affected the topographic surface and suggesting that the observed offset completely accumulated during the Quaternary. The throw distribution along the fault segments is asymmetric and reaches maximum values (500–550 m) within the zone of fault overlap, suggesting mechanical interaction between the studied faults. Maximum length-throw correlation suggests that the studied faults grew according to a linear scaling relationship.

Abstract An aromagnetic study was conducted over the Oceanographer Fracture Zone on, the Mid-Atlantic Rides between 33° to 37°N and 31° to 39°W. A seafloor-spreading interpretation of the magnetic anomalies reveals that the ridge crest is formed of short, en echelon segments 40 to 60 km Iong. These segments are offset by transform fractures. An average spreading rate of about 1.1 cm/yr active over the last 10 m.y. can be fitted to the ridge crest anomalies 2’ through 5. However, positive identification of the outer flank anomalies ts not possible. The ridge crest anomalies younger than 7 m.y. old (anomaly 4) show a general trend of N30°E, but anomalies be, tween 9.3 and 17.5 m.y. old (anomaly 5 to 5’) have trends of about N8 °E. The oldest flank anomalies (anomaly 6 ) trend about N35°E. Application of the anomaly trend superposirion technique to account for the offset anomaly and fracture-zone pattern. has allowed a new calculation of rotation pole parameters for the North American-AfricanNplate systems. For anomaly 2(2.7 m.y. ago), the finite rotation pole is located south of Iceland at 58.8°N, 17.4°W, with an angular rotation of 1.26°. For anomaly 5 and the older flank angmalies 5’ and 6 , the finite rotation poles are located near Svalbard at 78.6°N, 34.5°E; 80°N, 29.9°E; and 809N, 46.1°E with angular rotations of 2.67, 3.84, and 4.64 degrees, respectively. The major change in the pole location between anomalies 2’ and 5 about 7 m.y. ago appears to have been accompanied by the creation of a new transform fracture pattern with old fractures terminating and new ones being formed. Comparison of the two general pole locations deduced here with poles determined by others for the earlier opening history of the North American American- African plate system shows that all finite poles lie,in either of these locations. This suggests that a bi-stable dynamic equilibrium condition has prevailed throughout the opening history, with the rotation poles being located south of Iceland during the earliest period ( 200 to 80’ m.y. ago) and the latest period ( ~7 m.y. ago to the present) of opening. During the intervening period, the poles were located near Svalbard. Key words: marine geophysics, ocean ridges, geomagnetic survey, plate tectonics, sea-floor spreading.

Miocene-Pliocene history is recorded in south-central Colorado by sediments deposited in subsiding basins bounded by fault-block mountains and by faulted sedimentary and volcanic deposits lying on a channeled late Eocene erosion surface of regional extent. The San Luis Valley and upper Arkansas Valley are en echelon segments of the Rio Grande trough that are constricted and faulted south of Salida at the northern end of the Sangre de Cristo Range. Great movements on the bounding faults during Neogene time are indicated by clastic and volcanic trough fill, which may be 10,000 m thick near Alamosa and 1,500 m thick near Salida, and by adjoining mountains which stand as much as 1,500 m above the valley floors. The Sangre de Cristo Range owes most of its present elevation to Neogene faulting that cut the Miocene-Pliocene Dry Union Formation and some volcanic deposits in Oligocene-Miocene paleovalleys. Valley fill in the Wet Mountain Valley graben is as much as 1,500 m thick, and at least the upper 300 m of it consists of pinkish beds correlated with the Santa Fe(?) Formation. The crest of the Wet Mountains was uplifted about 400 m above the valley to the west of the mountains and nearly 1,200 m above the high plains which lie to the east. To the north, Neogene faulting elevated the Rampart Range at the front of the Rocky Mountains, dropped the Fourmile Creek graben nearly 400 m, formed complex fault-bounded basins at the southern margin of South Park, and segmented volcanic deposits in paleovalleys that once crossed the upper Arkansas Valley.

The Hosgri fault zone (HFZ) is the name given to the southern section of the major coastal fault in central California. The Hosgri separates Transverse Range structure from offshore Santa Maria Basin structure and is a key element for any tectonic model that includes this economically significant region. Previous published maps have not adequately defined the southern termination of the HFZ, the style of faulting on the HFZ, and the relation of the HFZ to surrounding structures. Using more than 1,500 mi of processed seismic reflection data, we have mapped upper Miocene and Pliocene structure in the region of the HFZ offshore from Point Sal in the north, to Point Conception in the south where the HFZ ends against east-west structures in the westernmost Santa Barbara Channel. In the same area, east-west-trending structures in the western Transverse Ranges north of the channel abut against the HFZ. The HFZ is an oblique right-slip fault along most of its length, but significant changes in the style of faulting are associated with variations in fault trend. North of Point Arguello, the HFZ appears to dip at a high angle in the upper 2,000 m of section and is distinguishable from thrust and reverse faults developed to its west. Between Point Arguello and Point Conception it may be a northeast-dipping thrust. Along its mapped length, east-side-up vertical separation is typical and may be more than 400 m on a Pliocene unconformity. Older horizons show more separation; the lower Miocene is up on the east by almost 1 km off Purisima Point. However, individual en echelon segments of the fault show west-side-up vertical separation where expected in an oblique right-slip fault system. No piercing points were found to define strike separation. Pliocene drag folds indicate dextral slip in Pliocene and later time.