The relatively well-preserved ancient crust of Mars provides a natural window into early planetary evolution not available on Earth due to sustained tectonic recycling and erosion on this planet. Mars has generally been considered a one-plate, basaltic planet though recent evidence suggests magmatic evolution resulting in felsic crust might have occurred sporadically1–6 and some structural data potentially support early forms of regional subduction7. Here we show multiple lines of evidence for diverse volcanism and complex volcanotectonics in the southern highlands of Mars within and around the Eridania basin. Infrared remote sensing reveals bimodal volcanism consisting of olivine-bearing basalts and voluminous, widespread dacitic (64-69% SiO2) volcanic deposits. The diverse igneous compositions are associated with an extraordinary number and morphological range of volcanic structures including domes, stratovolcanoes, calderas and pyroclastic shields. Tectonic escarpments of comparable morphology, length (100s-1000s of km) and height (1-3 km) to convergent tectonic boundaries on Earth are observed to overthrust hydrated crust in the Eridania region, including weathered ash8 and seafloor hydrothermal deposits9. Multiple lines of evidence point to a substantial, vigorous episode of volcanotectonics in the Eridania region (Terras Cimmeria and Sirenum) 3-4 billion years ago that could shed light on processes which operated on the early Earth, before a recognizable form of plate tectonics took hold on this planet c.a. 3 billion years ago10. The various environments associated with emergent volcanoes within and around the large (~2 x 105 km3) Eridania sea are analogous to all widely discussed scenarios for the origin of life for Earth11,12.