Peter B. Sak

Effect of subducting sea-floor roughness on fore-arc kinematics, Pacific coast, Costa Rica

Fault kinematics and uplift in the Costa Rican fore arc of the Middle America convergent margin are controlled to a large extent by roughness on the subducting Cocos plate. Along the northwest flank of the incoming Cocos Ridge, seafloor is characterized by short wavelength roughness related to northeast-trending seamount chains. Onland projection of the rough subducting crust coincides with a system of active faults oriented at high angles to the margin that segment the fore-arc thrust belt and separate blocks with contrasting uplift rates. Trunk segments of Pacific slope fluvial systems typically follow these margin-perpendicular faults. Regionally developed marine and fluvial terraces are correlated between drainages and across faults along the Costa Rican Pacific coast. Terrace separations across block-bounding faults reveal a pattern of fore-arc uplift that coincides roughly with the distribution of incoming seamounts. Magnitude and distribution of Quaternary uplift along the Costa Rican Pacific coast suggests that, despite a thin incoming sediment pile, the inner fore arc shows an accumulation of mass—a characteristic that may be due to underplating of seamounts beneath the fore-arc high.

Basalt weathering rates on Earth and the duration of liquid water on the plains of Gusev Crater, Mars

Where Martian rocks have been exposed to liquid water, chemistry versus depth profi les could elucidate both Martian climate history and potential for life. The persistence of primary minerals in weathered profi les constrains the exposure time to liquid water: on Earth, mineral persistence times range from ~10 k.y. (olivine) to ~250 k.y. (glass) to ~1 m.y. ( pyroxene) to ~5 m.y. (plagioclase). Such persistence times suggest mineral persistence minima on Mars. However, Martian solutions may have been more acidic than on Earth. Relative mineral weathering rates observed for basalt in Svalbard (Norway) and Costa Rica demonstrate that laboratory pH trends can be used to estimate exposure to liquid water both qualitatively (mineral absence or presence) and quantitatively (using reactive transport models). Qualitatively, if the Martian solution pH >~2, glass should persist longer than olivine; therefore, persistence of glass may be a pH indicator. With evidence for the pH of weathering, the reactive transport code CrunchFlow can quantitatively calculate the minimum duration of exposure to liquid water consistent with a chemical profi le. For the profi le measured on the surface of the exposed Martian rock known as Humphrey in Gusev Crater, the calculated exposure time is 22 k.y., which is a minimum due to physical erosion. If correct, this estimate is consistent with short-term, episodic alteration accompanied by ongoing surface erosion. More of these depth profi les should be measured to illuminate the weathering history of Mars.

Active thrusting in the inner forearc of an erosive convergent margin, Pacific coast, Costa Rica

(1) Structural and geomorphic analyses of the Fila Costena thrust belt in southwest Costa Rica indicate active thrusting within the inner forearc. The Fila Costena exposes three major thrust faults that imbricate the late Tertiary forearc basin sequence of the Terraba basin. The frontal thrust of the Fila Costena marks the boundary between an uplifting inner forearc and a subsiding outer forearc, with only local uplift astride the indenting Cocos Ridge. On the basis of surface constraints a cross section across the thrust belt suggests that all three thrusts flatten into parallelism with a low-angle decollement horizon near the contact between the basement and the cover sequence of the Terraba basin. This decollement lies at a depth of � 4 km. The minimum shortening recorded by restoration of fault-related folds is 17 km, or 45%. Observations of late Tertiary marine sediments, tilted and faulted late Quaternary fluvial terraces, and raised Holocene marine terraces indicate that Fila Costena uplift was likely initiated in the Quaternary and is ongoing. Given that the coastal mountains are separated from the Talamanca Range by a valley, the decollement that delaminates the forearc basin from the underthrusting forearc must continue as a flat beneath the valley but must link with the plate boundary along a crustal-scale ramp system, a structural geometry that has resulted in uplift of the Talamanca Range, the highest peaks in Central America. The dichotomy between uplift in the inner forearc and subsidence in the outer forearc is explained in terms of the response of an arcward thickening wedge to rough, subducting crust. INDEX TERMS: 8150 Tectonophysics: Plate boundary—general (3040); 8123 Tectonophysics: Dynamics, seismotectonics; 8010 Structural Geology: Fractures and faults; 8015 Structural Geology: Local crustal structure; 8005 Structural Geology: Folds and folding;

Rough crust subduction, forearc kinematics, and Quaternary uplift rates, Costa Rican segment of the Middle American Trench

