By Jonathan Stempel

NEW YORK (Reuters) - Thornburg Mortgage Inc (TMA.N: Quote, Profile, Research) shares sank 51 percent on Thursday on worries the "jumbo" mortgage lender might go bankrupt, after its failure to meet a margin call triggered defaults under other lending agreements.

The decline followed Thornburg's disclosure in a late Wednesday filing with the U.S. Securities and Exchange Commission that it failed to meet a $28 million margin call from JPMorgan Chase & Co (JPM.N: Quote, Profile, Research), and that the bank would exercise its rights under a $320 million loan.

Margin calls force borrowers to pay back loans or post more collateral.

Thornburg said the JPMorgan notification triggered defaults under its other reverse repurchase and secured loan agreements, and its obligations under those agreements were "material."

The Santa Fe, New Mexico-based company has suffered as the housing slump and tight credit led investors to shun securities they no longer consider safe. These include the higher-rated mortgages above $417,000, which Fannie Mae (FNM.N: Quote, Profile, Research) and Freddie Mac (FRE.N: Quote, Profile, Research) cannot buy and in which Thornburg specializes.

"Thornburg now appears to be on the ropes," wrote Jason Arnold, an analyst at RBC Capital Markets, in a report titled "Game-Over at Thornburg?"

"With other lenders seizing assets, Thornburg would have limited access to liquidity and limited means to resolve other capital and funding needs," Arnold continued. "A bankruptcy filing is a more likely outcome." The analyst rates Thornburg "underperform," and halved his price target to $1 per share.

Thornburg spokeswoman Suzanne O'Leary Lopez declined to comment.