Sedimentologic, provenance, oxygen isotope, and 40Ar/39Ar results provide insights into late Cenozoic evolution of an extensional hinterland basin in the Peruvian Andes. The 5–6.5 km Cordillera Blanca composes the glaciated footwall of a low‐angle normal fault parallel to active contractional structures in the Andean fold‐thrust belt. The ∼200 km long, WSW dipping (19°–36°) Cordillera Blanca detachment fault accommodated >12–15 km of dip‐slip displacement, inducing subsidence of the hanging‐wall supradetachment basin, which is filled by ∼1300 m of conglomerate, sandstone, siltstone, and limited carbonate of the upper Miocene‐Pliocene Lloclla Formation. Lithofacies associations are attributed to lacustrine fan‐delta to proximal, stream‐dominated alluvial fan sedimentation. Provenance data record footwall unroofing of Jurassic through Miocene volcanic and sedimentary rocks during NW directed, axial dispersal of sediment. Provenance data record the appearance of footwall‐derived granite clasts from the upper Miocene Cordillera Blanca batholith and a change to transverse, WSW directed transport. Variations in the character and preservation of basin fill suggest along‐strike propagation of the fault through time. Initial extension and subsidence is constrained to the latest Miocene by an 40Ar/39Ar biotite age of 5.4 ± 0.1 Ma for a basal tuff in the Lloclla Formation. Estimation of paleolake water composition shows very negative δ18O(VSMOW) values (−13.6‰ to−18.2‰), indicating that high elevations comparable to modern were already attained in the Cordillera Blanca during earliest normal faulting and basin evolution. These results lend support to models suggesting that substantial shortening, crustal thickening, and surface uplift were necessary precursor conditions for the generation of the Cordillera Blanca detachment fault.

During the course of an ongoing CCD monitoring program to investigate low-level light variations in subdwarf B (sdB) stars, we have serendipitously discovered a new class of low amplitude, multimode sdB pulsators with periods of the order of an hour. These periods are more than a factor of ten longer than those of previously known multimode sdB pulsators (EC 14026 stars), implying that they are due to gravity modes rather than pressure modes. The longer period pulsators are found only among cooler sdB stars, where they are surprisingly common. The iron opacity instability that drives the short period EC 14026 stars is effective only in hot sdB's, leaving the driving mechanism for the deeper gravity modes in cool sdB's currently unknown. We present the first observational results for our newly identified sdB variables, and discuss possible implications.

[1] The plate boundary between the South American and Nazca plate along the south-central Peru coast has been the site of large destructive earthquakes for many centuries, including the June 23, 2001 (M W = 8.4) event. This underthrusting event has a fault area of 320 km by 100 km based on relocated aftershocks during the first three weeks following the mainshock. Modeling of the teleseismic broadband P waves of the 2001 Peru earthquake indicates two pulses of moment release with the larger second pulse located 130 km southeast of the mainshock initiation, indicating a unilateral rupture to the southeast. Based on intensity and tsunami reports, previous earthquakes in 1868 and 1604 were larger than the 2001 earthquake, while an event in 1784 was smaller. This provides further evidence that the size of earthquakes along the Peru coast has changed between successive earthquake cycles. INDEX TERMS:7209 Seismology: Earthquake dynamics and mechanics; 7215 Seismology: Earthquake parameters; 7223 Seismology: Seismic hazard assessment and prediction; 7230 Seismology: Seismicity and seismotectonics.

The provenance of sedimentary strata that accumulate in foreland basins record the growth and denudation of the adjacent orogen. We use U-Pb geochronology of detrital zircon and monazite, Sm-Nd isotope geochemistry, trace-and rare earth element geochemistry, and petrographic data from synorogenic clastic sedimentary rocks in the Cordilleran foreland basin of southwestern Alberta to provide new perspectives on the evolution of the orogen. Foreland basin clastic rocks comprise three major pulses of sediment delivery: (1) upper Fernie Formation-Kootenay Group (154-142 Ma), (2) Blairmore Group (115-103 Ma), and (3) Milk River Group-Porcupine Hills Formation (78-58 Ma). Nd isotope data are dominated by ε Nd values of -7 to -12, interpreted to represent a well-mixed provenance from Devonian through Triassic strata and subordinate contributions from thrust-imbricated pre-Devonian strata of the Cordilleran miogeocline. Signifi cant deviations to less negative (more juvenile) values between -5 and +1 represent periods when the foreland was fl ooded by juvenile detritus from oceanic arc sources such as Quesnel terrane and from syndepositional continental magmatic arcs of mid-and Late Cretaceous ages.Detrital zircon and monazite from the Fernie-Kootenay clastic pulse (pulse 1) indicate derivation from Triassic-Ordovician sandstones imbricated within the thrust-and-fold belt, consistent with the Nd tracer results and petrography. U-Pb zircon ages from the Blairmore Group (pulse 2) confi rm a provenance from Triassic and Jurassic arc rocks of Quesnel terrane with only minor contributions from older miogeoclinal rocks; they also record the presence of syndepositional magmatic material. The upper part of the Blairmore Group shows a transition to less juvenile Nd isotopic signatures and the reappearance of detrital zircons of miogeoclinal derivation. A similar pattern occurs in the Milk River-Porcupine interval (pulse 3) with juvenile material occurring early in the sequence, accompanied by zircon grains from syndepositional volcanic sources and by more continental material in the upper part of the sequence.Pulse 1 records the erosion of thrust-imbricated miogeoclinal rocks during the creation and erosion of the foreland thrust-and-fold belt with no detectable material derived from the deeper parts of the hinterland. A significant unconformity of ~27 m.y. duration led to redistribution of the foreland basin fi ll and erosion of the adjacent thrust-and-fold belt and corresponds to a period of magmatic and tectonic quiescence in the southern Canadian Cordillera. Renewed contraction within the erosionally modifi ed thrust wedge led to development of out-of-sequence thrust structures which allowed juvenile terranes (Quesnel terrane) to become the dominant source for foreland (pulse 2) to the exclusion of miogeoclinal material. Reappearance of the miogeoclinal signature in upper pulse 2 is interpreted to record eastward propagation of the thrust-and-fold belt into miogeoclinal strata. The third pulse of sediment records signifi cant ...

A new class of pulsating subdwarf B stars has recently been announced by Green and coworkers. Here we present a follow-up paper describing our observations and the pulsation structure of the class prototype PG 1716+426. The oscillations are multiperiodic with periods between 0.8 and 1.4 hr (180-340 Hz) and semiamplitudes less than 0.2%. We also observe that the periods and amplitudes appear variable, making the pulsation structure of PG 1716 complicated. The periods are an order of magnitude longer than those seen in EC 14026 (sdBV) stars, implying that they are gravity modes rather than pressure modes. As such, they represent a new class of variable star.