Basin Research - Volume 12, Issue 2, 2000

Apatite fission‐track (FT) and (U–Th)/He analyses are used to constrain the low‐temperature thermal history of the San Gabriel and San Bernardino Mountains (SGM and SBM), which are part of the southern California Transverse Ranges. FT ages from 33 SGM samples range from 3 to 64 Ma. Helium ages, ranging from 3 to 43 Ma, were obtained from 13 of these samples: all of the He ages are the same or younger than their respective FT ages. FT ages from 10 SBM samples were older, ranging from 45 to 90 Ma. The FT and He data document at least three phases of cooling in the SGM, but only two in the SBM. Prior to ~7 Ma, the thermal history of the SGM appears to have been nearly identical to many of the core complexes in the Basin and Range of south‐eastern California: a major phase of cooling is indicated from ~60 to 40 Ma, with a more recent phase beginning at ~23 Ma and continuing until ~10 Ma. The similarity of this timing to that of core complexes suggests that the SGM also originated as a core complex, when the rocks were adjacent to the Chocolate–Orocopia Mountains, and that some of the range‐bounding faults were initially extensional. In the SBM, the two phases of cooling documented by the FT data occurred from ~65 to 55 Ma, and from ~18 Ma to the present. The timing on the second phase is very poorly constrained and, therefore, we do not speculate on the origin of the SBM.

The most recent phase of cooling appears to have begun at ~7 Ma in the SGM, as the result of the onset of contractional deformation. A more accelerated phase of cooling may have begun at ~3 Ma. Distinct variations in the total amounts and rates of cooling between different fault‐bounded blocks within the SGM are documented since 7 Ma. We use these variations in cooling rates to calculate denudation rates, which are then compared to topographic characteristics for each structural block. These comparisons suggest that more rapid bedrock uplift in the eastern and southern part of the range has strongly affected the present‐day physiography. Despite a higher mean elevation, the SBM are much less dissected than the SGM, suggesting that the most recent phase of cooling and bedrock uplift began in the last 3 Myr, much later than the initiation of recent bedrock uplift in the SGM.

The specific time window of apatite fission track thermochronology (AFTT) places it in a unique position to offer continuous time–temperature baseline histories for relatively stable shield, rift and passive margin environments, spanning several geological eras. Fission tracks anneal partially at temperatures between ≈110 °C and ≈60 °C and thereby provide information on residence times within specific levels of the crust. Most samples collected from these terrains, however, are usually found to have cooled out of the apatite partial annealing zone (APAZ) by the mid‐Cenozoic at the latest. Owing to the stability, at geological timescales, of tracks at temperatures <60 °C (i.e. within depths of 0–2.5 km at normal geothermal gradients), a significant loss of resolution must therefore be reckoned with at shallow, although geomorphologically crucial, crustal depths. Indeed, the Neogene and Quaternary are understood to have been most influential in generating the scenery of today, and a use of radiometric and stratigraphic techniques in a nested, multisystem approach can assist in bridging the resolution gap. This paper examines and illustrates, mostly with original examples, the uses and limitations of AFTT in addressing the response of Earth surface systems to event patterns in global tectonics, the controls of lithology and structure on denudation rates, the origin and evolution of passive margin escarpments, the mass‐balanced reconstruction of palaeoelief, the use of apatites as tracers for understanding provenance in sediment routing systems, and the tempo (or episodicity) of denudation as postulated by W. M. Davis’ canons of the ‘geographical cycle’. Alongside efforts towards standardizing the supply of analytically robust AFT results in the laboratory, a more standardized geoscientific interpretation of AFT data is also desirable in order to build a consistent world base of geomorphic rates which can be made available to – and used with confidence by – non‐AFTT specialists.

The extensive shallow‐water carbonate platform deposits of the Dolomia Principale Formation (Southern Alps, northern Italy) accumulated during the Late Triassic, a time of plate‐scale reorganization and rifting. Synsedimentary tensional features such as fractures, neptunian dykes, normal faults, shatter breccias and laterally discordant intraformational breccias have been studied within a well‐preserved platform‐to‐basin transition in the Monte Pramaggiore area (Carnian Prealps). These tensional features follow three preferential orientations: N–S, E–W and NE–SW. To fully explain these different arrays it is proposed that the study area experienced during the Late Triassic the waning rifting phase connected to the westward propagation of the NeoTethys (N–S extension) and the onset of the rifting phase that led in the Middle Jurassic to the opening of the Central Atlantic (E–W extension), with a contemporaneous reactivation of Early–Middle Triassic NE–SW‐orientated faults. This palaeostress analysis reveals the good potential of tensional features as reliable palaeostress indicators.

The vertical characteristics of underpressure, pore fluids and sealing condition in the Shiwu Fault Depression, south‐east Songliao Basin, are investigated. Based on the pore pressure data from drill stem tests collected from 40 wells, the vertical distribution of pore pressure in this depression consists of a shallow hydrostatic pressure system and a deep underpressure system. The observed low pressures range from about 4 to 10 MPa in the depth interval of about 1550–2800 m. The pore water chemistry data document that the ionic evaporite trends of the pore water in the underpressure zone are different from overlying sediments in the hydrostatically pressured section, indicating that the underpressure system is sealed. The study indicates that the depression has undergone rapid deposition in a rift period during the Cretaceous followed by a long‐term slow uplift and erosion in the Tertiary. On the basis of the modelled results of the abnormal pressure evolution in a typical cross‐section using a two‐dimensional numerical flow model, we believe that the predecessor of the underpressure system was an overpressure system. Since the end of the Cretaceous, the observed underpressure system has developed as a result of a geotemperature decrease ranging from 30 to 70 °C, owing to palaeoheat flow reduction and long‐term uplift and erosional cooling. The mudstones below a depth of 1550 m in the deep subnormal pressure system have small measured porosities ranging from 4% to 1.2% with calculated permeability of about 10⁻²¹–10⁻²⁴ m². The observed underpressure can be modelled if we assign permeabilities below 10⁻²⁰ m² with a linear reduction of geotemperature. The geotemperature decrease, in combination with very low permeability in the Lower Cretaceous mudstones, is therefore a possible origin for the formation of the underpressure system in this depression.

Subsidence and provenance analysis has been used as a tool to quantify and discriminate the role of tectonics and eustasy in the Veneto and Friuli Basin, north‐east Italy, using 17 sections distributed along east–west‐trending outcrops of Oligo‐Miocene deposits. The basin can be considered a two‐phase foreland; first, during late Oligocene to Langhian with respect to the NW–SE‐trending Dinaric Chain, and then with respect to the south‐vergent South‐Alpine Chain.The clastic succession is up to 4000 m thick, and was deposited in a generally shallow‐marine to nonmarine environment. Subsidence diagrams reconstructed for each section and E–W subsidence profiles indicate a compound effect of the Dinaric and South‐Alpine tectonics as well as interference with eustatic sea‐level changes.During the Oligocene and the early Miocene, the cycles recognized within the basin approximately match sea‐level curves, the inferred cyclicity being primarily eustatic. However, the westward migration of the sedimentary depocentre during the same interval of time indicates activity of Dinaric thrusts.From Burdigalian (20 Ma) onwards, differential subsidence between the northernmost and the southernmost sectors of the basin suggests initiation of South‐Alpine uplift in the frontal parts. During Tortonian and early Messinian uplift, erosion and southward migration of the thrust system was associated with the progressive closure of the basin from open marine influence. During Messinian sea‐level drop, up to 2500 m of alluvial sediments were deposited at the same time as the South‐Alpine thrusts were emerging, as confirmed by progressive angular unconformities within the continental succession.