Tim P. Dooley

Analog Modeling of Pull-Apart Basins

Scaled sandbox models have successfully simulated the geometries and progressive evolution of pull-apart basins developed in a weak sedimentary cover above right-stepping (releasing) dextral strike-slip fault systems in rigid basement. Synkinematic basin infill was added progressively as the models were deformed. Vertical and horizontal sectioning of the completed models has allowed the full three-dimensional architecture of the pull-apart system to be analyzed. We present three representative end-member experiments: 30° underlapping releasing sidestep, 90° sharp nonoverlapping releasing sidestep, and 150° overlapping releasing sidestep. The pull-apart basin geometries are typically sigmoidal to rhombic grabens, the geometries of which are dependent upon the architecture of the underlying basement fault systems. Underlapping releasing sidestepping faults (offset angles of 30-75°) typically form elongate rhomboidal grabens; 90° releasing offset sidesteps produce shorter, squat, rhomboid pull-apart basins; and overlapping releasing sidesteps (115-150°) produce box-like grabens with highly kinked basin sidewalls. The pull-apart basins are bounded by terraced oblique-slip extensional sidewall fault systems that link the laterally offset principal displacement zones (PDZ) of the main dextral strike-slip faults. The sidewall faults show changes in kinematics from dominantly dip-slip extension in ©Copyright 1997. The American Association of Petroleum Geologists. All rights reserved.1Manuscript received June 24, 1996; revised manuscript received November 20, 1996; final acceptance June 10, 1997. 2Fault Dynamics Project, Geology Department, Royal Holloway, University of London, Egham, Surrey TW20 OEX, United Kingdom. Supported by the Fault Dynamics Project [sponsored by ARCO British Limited, PETROBRAS U.K. Ltd., BP Exploration, Conoco (U.K.) Limited, Mobil North Sea Limited, and Sun Oil Britain]. We also acknowledge additional funding from Sun Oil Britain. Landsat Thematic Mapper data courtesy of RTZ Mining and Exploration Limited, South American Division. Jurrian Reijs is thanked for field data relating to the Salina del Fraile pull-apart basin. Chris Elders is thanked for reviewing earlier versions of this manuscript, and J. Crowell, C. Morley, and K. Biddle are thanked for constructive reviews of the manuscript. The contents of this paper have been

Deep-water polygonal fault systems as terrestrial analogs for large-scale Martian polygonal terrains

Discovery of giant polygonal terrains on Mars has prompted a 30-year debate over how they formed. The prevailing hypothesis is that small-scale Martian polygons formed by thermal contraction, as in terrestrial permafrost environments. Large-scale (>1 km) Martian polygons in the northern plains are visible in THEMIS, MOLA, Viking, and Mariner data, but how they formed remains enigmatic. We suggest that terrestrial deep-water marine polygons are morphological and perhaps genetic analogs to largescale Martian polygonal features. The terrestrial, deep-water polygons are imaged in three-dimensional seismic-reflection data acquired by the oil and gas industry in offshore Norway and the Gulf of Mexico. How deep-water polygonal fault systems form is a debated topic beyond the scope of this work. However, similarities between terrestrial deep-water polygonal fault systems and large-scale Martian polygonal terrains suggest that the latter could have formed during deep-water marine deposition. Deep-water polygonal faults form within fine-grained sediment at shallow burial depths. Increases in slope angles can trigger downslope disaggregation of deep-water polygons and mass wasting (forming debris flows). Physical models indicate that multidirectional extension can cause polygonal features to break up on a slope over a mobile substrate. Some knobby terrains in the Vastitas Borealis Formation seem to originate from disaggregation of large-scale Martian polygonal terrains. These analogies suggest a possible deep-water subaqueous origin for large-scale Martian polygonal terrains and support the idea of a late Hesperian–early Amazonian ocean on the northern plains of Mars.

