Basin Research - Volume 11, Issue 3, 1999

Integration of vitrinite reflectance (Ro) and apatite fission track (AFT) data from well sequences can provide a direct estimate of the geothermal gradient at the time of maximum palaeotemperatures and the time at which sequences began to cool from maximum palaeotemperatures. These values, plus an understanding of the effects of cooling in response to long‐term climatic changes, are particularly important when estimating the amount of denudation experienced by the sequences during cooling from maximum palaeotemperatures. In this case study, AFT data have been generated for subsurface samples from eight wells drilled within the North Slope foreland basin of northern Alaska in an effort to study the thermal history of the basin. The combination of Ro and AFT data establish that maximum palaeotemperatures were attained within the North Slope foreland basin prior to cooling beginning in the Palaeocene. Furthermore, they indicate that palaeogeothermal gradients when cooling began were close to the present‐day values, and that Cenozoic surface cooling resulted in a significant amount of ‘apparent’ denudation. These results suggest that heating throughout the basin was largely due to deeper burial, and that cooling was due to both removal of section by denudation and a drop in the mean annual surface temperature.

This study examines a thick section of Pliocene–Pleistocene sedimentary rocks exposed in the footwall of an active normal fault (Cañon Rojo fault) near its intersection with the dextral‐normal Laguna Salada fault in north‐western Mexico. These rocks are situated in the upper plate of an inactive strand of the Cañada David detachment fault, which is cut on the north‐east by the Laguna Salada fault. The stratigraphy is divided into three unconformity‐bounded sequences: (1) marine mudstone of the Pliocene Imperial Formation; (2) nonmarine Pliocene–Pleistocene redbeds, consisting of sedimentary breccia, conglomerate, conglomeratic sandstone (all un‐named) and fine‐grained sandstone and mudstone of the Palm Spring Formation; and (3) uncemented Pleistocene boulder gravel. Coarse deposits of the redbeds sequence were deposited in fault‐bounded, high‐ and low‐gradient alluvial fans that passed laterally into a low‐energy fluvial plain of the ancestral Colorado River (Palm Spring Formation) which occupied the present‐day Laguna Salada.

Detailed mapping reveals convergence and lap‐out of bedding surfaces in the redbeds sequence onto the west limb of a large anticline cored by Imperial Formation. These geometries, combined with fanning dips and thickening of stratigraphy into the flanking syncline, indicate that the anticline grew during deposition of the redbeds. Fold axes of the growth anticline and smaller related folds trend N to NNE, parallel to the strike of associated normal faults and perpendicular to the extension direction. Based on its orientation, large size and relationship to neighbouring structures, the anticline is interpreted to be a fault‐bend fold that grew in response to slip of the upper plate over a bend in the Cañada David detachment fault during deposition in a transtensional supradetachment basin. Localized subsidence in the flanking syncline resulted in deposition of >1000 m of alluvial sediments near its intersection with the Laguna Salada fault. Sedimentary detritus is derived exclusively from the north‐east (footwall) side of the dextral‐normal Laguna Salada fault, indicating that topographic relief was high in the Sierra Cucapa and was subdued or negligible in the footwall of the coeval Cañada David detachment. Following deposition of the redbeds and grey gravel units, the northern part of the detachment fault was abandoned and the modern Cañon Rojo fault was initiated, producing rapid footwall uplift and erosion of previously buried stratigraphy. Slip rate on the Cañon Rojo fault is estimated to be ≈2–4 mm yr⁻¹ since middle Pleistocene time, similar to the late Pleistocene to Holocene rate determined in previous studies.

Six time‐slice reconstructions in the form of palaeogeographical maps show the large‐scale tectonic and sedimentary evolution of the Hellenic outer‐arc basins in central and eastern Crete for the middle and late Miocene. The reconstructions are based on extensive field mapping and a detailed chronostratigraphy.

Latest compressional features related to subduction and associated crustal thickening are poorly dated and assigned a middle Miocene age. These are possibly contemporaneous with widespread occurrence of breccia deposits all over Crete. The precise date for the onset of extension, possibly controlled by the roll‐back of the subsiding African lithosphere, remains at this point a discussion. We present circumstantial evidence to place the beginning of the roll back in the middle Miocene, during the accumulation of an arc‐parallel, westward‐draining fluvial complex. The continental succession is transgressed steadily until it is interrupted by an important tectonic event at the boundary of the middle and late Miocene (normally seen as the onset of slab roll‐back). In the earliest late Miocene a few large‐sized fault blocks along arc‐parallel normal faults subsided rapidly causing a deepening of the half‐graben basins up to approximately 900 m. About 1 Myr later, a new N020E and N100E fault system developed fragmenting the existing half‐grabens into orthogonal horst and graben structure. The development of the new fault system caused original continental regions to subside and original deep basins to emerge, which is not easy to reconcile with roll back controlled extensional processes alone. Underplating and inherited basement structure may have played here an additional role, although evidence for firm conclusions is lacking. In late Miocene times (late Tortonian, ≈7.2 Ma), the extensional outer arc basins become deformed by N075E‐orientated strike‐slip. The new tectonic regime begins with strong uplift along existing N100E fault zones, which developed about E–W‐striking topographical highs (e.g. Central Iraklion Ridge and Anatoli anticline) in about 0.4 Myr. The strong uplift is contemporaneous with abundant landsliding observed along an important N075E fault zone crossing eastern Crete and with renewed volcanic activity of the arc. The origin of the ridges may be due to active folding related to the sinistral slip.

