Search

Within the uncertainties of involved astronomical and biological parameters, the Drake Equation typically predicts that there should be many exoplanets in our galaxy hosting active, communicative civilizations (ACCs). These optimistic calculations are however not supported by evidence, which is often referred to as the Fermi Paradox. Here, we elaborate on this long-standing enigma by showing the importance of planetary tectonic style for biological evolution. We summarize growing evidence that a prolonged transition from Mesoproterozoic active single lid tectonics (1.6 to 1.0 Ga) to modern plate tectonics occurred in the Neoproterozoic Era (1.0 to 0.541 Ga), which dramatically accelerated emergence and evolution of complex species. We further suggest that both continents and oceans are required for ACCs because early evolution of simple life must happen in water but late evolution of advanced life capable of creating technology must happen on land. We resolve the Fermi Paradox (1) by adding two additional terms to the Drake Equation: foc (the fraction of habitable exoplanets with significant continents and oceans) and fpt (the fraction of habitable exoplanets with significant continents and oceans that have had plate tectonics operating for at least 0.5 Ga); and (2) by demonstrating that the product of foc and fpt is very small (< 0.00003–0.002). We propose that the lack of evidence for ACCs reflects the scarcity of long-lived plate tectonics and/or continents and oceans on exoplanets with primitive life. [ABSTRACT FROM AUTHOR]

[Display omitted] • A scoring system proposed for ophiolite identification. • Oldest confident ophiolites appear in the Neoarchean. • Ophiolite frequency reflects regions of extensive study and preservation biases. • Most common ophiolite tectonic setting is forearcs. • Tectonic significance of Mesoproterozoic ophiolites is uncertain. Ophiolites are fragments of oceanic lithosphere that are produced at spreading centers at ocean ridges, back arc basins, or forearcs during subduction initiation, and are key indicators of plate tectonics. Although it is widely agreed that ophiolites are remnants of oceanic crust and associated depleted mantle preserved in orogens, the recognition of ophiolites and their tectonic significance is still a topic of discussion and disagreement. We propose that ophiolites can be recognized in the geologic record by some combination of genetically related pillow basalt, layered gabbro, sheeted dykes, podiform chromite, harzburgite or/and dunite. Mafic igneous rocks have either ocean-ridge basalt or immature oceanic arc basalt chemical compositions. Using a scoring system of 1–11 for ophiolite confidence level, scores of ≥ 8 are considered confident, 6–8 probable, and < 6 questionable or unlikely ophiolites. Most ophiolites with scores ≥ 6 are < 900 Ma. The oldest confident ophiolite (score of 8) is the Zunhua ophiolite in eastern China at 2550 Ma, and the oldest well documented sheeted dykes occur in the Jormua and Purtuniq ophiolites at 2000–1950 Ma. Ophiolites do not become geographically widespread until after 900 Ma, and most ophiolites of all ages formed in a forearc (subduction initiation) tectonic setting. If ophiolite production requires plate tectonics, subduction must have begun at least locally by 2700 Ma but did not become widespread until after 2000 Ma. The abundance of ophiolites after 900 Ma may reflect better preservation of subduction-related ophiolites, or to an increasing global network of interconnected plates. Ophiolite frequency peaks in the geologic record partially reflect geographic regions where ophiolites have been extensively studied rather than monitoring the production rate of ophiolites, but the scarcity of > 900 Ma ophiolites is probably real. [ABSTRACT FROM AUTHOR]

[Display omitted] • 8 lines of evidence indicate that plate tectonics occurred ∼1.8–2.0 Ga. • Tectonic evidence is adequate for the last 2 Ga of Earth history. • Earth alternates between plate and single lid tectonic regimes over past 2.5 Ga. Eight lines of evidence indicate that the Orosirian Period in mid-Paleoproterozoic time was characterized by plate tectonics: ophiolites, low T/P metamorphism including eclogites, passive margin formation, tall mountains, paleomagnetic constraints, ore deposits, abundant S-type granites, and seismic images of paleo-subduction zones. This plate tectonic episode occurred about 1 billion years earlier than the present plate tectonic episode began in Neoproterozoic time. The two plate tectonic episodes bracket the 'Boring Billion', which may have been a protracted single lid tectonic episode that began when the supercontinent Nuna or Columbia formed. Recognition of multiple lines of evidence for Orosirian plate tectonics demonstrates that Earth's tectonic style can be reconstructed with some confidence back to at least Early Paleoproterozoic time, and thus the absence of compelling evidence for Mesoproterozoic plate tectonics is not obvious due to poor preservation. A tectono-magmatic lull ∼2.3 Ga suggests an earlier episode of single lid tectonics. Evidence for two episodes of plate tectonics and two episodes of single lid tectonics indicates that Earth switched between single lid and plate tectonics multiple times during the last 2.4 Ga. [ABSTRACT FROM AUTHOR]

South of the latitude of Guam, the Mariana Trough exhibits both trenchparallel and trench-normal extension. In this study, we examined the locus of trench-normal extension separating the Philippine Sea plate from the broadly deforming Mariana platelet. Along this boundary, we identified three distinct modes of extension and described their distinguishing characteristics using deepand shallow-towed side-scan sonar and ship multibeam data along with regional geophysical, geochemical, and seismicity data. In the west, the Southwest Mariana Rift is an active tectonic rift exhibiting abundant strong earthquakes up to mb 6.7 and limited evidence of volcanism. In the east, the Malaguana-Gadao Ridge is a seafloor spreading center producing few and weak earthquakes less than mb 5. Between these zones, there is an ~20-40-km-wide and ~120-km-long area of high acoustic backscatter characterized by closely spaced volcano-tectonic ridges and small volcanic cones with distributed intermediate-strength seismicity up to mb 5.7. Fresh-looking volcanic rocks with high water contents and strong arc chemical affinities have been recovered from the high-backscatter zone. We interpret this morphologically and geophysically distinct zone as undergoing diffuse spreading, a distributed form of magmatic crustal accretion where new crust forms within a broad zone tens of kilometers across rather than along a narrow spreading axis. Diffuse spreading appears to be a rheological threshold effect enabled by slow opening rates and a high slab-fluid flux that facilitate the formation of a broad zone of weak hydrous lithosphere, within which new crust is accreted. Our findings describe a poorly understood process in plate tectonics, and observations of similar terrains in other backarc basins suggest that this process is not unique to the Mariana Trough. [ABSTRACT FROM AUTHOR]