4-D evolution of rift systems: Insights from scaled physical models

The four dimensional (4-D) evolution of brittle fault systems in orthogonal, oblique, and offset rift systems has been simulated by scaled sandbox models using dry, cohesionless, fine-grained quartz sand. Extensional deformation in the models was controlled by the orientation and geometry of a zone of stretching at the base of the model. The results of these analog model studies are compared with natural examples of rift fault systems. Rift basins produced by orthogonal and oblique rifting are defined by segmented border fault systems parallel to the rift axes and by intrarift fault systems that are subperpendicular to the extension direction. Segmentation of the rift margin increases with increase in obliquity of the rift axis, resulting in a consequent increase in displacement on intrarift fault systems. Offset rift models are characterized by highly segmented border faults and offset subbasins in the rift zone. Along-strike displacement transfer in the model rifts occurred as a result of formation of two types of accommodation zones. High-relief, extension-parallel accommodation zones typically are found in 60 degrees rifts and above left steps in offset rift systems. Changes in fault polarities in these accommodation zones were achieved by interlocking arrays of conjugate extensional faults. The second type of accommodation zone was generally oblique to the extension direction and consisted of conjugate fault arrays having rotated tips that bounded a low-relief oblique-slip zone or grabens. These typically are found in highly oblique rift systems (<45 degrees) and above right steps in offset rift models.

Analogue models of pull-apart basins

Sandbox analogue models of pull-apart basins that developed in sedimentary strata above releasing steps in underlying basement faults are characterized by rhombic basins that are flat-bottomed box grabens with a subhorizontal synkinematic basin infill. Steep to nearly vertical, sigmoidal oblique-slip and segmented oblique-extensional faults are the dominant bounding structures of the pull-apart basins. Cross-basin, short-cut faults link the offset principal displacement zones that are characterized by flower structure development. The structural architectures of the physical models compare directly in form and dimensions to natural examples of strike-slip pull-apart basins.

Analog modeling of progradational delta systems

Scaled physical models of progradational sand wedges above a ductile polymer substrate have successfully simulated the formation of delta-top graben systems synchronous with the development of delta-toe fold-thrust belts. delta-top graben systems are characterized by paired regional and counter-regional listric growth faults. The latter form without total removal of polymer from beneath the graben system. The delta-toe fold-thrust belts are formed by polymer bulges and fault-related detachment folds that have thickening of the ductile polymer in their cores. Multiload wedge models show the formation of younger delta-top extensional graben systems above older fold-thrust belts. The analog model results show structures similar to those found in many delta systems and provide kinematic models for their evolution.

Quantifying the origin and geometry of circular sag structures in northern Fort Worth Basin, Texas: Paleocave collapse, pull-apart fault systems, or hydrothermal alteration?

Three-dimensional seismic data reveal numerous subcircular sag structures in the northern Fort Worth Basin. The structures are defined by concentric faults that extend vertically upward 760–1060 m (2500–3500 ft) from the Lower Ordovician Ellenburger Group. The largest structures remained active into the lower Desmoinesian Strawn Group. Criteria are outlined for defining seismically resolvable sag structures, and a detailed quantitative analysis of the geometries of these circular features was undertaken. Results are compared and contrasted, with reviews of subsurface collapse mechanisms and strike-slip processes that are known to produce subsurface circular to subcircular sag geometries in plan view. In this manner, we develop several constraints for differentiating collapse-related sag structures from strike-slip–related sag structures. Qualitative analyses indicate that the geometries observed are strongly analogous to subsurface collapse features where material is removed at depth to create a void, into which the overburden subsequently sags and collapses. Quantitative analyses support the formation of these features by incremental collapse and suprastratal deformation above a linked system of coalesced, collapsed paleocaves within the Ellenburger Group. Observations and criteria presented herein provide valuable information in defining seismically resolvable collapse features worldwide and help distinguish sag features of collapse affinity from those of other origins.

