Thomas F. Bullard

Quaternary uplift astride the aseismic Cocos Ridge, Pacific coast, Costa Rica.

The Pacific coast of Costa Rica lies within the Central American forearc and magmatic-arc region that was created by northeastward subduction of the Cocos plate beneath the Caribbean plate at the Middle America Trench. From the Peninsula de Nicoya south-eastward toward the Peninsula de Osa and the Peninsula de Burica on the Panamanian border, the Middle America Trench loses its physiographic expression where it intersects the aseismic Cocos Ridge. Interaction between subduction of the buoyant, aseismic Cocos Ridge and the overriding Caribbean plate is invoked to explain the variation in rates of vertical crustal uplift along a coastal transect from Nicoya to Burica. The Pliocene and Pleistocene stratigraphic record and Holocene marine terraces and beach ridge complexes indicate that maximum rates of crustal uplift have occurred on the Peninsula de Osa, immediately landward of the aseismic Cocos Ridge. Crustal uplift rates decrease northwest toward the Peninsula de Nicoya, and to a lesser extent southwest toward the Peninsula de Burica. The late Quaternary stratigraphy on the Peninsula de Osa is subdivided into two major chronostratigraphic sequences from groupings of radiocarbon dates. Crustal uplift rates calculated from these sequences systematically decrease from 6.5 to 2.1 m/ka north-east across the peninsula. Deformation of the peninsula is modeled as uplifted and down-to-the-northeast-tilted fault blocks with an angular rotation rate of 0.03° to 0.06° per thousand years. Although less well constrained, crustal uplift rates on the Peninsula de Nicoya, 200 km to the northwest of the Peninsula de Osa, vary from <1 m/ka for Pliocene and Pleistocene sediments to 2.5 m/ka for Holocene marine terraces. In the Quepos region, 100 km to the northwest of the Peninsula de Osa, calculated uplift rates derived from incision of late Quaternary fluvial terraces range from 0.5 to 3.0 m/ka. On the Peninsula de Burica, only 60 km to the southwest of the Peninsula de Osa, calculated uplift rates range from 4.7 m/ka for a late Holocene marine terrace to 1.2 m/ka for post-late Pliocene deep-sea sediments. The variations in calculated uplift rates on the Peninsula de Osa constrain a dynamic model for subduction of the Cocos Ridge and the resulting uplift of the overriding Caribbean plate. Deflection of the Caribbean plate is modeled using various effective elastic thicknesses as the response of an elastic plate to the buoyant force of the subducted Cocos Ridge. Because the shape of the subducted end of the Cocos Ridge is unknown, two scenarios are evaluated: (1) a radially symmetric ridge with a slope similar to the slope of the flanks of the ridge and (2) a ridge where the subducted end was truncated by the Panama fracture zone. The best-fit model utilizes a truncated ridge that has been subducted during the past 0.5 m.y. ∼50 km beneath the overriding Caribbean plate, which has an effective elastic thickness of 5 km. The model predicts that the highest uplift rate should be ∼3.7 m/ka and occur on the southwest coast of the Peninsula de Osa. The rate of uplift slows considerably to the northeast and indicates that the Peninsula de Osa is tilting to the northeast, which agrees with observations in that region. The predicted uplift rate attributed to aseismic ridge subduction also decreases along the coast both north and south of the Peninsula de Osa, resulting in little uplift that can be attributed to Cocos Ridge subduction in the northwestern portions of the Peninsula de Nicoya.

Regional variations in tectonic geomorphology along a segmented convergent plate boundary, Pacific coast of Costa Rica

Abstract Pacific coastal mountain/piedmont landforms of Costa Rica extend across the tectonic boundary between the forearc and magnetic arc region of an active convergent margin. This plate boundary became segmented circa 1 million years ago when the aseismic Cocos Ridge impinged upon the Middle America Trench offshore from the southernmost coastal area of Costa Rica. Morphometric analyses of 100 mountain fronts and numerous river long-profiles, radiometric dating, and field studies were conducted in two study areas located arcward from the plate boundary where oceanic lithosphere of the Cocos plate is being subducted beneath the Caribbean plate (region I) and the partially subducted aseismic ridge is uplifting the plate margin by isostatic and collisional processes (region II). Values of tectonic geomorphic parameters [mountain front sinuosity (S), percent dissected facets (Ffd), river concavity (K)] are not only different statistically in regions I and II but are also different in the areas experiencing isostatic and collisional responses to the subducting aseismic ridge. In the area experiencing collisional responses, mountain fronts, developed along NE-dipping imbricate thrust and high-angle reverse faults, step upward and inland from the coast; morphometric data along with the divergence of river-terrace profiles from the coast piedmont inland toward the mountains indicates higher uplift rates along interior-range mountain fronts. Isostatic uplift in the outer forearc area in region II produces a distinctly different morphologic and neotectonic style characterized by regional uplift distributed across a number of blocks bounded by normal faults. Geomorphic analyses indicate a general southward trend of increasing tectonic uplift from region I into region II where the highest frequency of mountain fronts with low values of S and Ffd, as well as rivers with the highest values of K, occur over the crest of the subducted ridge. Field and historical seismic data for these regional trends include: (1) fault scarps displacing late Quaternary fluvial terraces and colluvial soils in areas of collisional responses; (2) Holocene-latest Pleistocene marine sediments uplifted tens of meters along normal faults in areas of isostatic response of region II; and (3) more frequent shallow, high-magnitude earthquakes in region II. This study indicates that spatial variations in the plate tectonic framework can be detected by regional morphometric analyses using techniques applied in extensional and compressional terranes of arid and semiarid regions but not previously applied to forearc systems along convergent plate boundaries in tropical areas.

