Mario Araujo

Basement seismicity beneath the Andean precordillera thin-skinned thrust belt and implications for crustal and lithospheric behavior

Data from a digitally recording seismic network in San Juan, Argentina, provide the first images of crustal scale basement faults beneath the Precordillera. This seismicity is near the boundary between the Precordillera (a thin-skinned thrust belt) and the Sierras Pampeanas (a region of thick-skinned basement deformation), two seismically active tectonic provinces of the Andean foreland. The seismicity data support models for this region in which crustal thickening, rather than magmatic addition or thermal uplift, plays the dominant mountain building role. The Precordillera seismicity occurs in three segments distributed north to south. The southern segment is an area of diffuse activity extending across the Precordillera and eastward into the Sierras Pampeanas that shows no patterns in map or cross section. The northern and central segments have well-defined dipping planes that define crustal scale faults extending from 5 to 35 km depth. It is clear from the relative fault geometries that the overlying Precordillera is not simply related to the basement activity. The seismicity here may result from reactivation of an ancient suture between the Precordillera and Pampeanas terranes or be occurring in basement of unknown affinity west of the suture. The seismicity provides the first constraints on basement fault geometries, and we present models integrating this information with the surface geology. These basement faults may have been responsible for the 1944 Ms 7.4 earthquake that destroyed the city of San Juan. The imaging of these faults suggests that seismic risk estimates for San Juan made on the basis of surface geologic studies may be too low.

Pegmatite Emplacement in the Seridó Belt, Northeastern Brazil: Late Stage Kinematics of the Brasiliano Orogen

Abstract Geometric and kinematic analysis was performed in an area located in the central part of the Serido Belt (NE Brazil), where supracrustal rocks affected by polyphase deformation are well exposed. The first event recognized in this area (and regionally known as the D 2 deformation) is characterized by top to the south thrust tectonics while a second one (D 3 deformation) is marked by upright folds, strike-slip or transpressive shear zones and the development of flower structures. Major pegmatite swarms were emplaced during and late as regards the second event (dated ca . 580 Ma), being part of the Brasiliano orogeny; similar dyke swarms are known from the Nigerian Shield. These pegmatite swarms provide reliable kinematic markers of the late evolutionary stage of the Neoproterozoic Trans-Sahara-Borborema collisional belt. Mineralogical, geometric and kinematic features support two stages of pegmatite emplacement during the strike-slip event: (i) older, syn-D 3 homogeneous pegmatites intruded mostly along lithological and structural discontinuities, such as foliation surfaces; (ii) late, D 3 heterogeneous pegmatites were emplaced along tension gashes and other dilation structures. The heterogeneous pegmatites are economically more important, being exploited for precious metals and stones, as well as industrial minerals.

The Sergipano Belt

The Neoproterozoic Sergipano is the main orogenic belt between the Borborema province in the north and the Sao Francisco craton in the south. The belt is divisible from north to south into the Caninde, Poco Redondo-Maranco, Macurure, Vaza Barris, and Estância lithostratigraphic domains; the first three are composed of plutonic, volcanic and sedimentary rocks, and the last three of sedimentary rocks. Field relationships combined with zircon geochronology of igneous and sedimentary rocks and whole-rock elemental and isotopic data allow us to propose the following evolution for the Sergipano belt. An early Neoproterozoic (~980–960 Ma) continental arc (Poco Redondo tonalitic gneisses) developed on the margin of the Paleoproterozoic-Neoproterozoic Pernambuco-Alagoas block in the north. Extension of this continental block gave rise to: (i) A-type crustal melt granites and associated sedimentary rocks on the stretched, rifted margin of the Poco Redondo-Maranco domain; (ii) the Caninde rift sequence between the Pernambuco-Alagoas block and the Poco Redondo/Maranco domain; (iii) a passive margin on the southern boundary of the Pernambuco-Alagoas block, upon which sediments were deposited soon after 900 Ma; and (iv) a second sedimentary shelf on the passive margin of the Sao Francisco craton. In the Caninde domain, rifting continued until ca. 640 Ma and led to emplacement of a bimodal association of A-type granites (715 Ma) and continental mafic volcanic rocks, a continental-type layered gabbroic complex (ca. 700 Ma), magma-mingled gabbro/quartz–monzodiorite (688 Ma), and rapakivi granites (684 and 641 Ma). Deformed pillow basalts and interleaved marble lenses are likely ocean floor relicts in the Caninde domain. Closure of the Caninde oceanic basin began at ca. 630 Ma with the intrusion of arc-type granitic plutons in the Macurure (628–625 Ma), Caninde (ca. 621 Ma) and Poco Redondo-Maranco (ca. 625 Ma) domains. Convergence of the Pernambuco-Alagoas block and the Sao Francisco craton led to deformation on the passive margins and granite emplacement (590–570 Ma) mainly in the Macurure domain. Exhumation of the Pernambuco-Alagoas block and the Caninde, Poco Redondo-Maranco, and Macurure domains in the north led to deposition of uppermost clastic sediments in the Estância and Vaza Barris domains in the south, interpreted as a foreland basin, and to final thrusting of the continental margin sedimentary rocks onto the Sao Francisco craton.

Lower Crustal Strength Controls on Melting and Serpentinization at Magma-Poor Margins: Potential Implications for the South Atlantic: LOWER CRUSTAL STRENGTH AND COT STYLE

Rifted continental margins may present a predominantly magmatic continent-ocean transition (COT), or one characterized by large exposures of serpentinized mantle. In this study we use numerical modeling to show the importance of the lower crustal strength in controlling the amount and onset of melting and serpentinization during rifting. We propose that the relative timing between both events controls the nature of the COT. Numerical experiments for half-extension velocities <510 mm/yr suggest there is a genetic link between margin tectonic style and COT nature that strongly depends on the lower crustal strength. Our results imply that very slow extension velocities (< 5 mm/yr) and a strong lower crust lead to margins characterized by large oceanward dipping faults, strong syn-rift subsidence and abrupt crustal tapering beneath the continental shelf. These margins can be either narrow symmetric or asymmetric and present a COT with exhumed serpentinized mantle underlain by some magmatic products. In contrast, a weak lower crust promotes margins with a gentle crustal tapering, small faults dipping both oceanand landward and small syn-rift subsidence. Their COT is predominantly magmatic at any ultra-slow extension velocity and perhaps underlain by some serpentinized mantle. These margins can also be either symmetric or asymmetric. Our models predict that magmatic underplating mostly underlies the wide margin at weak asymmetric conjugates, whereas the wide margin is mainly underlain by serpentinized mantle at strong asymmetric margins. Based on this conceptual template, we propose different natures for the COTs in the South Atlantic.