Strain partitioning in gravity-driven shortening of a thick, multilayered evaporite sequence

Abstract Three-dimensional seismic data from the Levant Basin, eastern Mediterranean, was used to quantify longitudinal strains in thick, multilayered Messinian evaporites at an early stage of salt tectonics. Gravity-spreading is driven by basin subsidence and tilting of the Levant margin and by progradation of the Nile Cone. Similar styles of shortening in two separate 3D survey areas comprise detachment folds, thrust-ramp folds and conjugate arrays of strike-slip faults. These Pleistocene structures can be explained with a single deformation phase with a tectonic transport direction of NE to ENE, obliquely opposed to the extension updip, which began in the Late Pliocene. Four major detachments within the Messinian are probably halite-rich intervals in the multilayer. Shortening of competent interlayers varies from 1–2% near the base to c. 7% near the top of the Messinian, with a sharp reduction in shortening at the top Messinian and roof to 1–2%. This shortening profile is attributed to asymmetric Poiseuille flow, indicating that salt is flowing downdip faster than the overburden is translating. Physical modelling supports the inferred flow profile, showing that each mobile layer flows faster than adjoining competent layers and that strains in evaporites can be far greater than in the overburden. This is the first published use of seismic data to demonstrate the flow regime within salt on a regional scale.

The structure and evolution of sutures in allochthonous salt

Salt canopies, formed by the coalescence of salt sheets, are an integral part of the slope and deep-water areas of many passive margin salt basins. A suture separates the two coalesced salt sheets (allosuture) or two lobes from a single salt sheet (autosuture), including any trapped sediments. Autosutures can form in two ways. An overriding autosuture is produced when part of a salt sheet overrides its neighbor in the direction of salt movement. The overridden roof subsides into the salt sheet, and these trapped sediments appear as intrasalt reflections on seismic data. An encircling autosuture forms when two lobes of a salt sheet separate to bypass an obstacle and then rejoin on the downstream side of the obstacle. Encircling autosutures tend to be short and parallel to the dominant salt-flow direction. Allosutures separate sheets sourced from two different feeders. If neither salt sheet overrides the other, the resulting suture is symmetric, forming an upright zone of roof sediments trapped between the two sheets. More typically, one salt sheet is more vigorous (generally the larger sheet or the one whose feeder is farther updip) and overrides the other. Sediments trapped in an asymmetric allosuture are mostly from the roof of the overridden sheet. The overriding sheet shears and extends the roof of the overridden sheet, detaching it from the base of the canopy and obscuring its origin. We present diagnostic criteria to distinguish between suture types and provide physical-model examples of each. This distinction between suture types is important because autosutures and allosutures have very different implications for canopy dynamics and evolution.

Salt tectonics and collapse of Hebes Chasma, Valles Marineris, Mars

A photogeologic and physical modeling study indicates that Hebes Chasma, Mars, formed by collapse of the megaregolith. Local heating facilitated drainage of ~10 5 km 3 of brines and entrained particulates through fractures in the chasma floor and into a regional aquifer. A megaregolith rich in salts and water is implied by massive, low-gradient allochthonous flows that terminate in deep pits and troughs, by emergent diapirs, and by arching of Hebes Mensa. These structures are consistent with plastic and viscous deformation but inconsistent with collapse of basalt flows and/or tephra. Spectral measurements confirm that hydrated sulfate salts are spatially associated with allochthonous flows from depth and light-toned deposits. Collapse features and flows are present in many other chasmata in Valles Marineris, suggesting that widespread salt tectonics and dissolution may have shaped the region.

Analogue models of delta systems above ductile substrates

Abstract Delta systems developed above ductile substrates such as overpressured shales and salt have been modelled using layered sand-packs above ductile silicone polymer layers. Gravity spreading of progradational sedimentary wedges produces delta-top and upper delta-slope grabens linked to delta-toe contractional fold-thrust and diapir zones. The delta-top grabens are bound by both regional and counter-regional listric growth faults. A basinward-stepping sequence of regional, counter-regional followed by regional faulting is commonly developed. Polymer pillows and ridges commonly develop in the footwalls of the major listric extensional faults and may evolve into reactive diapirs. Successive progradational loads generate new delta-top or upper delta-slope graben systems on top of older contractional belts where the ductile polymer layer has been thickened significantly. The analogue model results in cross-section show many similarities to examples of natural deltas and differential sedimentary load systems such as offshore Angola, the Niger and Nile Deltas, Kutai Basin, Kalimantan, the Baram delta, Brunei and the Orinoco delta, Columbus basin and offshore Trinidad.