Crustal thickening north of the Hellenic subduction zone continued in the most external zones (e.g. Crete) probably until the late middle Miocene. The following period of predominant extension has been related by various workers to a number of causes such as: (1) trench retreat (roll back) driven by the pull of the African slab and (2) gravitational body forces associated with the thickened crust, both in combination with NNE motion of the African plate combined with westward extrusion of the Anatolian block along the North Anatolian Fault. To verify these hypotheses an inventory of fault orientations and fault‐block kinematics was carried out for central and eastern Crete and adjoining offshore areas by combining satellite imagery, digital terrain models, and structural, seismic, sedimentary and stratigraphical field data, all set up in a GIS. The GIS data set enabled easy visualization and combination of data, which resulted in a relatively objective analysis. The geological results are discussed in the light of a numerical model that investigated the intraplate stresses resulting from the above mentioned forces.

Our tectonostratigraphic results for the late Neogene of central and eastern Crete show three episodes of basin extension following a period of approximately N–S compression. In the earliest Tortonian, N130E‐ to N100E‐trending normal faults developed, resulting in a roughly planar, arc‐parallel fault system aligning strongly asymmetric half‐grabens. The early Tortonian to early Messinian period was characterized by an orthogonal fault system of N100E and N020E faults resulting in rectangular grabens and half‐grabens. From the late Tortonian to early Pleistocene, deformation occurred along a pattern of closely spaced, left‐lateral oblique N075E faults, orientated parallel to the south Cretan trenches. Deformation phases younger than early Pleistocene are dominated by normal to oblique faulting along WSW–ENE (N050E) faults and dextral, oblique motions along NNW–SSE (N160E) faults. Many faults that were generated during previous deformational episodes appear to be reactivated in later periods.

Our tectonostratigraphy points to a three step anticlockwise rotation of active fault systems since the late middle Miocene compressional phase. We suggest here that the rotation is associated with a reorganization of the stress field going from SSW–NNE tension in the early late Miocene to NE–SW left‐lateral shear in the Quaternary. The rotation is likely to be a response to arc‐normal pull forces combined with a progressive increase of the curvature of the arc. During the Pliocene to Recent period, the SSW‐ward retreat of the arc and trench system relative to the African plate was accomplished by transform motions in the eastern (Levantine) segment of the Hellenic Arc, resulting in, respectively, NNW–SSE and NE–SW left‐lateral shear on Crete.

Anticlinal ridges of the actively deforming Umbria–Marche Apennines fold‐thrust belt are transected by deep gorges, accommodating a drainage pattern which almost completely ignores the presence of pronounced anticlinal mountains. Because the region was below sea level until the folds began to form, simple antecedence cannot explain these transverse canyons. In addition, the fold belt is too young for there to have been a flat‐lying cover from which the rivers could have been superposed.

In 1978, Mazzanti & Trevisan proposed an explanation for these gorges which deserves wider recognition. They suggested that the Apennine fold ridges emerged from the sea in sequence, with the erosional debris from each ridge piling up against the next incipient ridge to emerge, gradually extending the coastal plain seaward. The new coastal plain adjacent to each incipient anticline provided a flat surface on which a newly elongated river could cross the fold, positioning it to cut a gorge as the fold grew. Their mechanism is thus a combination of antecedence and superposition in which folds, overlying sedimentary cover and downstream elongations of the rivers all form at the same time.

A study of Apennine drainage, using the sequence of older‐to‐younger transected Apennine folds as a proxy for the historical evolution of drainage cutting through a single fold, shows that transverse drainage forms when sedimentation dominates at the advancing coastline. Longitudinal drainage forms when uplift dominates, the folds first emerge as offshore islands and the Mazzanti–Trevisan mechanism is suppressed.

Complicating factors include several departures from steady‐state growth of the fold‐thrust belt, a possible case of precursory submarine drainage, early emergence of anticlinal culminations and the location of several transverse canyons at the structurally highest point along anticlinal axes.