Orthogonal subduction of bathymetrically rough oceanic lithosphere along the northwestern flank of the Cocos Ridge imprints a distinctive style of deformation on the overriding Costa Rican forearc. We divide the Costa Rican forearc into three 100–160-km-long deformational domains based on the bathymetric roughness and thickness of the Cocos plate entering the Middle American Trench, the dip of the subducting plate, the variation in surface uplift rates of late Quaternary coastal deposits, and the orientations and types of faults deforming Paleogene and Neogene sedimentary rocks. In the ~100-km-long Nicoya domain, coastal deposits show localized surface uplift and arcward tilting above the downdip projections of the fossil trace of the Cocos-Nazca-Panama (CO-NZ-PA) triple junction and the Fisher seamount and ridge. In the ~120-km-long central Pacific forearc domain between the Nicoya Peninsula and Quepos, shallower (~60°) subduction of seamounts and plateaus is accompanied by trench-perpendicular late Quaternary normal faults. Steeply dipping, northeast-striking, margin-perpendicular faults accommodate differential uplift associated with seamount subduction. Uplift and faulting differ between the segments of the forearc facing subducting seamounts and ridges. Inner forearc uplift along the seamount-dominated segment is greatest inboard of the largest furrows across the lower slope. Localized uplift and arcward tilting of coastal deposits is present adjacent to subducting seamounts. In contrast, inboard of the underthrusting aseismic Cocos Ridge, along the ~160-km-long Fila Costena domain between Quepos and the Burica Peninsula, mesoscale fault populations record active shortening related to the ~100-km-long Fila Costena fold-and-thrust belt. The observed patterns of faulting and permanent uplift are best explained by crustal thickening. The uplifted terraces provide a first-order estimate of permanent strain along the forearc in Costa Rica. The permanent strain recorded by uplift of these Quaternary surfaces exceeds the predicted rebound of stored elastic strain released during subduction-zone earthquakes.

Holocene forearc block rotation in response to seamount subduction, southeastern Península de Nicoya, Costa Rica

The southeastern tip of the Peninsula de Nicoya, Costa Rica, on the Caribbean plate margin lies inboard of the rough bathy- metric terrain on the subducting Cocos plate and along the land- ward projection of the convergence vector for the Fisher seamount group. The southern tip of the peninsula has nearly orthogonal coastlines and extensive, well-preserved, Holocene marine terraces,

Effects of subducting seafloor roughness on upper plate vertical tectonism: Osa Peninsula, Costa Rica

[1]xa0Subduction of seamounts and ridges along thinly sedimented convergent margins results in deformation of the overriding forearc. Exposures of newly recognized late Pleistocene, shallow water deposits (i.e., Marenco formation) record intervals of rapid subsidence and uplift across the Costa Rican forearc inboard of the subducting Cocos Ridge. In general, the Marenco formation is a fining upward, fossiliferious, late Pleistocene, marine sand disconformably overlying beveled surfaces cut across the competent Osa melange basement. The ∼50 to 27 ka age of the Marenco formation is constrained by 12 accelerator mass spectrometry and two conventional 14C dates obtained on marine macrofossils. The deposition of this sequence coincident with a general fall in sea level during oxygen isotope stage 3 requires >6 mm yr−1 subsidence inboard of the northwest flank of the subducting Cocos Ridge. Presently, exposures of the Marenco formation are found at >75 m above sea level, requiring uplift rates in excess of 6 mm yr−1. We interpret the down and up history of vertical tectonism recorded by the Marenco formation as the response of the upper plate to variations in the elevation of the subducting Cocos Ridge. On the basis of a model where the upper plate deforms through bends because of roughness on a rigid downgoing plate, the rate, duration, and spatial distribution of vertical tectonism across the forearc are determined by the magnitude of the orthogonal component of the relative convergence vector and the bathymetry of the underthrusting plate. Application of this model to bathymetric data for the Cocos plate offshore yields a broad agreement between predicted future rates of subsidence and uplift and rates over the last 50 kyr recorded by the Marenco formation. Furthermore, analysis suggests that the arrival of the blunt-tipped leading edge of the Cocos Ridge (0.5–3 Ma) resulted in an initial period of very rapid (∼30 mm yr−1) uplift.

Rapid pulses of uplift, subsidence, and subduction erosion offshore Central America: Implications for building the rock record of convergent margins

Integrated Ocean Drilling Program (IODP) Expedition 334 to southern Costa Rica, Central America, documented unprecedented subduction erosion in an area of active seismic slip. Widespread subduction erosion of the upper plate initiated when the Cocos Ridge, an overthickened aseismic ridge, arrived at the Middle America Trench. Subduction erosion was coeval with the rapid formation of deposition centers on the forearc of the upper plate. The completely recovered shelf sequence constrains a short (<2 m.y.) interval of extreme subsidence (∼1200 m) with a rapid pulse occurring during the first ∼0.3 m.y. This event removed an estimated 1.2 × 106 km3 of forearc material at a rate of ∼1690 km3/m.y./km of trench during a time of rapid (∼1035 m/m.y.) shelf sediment accumulation. At this erosive margin, a sediment-starved trench persisted, in spite of abundant sediment supply, because subduction erosion led to the creation of forearc basins. Similar rapid pulses of subduction erosion may punctuate the evolution of many margins, contributing disproportionately to crustal recycling at subduction zones with implications for the evolution of continental crust and mountain belts, and recycling of continental material into the mantle.

