The sudden collapse of a little-known, California-based tech lender fueled market chaos on Friday and sparked fears of a wider contagion that some experts worry could upend the US banking sector. Silicon Valley Bank — a 40-year-old lender to startups and venture capitalists — became the second-biggest bank casualty in US history as it was abruptly shut down on Friday by the California Department of Financial Protection and Innovation, which placed its remaining assets under the Federal Deposit Insurance Corp.’s control.