Predictive Soil Maps Based on Geomorphic Mapping, Remote Sensing, and Soil Databases in the Desert Southwest

We present an expert based system to rapidly predict the shallow soil attributes that control dust emissions in the arid southwest U.S. Our system’s framework integrates geomorphic mapping, remote sensing, and the assignment of soil properties to geomorphic map units using a soil database within a geographic information systems (GIS) framework. This expert based system is based on soil state factor-forming model parameters that include: (1) climate data, (2) landform, (3) parent material, and (4) soil age. The four soil-forming data layers are integrated together to query the soil database. To validate the accuracy of the expert based model and resultant predictive soil map, a blind test was performed at Cadiz Valley in the Mojave Desert, California. The desert terrain in Cadiz Valley consists of alluvial fans, fan remnants, sand dunes, and playa features. The test began with three users independently mapping an area of over 335 km2 using 1:40,000-scale base maps to rapidly create geomorphic and age class layers, and then integrating these with climate and parent material layers. The results of the four data layers were then queried in the soil data base and soil attributes assigned to map unit layers. The soil-forming model presented here is geomorphic-based, and considers soil age as a significant factor in accurately predicting soil conditions in hyper arid to mildly arid regions. This work comprises a successful first step in the development of an expert-based system to map shallow soil conditions in support of dust emission models in remote desert regions.

Military Geosciences and Desert Warfare

Gallipoli continues to be a cause célèbre for those seeking to assign blame for this ill-fated military campaign fought against the Ottoman Empire from April to December 1915. Variously blamed are weak generals, poor planning and preparation—and even inadequate topographical mapping. Intended to assist the Allied naval fleet in breaking through the Dardanelles Straits, thereby threatening the Ottoman Capital of Constantinople (and, it was hoped, forcing the Ottomans out of the war), the military campaign was certainly hastily conceived and underresourced. Commencing on 25 April 1915 as an amphibious landing, the campaign soon degenerated into a desperate struggle, as the Allies attempted in vain to break out of tightly constrained beachheads. This study investigates the role of terrain in the warfare of the ANZAC (Australian and New Zealand Army Corps) Sector, from initial landings in April, to attempted breakout in August. At ANZAC, an ‘unfortunate accident of geography’ brought, dry, mostly fine-grained Pliocene sediments to the coast. An upland area created by the North Anatolian Fault System, the fine sediments were (and are) quickly weathered and eroded to form topographically complex gullied surfaces. This would be the almost hopeless battleground of the Australians and New Zealanders in April–December 1915. With the Ottomans holding a firm grip on the ridge top, the ANZAC troops were constrained to a small, deeply dissected and mostly waterless sector of the scarp slope of the Sari Bair Plateau and ridge system. The war here would be hard fought and bloody, with geology having a major impact on its outcome; the withdrawal of ANZAC troops in December 1915.

Development of an Archeological Predictive Model for Management of Military Lands

A framework has been developed for an archeological predictive model based on demonstrated relations among multiple geologic variables (e.g., topography, geochronology, rock type, geomorphology) for 81 previously identified cultural resource sites across diverse desert terrain. Results indicated that the most useful variables are deposit type, piedmont setting, geometric form, deposit age, surface age, desert pavement, surface horizon, and strongest subsoil horizon. Traditional sources for geologic and soil data available to most military installations are of low resolution or are incomplete. Rapid and cost-effective methods for collecting these data in desert terrains will be required to advance and implement archeological predictive models. The new model was applied initially to desert terrain conditions at the US Army National Training Center, Fort Irwin, CA, but it has potential for application across military lands throughout the southwestern US where as much as 80% of the holdings have not been inventoried for cultural resources.