Coal-Tar-Based Sealcoated Pavement: A Major PAH Source to Urban Stream Sediments

We used land-use analysis, PAH concentrations and assemblages, and multivariate statistics to identify sediment PAH sources in a small (~1303 km(2)) urbanizing watershed located in South-Central, Pennsylvania, USA. A geographic information system (GIS) was employed to quantify land-use features that may serve as PAH sources. Urban PAH concentrations were three times higher than rural levels, and were significantly and highly correlated with combined residential/commercial/industrial land use. Principal components analysis (PCA) was used to group sediments with similar PAH assemblages, and correlation analysis compared PAH sediment assemblages to common PAH sources. The strongest correlations were observed between rural sediments (n = 7) and coke-oven emissions sources (r = 0.69-0.78, n = 5), and between urban sediments (n = 22) and coal-tar-based sealcoat dust (r = 0.94, n = 47) suggesting that coal-tar-based sealcoat is an important urban PAH source in this watershed linked to residential and commercial/industrial land use.

Unraveling the central Appalachian fold-thrust belt, Pennsylvania: The power of sequentially restored balanced cross sections for a blind fold-thrust belt

We present a kinematic model for the sequential development of the Appalachian fold-thrust belt (eastern U.S.) across a classic transect through the Pennsylvania salient. New map and strain data are used to create a balanced geologic cross section from the southern edge of the Valley and Ridge Province to the northern Appalachian Plateau. This region of the central Appalachian fold-thrust belt is an ideal location to illustrate the incorporation of strain data in balanced cross sections, because it cannot be balanced without quantifying grain-scale strain. We use a sequentially restored, balanced cross section to show how layer-parallel shortening (LPS) is distributed above and ahead of thrust and fold shortening and constrain the geometric and kinematic evolution of a passive roof duplex. By combining line length and area balancing of a kinematically viable cross section with LPS estimates in both the Valley and Ridge Province (20%) and Appalachian Plateau (13%), we document the total magnitude of shortening in both the folded cover sequence and the duplexed lower layer of the fold-thrust belt. Restoration of the cross section indicates a total of 77 km (22%) of shortening between the southern margin of the Valley and Ridge Province in central Pennsylvania and a pin line immediately north of the northern limit of documented LPS in the foreland. The 24 km (13%) of LPS on the Appalachian Plateau is interpreted as being above the Salina (salt) decollement. This magnitude of shortening is 14 km greater than the amount of displacement on the Nittany Anticlinorium, the northernmost structure of the fold-thrust belt that cuts upsection from the Cambrian Waynesboro Formation to the Silurian Salina decollement. Because the fault that cores the Nittany Anticlinorium can only facilitate 10 km of shortening on the plateau, an early history of Appalachian Plateau LPS in Silurian and younger rocks is required to balance the section. We propose that the additional 14 km of LPS on the plateau occurred early in the deformation history and was kinematically linked to two fault-bend folds that have a lower decollement in the Cambrian Waynesboro Formation and an upper, subhorizontal detachment in the Silurian Wills Creek Formation (in the Valley and Ridge) and the Salina Group on the Appalachian Plateau. This upper detachment feeds displacement from these early horses in the duplex system onto the Appalachian Plateau and is expressed there as LPS shortening. This early shortening is followed by the development of in-sequence horses that repeat the mainly thrust-faulted Cambrian–Ordovician sequence using both the main decollement in the Cambrian Waynesboro and the Ordovician Reedsville Formations as an upper detachment horizon. In the south, shortening in the Late Ordovician through Devonian layers is accommodated by both LPS and forced folding of the overlying folded cover sequence. We propose that the Reedsville Formation becomes weaker to the north, facilitating shorter wavelength detachment folds. The development of gentle open folds on the Appalachian Plateau, as well as the last 10 km of LPS on the plateau, is linked to the most forelandward horse in the duplex. This horse forms the broad Nittany Anticlinorium, the northern boundary of the Valley and Ridge.

Significance of the Deformation History within the Hinge Zone of the Pennsylvania Salient, Appalachian Mountains

Two competing models exist for the formation of the Pennsylvania salient, a widely studied area of pronounced curvature in the Appalachian mountain belt. The viability of these models can be tested by compiling and analyzing the patterns of structures within the general hinge zone of the Pennsylvania salient. One end-member model suggests a NW-directed maximum shortening direction and no rotation through time in the culmination. An alternative model requires a two-phase development of the culmination involving NNW-directed maximum shortening overprinted by WNW-directed maximum shortening. Structural analysis at 22 locations throughout the Valley and Ridge and southern Appalachian Plateau Provinces of Pennsylvania are used to constrain orientations of the maximum shortening direction and establish whether these orientations have rotated during progressive deformation in the Pennsylvania salient’s hinge. Outcrops of Paleozoic sedimentary rocks contain several orders of folds, conjugate faults, steeply dipping strike-slip faults, joints, conjugate en echelon gash vein arrays, spaced cleavage, and grain-scale finite strain indicators. This suite of structures records a complex deformation history similar to the Bear Valley sequence of progressive deformation. The available structural data from the Juniata culmination do not show a consistent temporal rotation of shortening directions and generally indicate uniform, parallel shortening directions consistent with the single-phase model for development of the Pennsylvania salient.